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Chemical Formulas, the Periodic Table & Compounds

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Chem 115 — Ch 2.4–2.6

CHEM 115 • Lecture 4 • Sep 12, 2026

Chemical Formulas · The Periodic Table · Molecular & Ionic Compounds

Key Points

Molecular formula shows how many of each atom; empirical formula is the simplest whole-number ratio; structural formula shows how atoms are connected.
A subscript is written only when more than one atom of that type is present, and it always follows the symbol — never in front.
2H (two separate atoms) and H₂ (one bonded molecule) are not the same species.
The seven diatomic elements are H, O, N, Cl, Br, I, F (HONClBrIF).
Period = row; group = column. Elements in the same group share similar properties.
Charge follows the column: groups 1, 2, 13 form +1, +2, +3; groups 15, 16, 17 form −3, −2, −1; group 18 forms no ions.
Metal + nonmetal = ionic bond (electrons stolen); nonmetal + nonmetal = covalent bond (electrons shared).
Ionic compounds only conduct electricity when molten or dissolved — never as a dry solid.

The Three Kinds of Formula

Molecular formula = how many of each atom are actually in one molecule. Example: benzene = C₆H₆.
Empirical formula = the simplest whole-number ratio of atoms. Benzene = CH.
Structural formula = how the atoms are attached — it draws the connections, not just the counts.

Formula type	Answers the question	Benzene
Molecular	How many of each atom?	C₆H₆
Empirical	Simplest ratio?	CH
Structural	What is bonded to what?	ring drawing

Trick Empirical =

\frac{{"reduce it like a fraction." C}_{6} H_{12} {Br}_{4}}{2 \to C_{3} H_{6} {Br}_{2}}

. If the numbers share no common factor, the empirical and molecular formulas are the same.

Reading and Writing Symbols

Chemical symbols = the types of atoms; subscripts after a symbol = how many of that atom.
A subscript is written only when there is more than one atom of that type. CO means 1 C and 1 O — never "C₁O₁".
The element symbol always comes first, the number after it: C₄H₈F₂, never "4C 8H 2F".

Test trap Writing the count in front of the symbol. "4C₈H₂F" is not a formula — the numbers belong after each symbol as subscripts: C₄H₈F₂.

Subscripts vs. Coefficients

A number in front (coefficient) counts whole particles. A number below (subscript) counts atoms inside one particle.

Symbol	What it is
H	one H atom
2H	two separate H atoms
H₂	one H₂ molecule (2 atoms bonded)
2H₂	two H₂ molecules (4 atoms total)

Two loose H atoms next to a bonded H2 molecule — Slide 4 Notice: 2H is two loose balls; H₂ is two balls stuck together. Same two atoms, completely different species.

Test trap Treating 2H and H₂ as the same thing. 2H = two unbonded atoms; H₂ = one bonded molecule. Exam questions live on this difference.

Elements That Exist as Molecules

Some elements exist as individual atoms: He, Ar, and the other noble gases.
Diatomic molecules (molecular elements) come in pairs: H₂, N₂, O₂, F₂, Cl₂ — bonded H–H, N≡N, O=O, F–F, Cl–Cl.
Larger molecular element: sulfur, S₈.

Trick The seven diatomics spell HONClBrIF ("Have No Fear Of Ice Cold Beer") = H, O, N, Cl, Br, I, F. Your slides list five of them (H₂, N₂, O₂, F₂, Cl₂) — Br₂ and I₂ behave the same way.

Parentheses in Formulas

When a group of atoms repeats, put it in parentheses — the subscript then applies to everything inside.
Worked example: Ca(NO₂)₃ → 1 Ca; 3 NO₂ groups; 3 × 1 N = 3 N; 3 × 2 O = 6 O.

Test trap Forgetting to multiply every atom inside the parentheses. In Ca(NO₂)₃ the 3 hits both the N and the O — it is not 3 N and 2 O.

Test trap Separately: a real calcium + nitrite compound is Ca(NO₂)₂, because Ca²⁺ needs two NO₂⁻ to balance (Ch 2.6). The Ca(NO₂)₃ on the slide is a counting drill, not a real formula.

Worked example

Total atoms inside parentheses = subscript outside × atoms inside the group

How many N and O atoms are in Ca(NO₂)₃?

1 Ca outside the parentheses
The group (NO₂) has 1 N and 2 O
The subscript 3 multiplies everything inside the group: 3 × 1 N = 3 N
3 × 2 O = 6 O

Answer 1 Ca, 3 N, 6 O

Isomers

Isomers = compounds with the same chemical formula but different structures — so different properties and different uses.
Slide example: C₂H₄O₂ has more than one possible structure.
All three of these are C₆H₆O₂: catechol (pesticide precursor), resorcinol (acne treatment), hydroquinone (skin bleacher).

Trick Isomer = "same parts, different build." Like the same Lego bricks assembled two ways.

Test trap Thinking isomers are the same substance because the formula matches. A skin bleacher and a pesticide precursor share C₆H₆O₂ — the molecular formula alone never identifies a compound.

