Difficulty: Intermediate | Prerequisites: Matter, Atomic Structure, and the Periodic Table study notes
Ionic bonds form between metals and non-metals; covalent bonds form between non-metals. VSEPR theory predicts molecular shapes from the number of bonded pairs and lone pairs around a central atom. The five main reaction types (synthesis, decomposition, single replacement, double replacement, combustion) each follow a recognisable pattern. Quantitative chemistry ties it all together through the mole concept, empirical and molecular formulas, and percent calculations. Nuclear chemistry and the light/energy equations round out the midterm material.
Ionic compound
A compound formed by the electrostatic attraction between a metal (or transition metal) cation and a non-metal anion. Charges must balance.
In simple terms, one atom gives electrons to the other, and the opposite charges hold them together.
Covalent compound
A compound formed when two non-metals share electrons.
Think of it as: neither atom wants to give up electrons, so they share.
VSEPR (Valence Shell Electron Pair Repulsion)
A model that predicts molecular geometry based on the idea that electron pairs around a central atom repel each other and arrange themselves as far apart as possible.
Hybridisation
The mixing of atomic orbitals to form new, equivalent hybrid orbitals (sp, sp², sp³, sp³d, sp³d²).
Polarity
A molecule is polar if it has an uneven distribution of electron density, resulting in a net dipole moment. Determined by both bond polarity and molecular shape.
Mole (mol)
The SI unit for amount of substance. One mole contains 6.02 × 10²³ particles (Avogadro’s number). At STP, 1 mol of an ideal gas occupies 22.4 L.
Empirical formula
The simplest whole-number ratio of elements in a compound.
In simple terms, it is the most reduced version of the formula.
Molecular formula
The actual number of atoms of each element in one molecule. It is a whole-number multiple of the