Quick self-check

Write the molecular formula for a compound with 4 carbons, 8 hydrogens and 2 fluorines.
Give the empirical formula for a compound with 6 C, 12 H and 4 Br.
How many atoms of each element are in Ca(NO₂)₃?
Explain the difference between 2H and H₂.
Benzene is C₆H₆. What is its empirical formula?
Name the five diatomic molecular elements given on the slides.
Two compounds are both C₆H₆O₂ but one treats acne and one bleaches skin. What is the term for this relationship?

Answers

C₄H₈F₂
C₃H₆Br₂ (divide by 2)
1 Ca, 3 N, 6 O
2H = two separate H atoms; H₂ = one molecule of two bonded H atoms
CH
H₂, N₂, O₂, F₂, Cl₂
They are isomers

How It Is Organised

The periodic table is an organised representation of all the elements; the arrangement itself summarises their properties.
Period = a row (across).
Group = a column (down).
Elements in the same group typically have similar properties — that is the whole point of the layout.

Trick "Period" is a row like a sentence reads across; "GrOUp" goes dOwn the cOlumn. Similar properties run vertically.

Test trap Swapping group and period on the exam. Period = horizontal row; group = vertical column. Similar behaviour = same group, not same period.

Metals, Nonmetals, Metalloids

Class	Appearance	Conducts heat & electricity
Metals	shiny, malleable	good conductors
Nonmetals	dull	poor conductors
Metalloids	mixed — some metal, some nonmetal properties	moderate conductors

Periodic table with the metal/metalloid/nonmetal staircase highlighted — Slide 12 Notice the heavy staircase line on the right: metals sit to its **left**, nonmetals to its **right**, and the metalloids are the elements the line touches. Group numbers run 1A–8A across the main-group columns.

Trick Metalloids hug the staircase: B, Si, Ge, As, Sb, Te ("Boys Should Get An Excellent Test"). Everything left of the stairs conducts; everything right of it does not.

Main Group, Transition, Inner Transition

Main group elements (also called representative elements): groups 1, 2, and 13–18 — labelled 1A–8A.
Transition metals: groups 3–13 (labelled 1B–8B) — the wide middle block.
Inner transition metals: the two rows pulled out at the bottom.
- Lanthanides = top row
- Actinides = bottom row

Trick The two bottom rows are named after the element each one starts with: the Lanthanides begin at La (57) on top, the Actinides at Ac (89) below.

The Special Groups (Memorise These)

Name	Group	Behaviour
Alkali metals	1 (except H)	highly reactive
Alkaline earth metals	2	fairly reactive
Halogens	17 (7A)	typically molecular elements (F₂, Cl₂)
Noble gases	18 (8A)	inert — do not react

Trick Left to right, reactivity flips: group 1 is the most eager to react, group 18 refuses entirely. "Alkali are wild, Noble are snobs." Halogens (17) are one step from being noble, so they grab one electron — remember that for Ch 2.6.

Test trap Calling hydrogen an alkali metal. H sits at the top of group 1 but is not an alkali metal — it is a nonmetal. The slide says "group 1 (except H)" for exactly this reason.

Test trap Softening "inert" to "unreactive-ish." Noble gases do not react — which is why they exist as single atoms (He, Ar), never as diatomic molecules.

Quick self-check

Is a period a row or a column? Which one groups elements with similar properties?
Which group are the halogens, and what is a typical form they take?
Name the three property classes and state which conducts electricity well.
Which groups make up the main group (representative) elements?
Lanthanides or actinides — which is the top row?
Why is hydrogen excluded from the alkali metals?
Group 18 elements are described by what one-word property?

Answers

Period = row; same group (column) = similar properties
Group 17 (7A); usually diatomic molecular elements like F₂, Cl₂
Metals (good conductors), nonmetals (poor), metalloids (moderate)
Groups 1, 2 and 13–18 (1A–8A)
Lanthanides
It is a nonmetal, not a metal
Inert

Chemical Bonds — the Two Kinds

Chemical bond = the force holding atoms together in a compound. Two types: ionic and covalent.
Ionic bond = electrons are stolen (transferred). Metal + nonmetal. Produces + and − ions.
Covalent bond = electrons are shared. Usually two or more nonmetals. Produces a molecule.

	Ionic compound	Molecular compound
Made of	metal + nonmetal	nonmetal + nonmetal
Electrons	transferred (stolen)	shared
Particles	+ and − ions	neutral molecules

Trick Metal + nonmetal = steal (ionic). Nonmetal + nonmetal = share (covalent). Metals are the thieves because they give electrons up easily.

Classifying Any Substance (4 Boxes)

Category	Looks like	Example
Atomic element	one element, single atoms	Al, Co
Molecular element	one element, bonded pair/group	Cl₂, N₂
Molecular compound	2+ elements, all nonmetals	CO, C₃H₆O
Ionic compound	2+ elements, metal + nonmetal	AlCl₃

Trick Two questions, in order: (1) One element or more than one? → element vs compound. (2) Is a metal involved? → ionic vs molecular. That pair of questions sorts every case on the slide.

Test trap Confusing molecular element with molecular compound. N₂ is a molecular element — it is still only one element, however many atoms are bonded. A compound needs two different elements.

Predicting Ionic Charge

Main-group metals lose electrons to match the preceding noble gas.
- Group 1: lose 1 e^- → 1+
- Group 2: lose 2 e^- → 2+
Some nonmetals gain electrons to match the next noble gas.
- Group 17: gain 1 e^- → 1−
- Group 16: gain 2 e^- → 2−
Transition metals cannot be predicted this way — many have more than one possible charge (Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺).

Periodic table columns labeled with typical ion charges — Slide 20 Notice the pattern by column: 1+, 2+ … then 3−, 2−, 1− as you approach group 18, which forms no ions. The middle block is irregular — that is why transition-metal charges must be memorised or given to you.

Trick Charge follows the column. Groups 1, 2, 13 → +1, +2, +3. Groups 15, 16, 17 → −3, −2, −1. Group 18 → none. Count how far the element sits from the nearest noble gas and that is the size of the charge.

Test trap Your slide prints "Group 16: gain 1 e^- → 2− charge." The charge is right but the electron count is a typo — group 16 gains 2 electrons to reach a 2− charge. O²⁻ and S²⁻ each take on two.

Cations, Anions, and Balancing

Cation = a positive ion (Mg²⁺, NH₄⁺).
Anion = a negative ion (Cl⁻, SO₄²⁻).
Ionic bonding = the force of attraction between oppositely charged ions.
Charges must balance — every ionic formula is overall neutral (net charge 0).

Trick Cation = "paw-sitive" (a cat has paws). Anion = A Negative ion. The cation is always written first in the formula.

Writing Ionic Formulas — the Crisscross

Net charge must be zero. Shortcut: the charge of one ion becomes the subscript of the other, then reduce.

Al³⁺ and Cl⁻ → the 3 goes to Cl → AlCl₃
Al + S: Al³⁺, S²⁻ → Al₂S₃
Ca + O: Ca²⁺, O²⁻ → Ca₂O₂ → reduce → CaO
Na + O → Na₂O · Al + O → Al₂O₃ · Sr + I → SrI₂

Test trap Forgetting to reduce. Mg²⁺ + S²⁻ crisscrosses to Mg₂S₂, but the answer is MgS. Always divide the subscripts by any common factor.

Worked example

Net charge = Σ(cation charge × count) + Σ(anion charge × count) = 0

Find the formula for aluminum and sulfur.

Aluminum forms Al³⁺, sulfur forms S^2-
Crisscross: the 3 from Al becomes the subscript on S, the 2 from S becomes the subscript on Al
Al₂S₃: net charge = 2×(+3) + 3×(−2) = 6 − 6 = 0
Check for a common factor to reduce — 2 and 3 share none, so Al₂S₃ is final

Answer Al₂S₃

Polyatomic Ions (Memorise)

Name	Formula	Name	Formula
ammonium	NH₄⁺	chromate	CrO₄^2-
hydronium	H₃O⁺	dichromate	Cr₂O₇^2-
peroxide	O₂^2-	permanganate	MnO₄^-
hydroxide	OH^-	nitrate	NO₃^-
acetate	CH₃COO^-	nitrite	NO₂^-
cyanide	CN^-	sulfate	SO₄^2-
azide	N₃^-	hydrogen sulfate	HSO₄^-
carbonate	CO₃^2-	sulfite	SO₃^2-
bicarbonate	HCO₃^-	hydrogen sulfite	HSO₃^-
dihydrogen phosphate	H₂PO₄^-	phosphate	PO₄^3-
hydrogen phosphate	HPO₄^2-	perchlorate	ClO₄^-
chlorate	ClO₃^-	chlorite	ClO₂^-
hypochlorite	ClO^-

Trick The oxygen ladder for chlorine: perchlorate ClO₄^- → chlorate ClO₃^- → chlorite ClO₂^- → hypochlorite ClO^-. Same rule everywhere: -ate has one more O than -ite (nitrate NO₃^- vs nitrite NO₂^-; sulfate SO₄^2- vs sulfite SO₃^2-). Adding an H⁺ raises the charge by one: SO₄^2- → HSO₄^-.

Test trap Mixing up -ate and -ite. They differ by one oxygen, not by charge: NO₃^- and NO₂^- are both 1−.

Properties — Ionic vs Covalent

Property	Ionic compounds	Covalent (molecular)
State	usually solids	often gases, low-boiling liquids, low-melting solids
Melting/boiling point	high	low
Conducts as a solid	no	no
Conducts molten or aqueous	yes	no
Basic unit	ions in a lattice	discrete neutral molecules

Slide example: solid NaCl does not conduct electricity; molten NaCl (above 800°C) does.

Trick Ions must be free to move to carry current. Locked in a solid lattice = no conduction; melted or dissolved = conduction. Frozen ions cannot run.

Test trap Saying "ionic compounds conduct electricity." Only when molten or dissolved. As a dry solid they are nonconductive — that distinction is the exam question.

Compounds With Both Bond Types

A compound containing a polyatomic ion has ionic bonding between the ions and covalent bonding inside the polyatomic ion. Example: Na₂SO₄ — ionic between Na⁺ and SO₄^2-, covalent within SO₄^2-.

Test trap Answering "NaCl" for a both-bond-types question. NaCl is purely ionic — it has no polyatomic group. Look for a metal plus a polyatomic ion.

Quick self-check

Classify each: Al · AlCl₃ · Cl₂ · C₃H₆O · CO · Co.
Is N₂ an atomic element, molecular element, molecular compound or ionic compound?
Which pair forms an ionic bond: Li+Na, N+Cl, Mg+Br, Ar+K?
Write the formula from potassium and oxygen.
Which pair forms a covalent bond: Ca+Cl, Mg+Na, C+S, C+Cs?
Which compound shows both ionic and covalent bonding: SO₂, CF₄, NaCl, Na₂SO₄, P₄O₁₀?
Why does molten NaCl conduct electricity when solid NaCl does not?
What charge does a group 2 metal form, and why?

Answers

Al = atomic element; AlCl₃ = ionic compound; Cl₂ = molecular element; C₃H₆O = molecular compound; CO = molecular compound; Co = atomic element
Molecular element
Mg + Br (metal + nonmetal)
K₂O
C + S (two nonmetals)
Na₂SO₄
Melting frees the ions to move; ions locked in the solid lattice cannot carry charge
2+, by losing 2 electrons to match the preceding noble gas

Added beyond your slides (6)

HONClBrIF mnemonic adds Br₂ and I₂ to the five diatomics the slides list explicitly.
The note that the real calcium nitrite formula is Ca(NO₂)₂, not the Ca(NO₂)₃ counting drill on the slide.
The correction that group 16 nonmetals gain 2 electrons (not 1, as the slide's typo states) to reach a 2− charge.
Worked formula blocks for the Ca(NO₂)₃ atom count and the Al+S crisscross (Al₂S₃) — step-by-step derivations not spelled out on the slides.
All memory mnemonics (HONClBrIF, 'Boys Should Get An Excellent Test', 'Alkali are wild, Noble are snobs', the oxygen ladder for chlorine, cation='paw-sitive') are study aids added beyond the slide content.
The staircase callouts (Metals / Nonmetals / Staircase = metalloids) on the periodic table figure are added annotations, not part of the original slide image.

In short

Chem 115 — Ch 2.4–2.6

Chapters 2.4–2.6 build the vocabulary of chemical formulas, then use the periodic table to predict how atoms combine into compounds. A chemical formula can describe atom counts (molecular), the simplest ratio (empirical), or the actual bonding pattern (structural) — and compounds with the same formula can still be different substances (isomers). The periodic table's rows (periods) and columns (groups) organize elements by shared behavior, letting main-group charges be predicted directly from column position. Ionic and covalent bonds explain why some compounds are solid lattices that only conduct when melted or dissolved, while others are discrete, low-melting molecules.

Molecular formula = atom counts; empirical formula = simplest ratio; structural formula = how atoms connect.
Subscripts go after the symbol and are omitted when the count is 1; coefficients count whole particles, subscripts count atoms inside one particle.
Seven elements are diatomic: H, N, O, F, Cl, Br, I (HONClBrIF); sulfur forms S₈.
Parentheses group repeating atom clusters — the outside subscript multiplies every atom inside.

Show more →

Isomers share a molecular formula but differ in structure, giving different properties (catechol, resorcinol, hydroquinone are all C₆H₆O₂).
Period = row, group = column; elements in the same group share properties.
Metals sit left of the staircase (B, Si, Ge, As, Sb, Te), nonmetals to its right; metalloids straddle it.
Main group = groups 1, 2, 13–18; transition metals = groups 3–13; lanthanides/actinides are the two pulled-out rows.
Alkali metals (group 1, not H), alkaline earths (group 2), halogens (group 17), and noble gases (group 18) are the four special groups to memorize.
Metal + nonmetal → ionic bond (electrons stolen); nonmetal + nonmetal → covalent bond (electrons shared).
Main-group ion charge follows column distance from the nearest noble gas; transition metal charges must be memorized.
The crisscross method builds ionic formulas from ion charges, then the ratio must be reduced to lowest terms.

Remember

Charge follows the column.
2H and H₂ are not the same species.
Ionic compounds conduct only when molten or dissolved.
-ate has one more oxygen than -ite.
Metal+nonmetal steals; nonmetal+nonmetal shares.

Practice questions

What is the molecular formula for a compound with 4 carbons, 8 hydrogens and 2 fluorines?
- A4C8H2F
- BC₄H₈F₂
- C4C 8H 2F
- DC₂H₄F
Show answer
C₄H₈F₂ — The element symbol comes first with the count as a subscript after it; the count never goes in front of the symbol.Slide 6
What is the empirical formula for a compound with 6 carbons, 12 hydrogens and 4 bromines?
- AC₆H₁₂Br₄
- BC₃H₆Br₂
- CC₆H₁₂B₄
- DC₃H₆B₂
Show answer
C₃H₆Br₂ — Divide every subscript by the common factor 2 to reduce to the simplest whole-number ratio.Slide 6
How many atoms of each element are in Ca(NO₂)₃?
- A1 Ca, 3 N, 3 O
- B1 Ca, 3 N, 6 O
- C1 Ca, 1 N, 6 O
- D3 Ca, 3 N, 6 O
Show answer
1 Ca, 3 N, 6 O — The subscript 3 outside the parentheses multiplies every atom inside the NO₂ group: 3×1 N and 3×2 O.Slide 7
Which best describes the relationship between catechol, resorcinol, and hydroquinone?
- AThey are the same compound
- BThey are isomers of C₆H₆O₂
- CThey have different molecular formulas
- DThey are diatomic elements
Show answer
They are isomers of C₆H₆O₂ — All three share the formula C₆H₆O₂ but differ in the positions of the OH groups, giving them different properties.Slide 8
Is a period a row or a column, and which one groups similar properties?
- APeriod = column; group = row groups properties
- BPeriod = row; group (column) groups properties
- CBoth are rows
- DBoth are columns
Show answer
Period = row; group (column) groups properties — Period runs horizontally across the table; elements sharing a group (a vertical column) share similar properties.Slide 11
Which elements are the metalloids?
- AB, Si, Ge, As, Sb, Te
- BLi, Na, K, Rb, Cs, Fr
- CF, Cl, Br, I, At
- DHe, Ne, Ar, Kr, Xe
Show answer
B, Si, Ge, As, Sb, Te — "Boys Should Get An Excellent Test" — these six elements straddle the staircase line and have mixed metal/nonmetal properties.Slide 12
Why is hydrogen not classified as an alkali metal?
- AIt is radioactive
- BIt is a nonmetal, not a metal
- CIt has no electrons
- DIt is a noble gas
Show answer
It is a nonmetal, not a metal — Hydrogen sits atop group 1 but behaves as a nonmetal, so slides exclude it with "group 1 (except H)."Slide 14
Which pair of elements forms an ionic bond?
- ALi + Na
- BN + Cl
- CMg + Br
- DAr + K
Show answer
Mg + Br — Ionic bonds form between a metal and a nonmetal; Mg is a metal and Br is a nonmetal.Slide 22
What is the formula for potassium and oxygen?
- AK₂O
- BKO₂
- CP₂O
- DPO₂
Show answer
K₂O — K⁺ and O^2- crisscross: the 2 from oxygen becomes the subscript on potassium, giving K₂O.Slide 23
Which compound exhibits both ionic and covalent bonding?
- ASO₂
- BCF₄
- CNaCl
- DNa₂SO₄
Show answer
Na₂SO₄ — Na₂SO₄ has ionic bonding between Na⁺ and the SO₄^2- polyatomic ion, and covalent bonding within the SO₄^2- ion itself.Slide 27
Why does molten NaCl conduct electricity but solid NaCl does not?
- AMolten NaCl has more ions
- BMelting frees the ions to move; they are locked in place as a solid lattice
- CSolid NaCl has no ions at all
- DElectricity only flows through covalent bonds
Show answer
Melting frees the ions to move; they are locked in place as a solid lattice — Ionic compounds only conduct when the ions are mobile — molten or dissolved, never as a dry solid.Slide 24
Group 16 nonmetals gain how many electrons to form their typical ion?
- A1 electron, forming a 1− charge
- B2 electrons, forming a 2− charge
- C3 electrons, forming a 3− charge
- D0 electrons, forming a neutral atom
Show answer
2 electrons, forming a 2− charge — The source slide's "gain 1 e-" for group 16 is a typo — the charge (2−) is correct, but reaching it requires gaining 2 electrons, as with O^2- and S^2-.Slide 20

Chem 115 — Ch 2.4–2.6

CHEM 115 • Lecture 4 • Sep 12, 2026

Chemical Formulas · The Periodic Table · Molecular & Ionic Compounds

Key Points

Molecular formula shows how many of each atom; empirical formula is the simplest whole-number ratio; structural formula shows how atoms are connected.
A subscript is written only when more than one atom of that type is present, and it always follows the symbol — never in front.
2H (two separate atoms) and H₂ (one bonded molecule) are not the same species.
The seven diatomic elements are H, O, N, Cl, Br, I, F (HONClBrIF).
Period = row; group = column. Elements in the same group share similar properties.
Charge follows the column: groups 1, 2, 13 form +1, +2, +3; groups 15, 16, 17 form −3, −2, −1; group 18 forms no ions.
Metal + nonmetal = ionic bond (electrons stolen); nonmetal + nonmetal = covalent bond (electrons shared).
Ionic compounds only conduct electricity when molten or dissolved — never as a dry solid.

The Three Kinds of Formula

Molecular formula = how many of each atom are actually in one molecule. Example: benzene = C₆H₆.
Empirical formula = the simplest whole-number ratio of atoms. Benzene = CH.
Structural formula = how the atoms are attached — it draws the connections, not just the counts.

Formula type	Answers the question	Benzene
Molecular	How many of each atom?	C₆H₆
Empirical	Simplest ratio?	CH
Structural	What is bonded to what?	ring drawing

Trick Empirical =

\frac{{"reduce it like a fraction." C}_{6} H_{12} {Br}_{4}}{2 \to C_{3} H_{6} {Br}_{2}}

. If the numbers share no common factor, the empirical and molecular formulas are the same.

Reading and Writing Symbols

Chemical symbols = the types of atoms; subscripts after a symbol = how many of that atom.
A subscript is written only when there is more than one atom of that type. CO means 1 C and 1 O — never "C₁O₁".
The element symbol always comes first, the number after it: C₄H₈F₂, never "4C 8H 2F".

Test trap Writing the count in front of the symbol. "4C₈H₂F" is not a formula — the numbers belong after each symbol as subscripts: C₄H₈F₂.

Subscripts vs. Coefficients

A number in front (coefficient) counts whole particles. A number below (subscript) counts atoms inside one particle.

Symbol	What it is
H	one H atom
2H	two separate H atoms
H₂	one H₂ molecule (2 atoms bonded)
2H₂	two H₂ molecules (4 atoms total)

Test trap Treating 2H and H₂ as the same thing. 2H = two unbonded atoms; H₂ = one bonded molecule. Exam questions live on this difference.

Elements That Exist as Molecules

Some elements exist as individual atoms: He, Ar, and the other noble gases.
Diatomic molecules (molecular elements) come in pairs: H₂, N₂, O₂, F₂, Cl₂ — bonded H–H, N≡N, O=O, F–F, Cl–Cl.
Larger molecular element: sulfur, S₈.

Parentheses in Formulas

When a group of atoms repeats, put it in parentheses — the subscript then applies to everything inside.
Worked example: Ca(NO₂)₃ → 1 Ca; 3 NO₂ groups; 3 × 1 N = 3 N; 3 × 2 O = 6 O.

Test trap Forgetting to multiply every atom inside the parentheses. In Ca(NO₂)₃ the 3 hits both the N and the O — it is not 3 N and 2 O.

Worked example

Total atoms inside parentheses = subscript outside × atoms inside the group

How many N and O atoms are in Ca(NO₂)₃?

1 Ca outside the parentheses
The group (NO₂) has 1 N and 2 O
The subscript 3 multiplies everything inside the group: 3 × 1 N = 3 N
3 × 2 O = 6 O

Answer 1 Ca, 3 N, 6 O

Isomers

Isomers = compounds with the same chemical formula but different structures — so different properties and different uses.
Slide example: C₂H₄O₂ has more than one possible structure.
All three of these are C₆H₆O₂: catechol (pesticide precursor), resorcinol (acne treatment), hydroquinone (skin bleacher).

Trick Isomer = "same parts, different build." Like the same Lego bricks assembled two ways.

Quick self-check

Write the molecular formula for a compound with 4 carbons, 8 hydrogens and 2 fluorines.
Give the empirical formula for a compound with 6 C, 12 H and 4 Br.
How many atoms of each element are in Ca(NO₂)₃?
Explain the difference between 2H and H₂.
Benzene is C₆H₆. What is its empirical formula?
Name the five diatomic molecular elements given on the slides.
Two compounds are both C₆H₆O₂ but one treats acne and one bleaches skin. What is the term for this relationship?

Answers

C₄H₈F₂
C₃H₆Br₂ (divide by 2)
1 Ca, 3 N, 6 O
2H = two separate H atoms; H₂ = one molecule of two bonded H atoms
CH
H₂, N₂, O₂, F₂, Cl₂
They are isomers

How It Is Organised

The periodic table is an organised representation of all the elements; the arrangement itself summarises their properties.
Period = a row (across).
Group = a column (down).
Elements in the same group typically have similar properties — that is the whole point of the layout.

Trick "Period" is a row like a sentence reads across; "GrOUp" goes dOwn the cOlumn. Similar properties run vertically.

Test trap Swapping group and period on the exam. Period = horizontal row; group = vertical column. Similar behaviour = same group, not same period.

Metals, Nonmetals, Metalloids

Class	Appearance	Conducts heat & electricity
Metals	shiny, malleable	good conductors
Nonmetals	dull	poor conductors
Metalloids	mixed — some metal, some nonmetal properties	moderate conductors

Trick Metalloids hug the staircase: B, Si, Ge, As, Sb, Te ("Boys Should Get An Excellent Test"). Everything left of the stairs conducts; everything right of it does not.

Main Group, Transition, Inner Transition

Main group elements (also called representative elements): groups 1, 2, and 13–18 — labelled 1A–8A.
Transition metals: groups 3–13 (labelled 1B–8B) — the wide middle block.
Inner transition metals: the two rows pulled out at the bottom.
- Lanthanides = top row
- Actinides = bottom row

Trick The two bottom rows are named after the element each one starts with: the Lanthanides begin at La (57) on top, the Actinides at Ac (89) below.

The Special Groups (Memorise These)

Name	Group	Behaviour
Alkali metals	1 (except H)	highly reactive
Alkaline earth metals	2	fairly reactive
Halogens	17 (7A)	typically molecular elements (F₂, Cl₂)
Noble gases	18 (8A)	inert — do not react

Test trap Calling hydrogen an alkali metal. H sits at the top of group 1 but is not an alkali metal — it is a nonmetal. The slide says "group 1 (except H)" for exactly this reason.

Test trap Softening "inert" to "unreactive-ish." Noble gases do not react — which is why they exist as single atoms (He, Ar), never as diatomic molecules.

Quick self-check

Is a period a row or a column? Which one groups elements with similar properties?
Which group are the halogens, and what is a typical form they take?
Name the three property classes and state which conducts electricity well.
Which groups make up the main group (representative) elements?
Lanthanides or actinides — which is the top row?
Why is hydrogen excluded from the alkali metals?
Group 18 elements are described by what one-word property?

Answers

Period = row; same group (column) = similar properties
Group 17 (7A); usually diatomic molecular elements like F₂, Cl₂
Metals (good conductors), nonmetals (poor), metalloids (moderate)
Groups 1, 2 and 13–18 (1A–8A)
Lanthanides
It is a nonmetal, not a metal
Inert

Chemical Bonds — the Two Kinds

Chemical bond = the force holding atoms together in a compound. Two types: ionic and covalent.
Ionic bond = electrons are stolen (transferred). Metal + nonmetal. Produces + and − ions.
Covalent bond = electrons are shared. Usually two or more nonmetals. Produces a molecule.

	Ionic compound	Molecular compound
Made of	metal + nonmetal	nonmetal + nonmetal
Electrons	transferred (stolen)	shared
Particles	+ and − ions	neutral molecules

Trick Metal + nonmetal = steal (ionic). Nonmetal + nonmetal = share (covalent). Metals are the thieves because they give electrons up easily.

Classifying Any Substance (4 Boxes)

Category	Looks like	Example
Atomic element	one element, single atoms	Al, Co
Molecular element	one element, bonded pair/group	Cl₂, N₂
Molecular compound	2+ elements, all nonmetals	CO, C₃H₆O
Ionic compound	2+ elements, metal + nonmetal	AlCl₃

Predicting Ionic Charge

Main-group metals lose electrons to match the preceding noble gas.
- Group 1: lose 1 e^- → 1+
- Group 2: lose 2 e^- → 2+
Some nonmetals gain electrons to match the next noble gas.
- Group 17: gain 1 e^- → 1−
- Group 16: gain 2 e^- → 2−
Transition metals cannot be predicted this way — many have more than one possible charge (Fe²⁺/Fe³⁺, Cu⁺/Cu²⁺).

Cations, Anions, and Balancing

Cation = a positive ion (Mg²⁺, NH₄⁺).
Anion = a negative ion (Cl⁻, SO₄²⁻).
Ionic bonding = the force of attraction between oppositely charged ions.
Charges must balance — every ionic formula is overall neutral (net charge 0).

Trick Cation = "paw-sitive" (a cat has paws). Anion = A Negative ion. The cation is always written first in the formula.

Writing Ionic Formulas — the Crisscross

Net charge must be zero. Shortcut: the charge of one ion becomes the subscript of the other, then reduce.

Al³⁺ and Cl⁻ → the 3 goes to Cl → AlCl₃
Al + S: Al³⁺, S²⁻ → Al₂S₃
Ca + O: Ca²⁺, O²⁻ → Ca₂O₂ → reduce → CaO
Na + O → Na₂O · Al + O → Al₂O₃ · Sr + I → SrI₂

Test trap Forgetting to reduce. Mg²⁺ + S²⁻ crisscrosses to Mg₂S₂, but the answer is MgS. Always divide the subscripts by any common factor.

Worked example

Net charge = Σ(cation charge × count) + Σ(anion charge × count) = 0

Find the formula for aluminum and sulfur.

Aluminum forms Al³⁺, sulfur forms S^2-
Crisscross: the 3 from Al becomes the subscript on S, the 2 from S becomes the subscript on Al
Al₂S₃: net charge = 2×(+3) + 3×(−2) = 6 − 6 = 0
Check for a common factor to reduce — 2 and 3 share none, so Al₂S₃ is final

Answer Al₂S₃

Polyatomic Ions (Memorise)

Name	Formula	Name	Formula
ammonium	NH₄⁺	chromate	CrO₄^2-
hydronium	H₃O⁺	dichromate	Cr₂O₇^2-
peroxide	O₂^2-	permanganate	MnO₄^-
hydroxide	OH^-	nitrate	NO₃^-
acetate	CH₃COO^-	nitrite	NO₂^-
cyanide	CN^-	sulfate	SO₄^2-
azide	N₃^-	hydrogen sulfate	HSO₄^-
carbonate	CO₃^2-	sulfite	SO₃^2-
bicarbonate	HCO₃^-	hydrogen sulfite	HSO₃^-
dihydrogen phosphate	H₂PO₄^-	phosphate	PO₄^3-
hydrogen phosphate	HPO₄^2-	perchlorate	ClO₄^-
chlorate	ClO₃^-	chlorite	ClO₂^-
hypochlorite	ClO^-

Test trap Mixing up -ate and -ite. They differ by one oxygen, not by charge: NO₃^- and NO₂^- are both 1−.

Properties — Ionic vs Covalent

Property	Ionic compounds	Covalent (molecular)
State	usually solids	often gases, low-boiling liquids, low-melting solids
Melting/boiling point	high	low
Conducts as a solid	no	no
Conducts molten or aqueous	yes	no
Basic unit	ions in a lattice	discrete neutral molecules

Slide example: solid NaCl does not conduct electricity; molten NaCl (above 800°C) does.

Trick Ions must be free to move to carry current. Locked in a solid lattice = no conduction; melted or dissolved = conduction. Frozen ions cannot run.

Test trap Saying "ionic compounds conduct electricity." Only when molten or dissolved. As a dry solid they are nonconductive — that distinction is the exam question.

Compounds With Both Bond Types

Test trap Answering "NaCl" for a both-bond-types question. NaCl is purely ionic — it has no polyatomic group. Look for a metal plus a polyatomic ion.

Quick self-check

Classify each: Al · AlCl₃ · Cl₂ · C₃H₆O · CO · Co.
Is N₂ an atomic element, molecular element, molecular compound or ionic compound?
Which pair forms an ionic bond: Li+Na, N+Cl, Mg+Br, Ar+K?
Write the formula from potassium and oxygen.
Which pair forms a covalent bond: Ca+Cl, Mg+Na, C+S, C+Cs?
Which compound shows both ionic and covalent bonding: SO₂, CF₄, NaCl, Na₂SO₄, P₄O₁₀?
Why does molten NaCl conduct electricity when solid NaCl does not?
What charge does a group 2 metal form, and why?

Answers

Al = atomic element; AlCl₃ = ionic compound; Cl₂ = molecular element; C₃H₆O = molecular compound; CO = molecular compound; Co = atomic element
Molecular element
Mg + Br (metal + nonmetal)
K₂O
C + S (two nonmetals)
Na₂SO₄
Melting frees the ions to move; ions locked in the solid lattice cannot carry charge
2+, by losing 2 electrons to match the preceding noble gas

Added beyond your slides (6)

HONClBrIF mnemonic adds Br₂ and I₂ to the five diatomics the slides list explicitly.
The note that the real calcium nitrite formula is Ca(NO₂)₂, not the Ca(NO₂)₃ counting drill on the slide.
The correction that group 16 nonmetals gain 2 electrons (not 1, as the slide's typo states) to reach a 2− charge.
Worked formula blocks for the Ca(NO₂)₃ atom count and the Al+S crisscross (Al₂S₃) — step-by-step derivations not spelled out on the slides.
All memory mnemonics (HONClBrIF, 'Boys Should Get An Excellent Test', 'Alkali are wild, Noble are snobs', the oxygen ladder for chlorine, cation='paw-sitive') are study aids added beyond the slide content.
The staircase callouts (Metals / Nonmetals / Staircase = metalloids) on the periodic table figure are added annotations, not part of the original slide image.

In short

Chem 115 — Ch 2.4–2.6

Molecular formula = atom counts; empirical formula = simplest ratio; structural formula = how atoms connect.
Subscripts go after the symbol and are omitted when the count is 1; coefficients count whole particles, subscripts count atoms inside one particle.
Seven elements are diatomic: H, N, O, F, Cl, Br, I (HONClBrIF); sulfur forms S₈.
Parentheses group repeating atom clusters — the outside subscript multiplies every atom inside.

Show more →

Isomers share a molecular formula but differ in structure, giving different properties (catechol, resorcinol, hydroquinone are all C₆H₆O₂).
Period = row, group = column; elements in the same group share properties.
Metals sit left of the staircase (B, Si, Ge, As, Sb, Te), nonmetals to its right; metalloids straddle it.
Main group = groups 1, 2, 13–18; transition metals = groups 3–13; lanthanides/actinides are the two pulled-out rows.
Alkali metals (group 1, not H), alkaline earths (group 2), halogens (group 17), and noble gases (group 18) are the four special groups to memorize.
Metal + nonmetal → ionic bond (electrons stolen); nonmetal + nonmetal → covalent bond (electrons shared).
Main-group ion charge follows column distance from the nearest noble gas; transition metal charges must be memorized.
The crisscross method builds ionic formulas from ion charges, then the ratio must be reduced to lowest terms.

Remember

Charge follows the column.
2H and H₂ are not the same species.
Ionic compounds conduct only when molten or dissolved.
-ate has one more oxygen than -ite.
Metal+nonmetal steals; nonmetal+nonmetal shares.

Chemical Formulas, the Periodic Table & Compounds

Key Points

2.4 Chemical Formulas

The Three Kinds of Formula

Reading and Writing Symbols

Subscripts vs. Coefficients

Elements That Exist as Molecules

Parentheses in Formulas

Isomers

2.5 The Periodic Table

How It Is Organised

Metals, Nonmetals, Metalloids

Main Group, Transition, Inner Transition

The Special Groups (Memorise These)

2.6 Molecular and Ionic Compounds

Chemical Bonds — the Two Kinds

Classifying Any Substance (4 Boxes)

Predicting Ionic Charge

Cations, Anions, and Balancing

Writing Ionic Formulas — the Crisscross

Polyatomic Ions (Memorise)

Properties — Ionic vs Covalent

Compounds With Both Bond Types

In short

Chemical Formulas, the Periodic Table & Compounds

Key Points

2.4 Chemical Formulas

The Three Kinds of Formula

Reading and Writing Symbols

Subscripts vs. Coefficients

Elements That Exist as Molecules

Parentheses in Formulas

Isomers

2.5 The Periodic Table

How It Is Organised

Metals, Nonmetals, Metalloids

Main Group, Transition, Inner Transition

The Special Groups (Memorise These)

2.6 Molecular and Ionic Compounds

Chemical Bonds — the Two Kinds

Classifying Any Substance (4 Boxes)

Predicting Ionic Charge

Cations, Anions, and Balancing

Writing Ionic Formulas — the Crisscross

Polyatomic Ions (Memorise)

Properties — Ionic vs Covalent

Compounds With Both Bond Types

In short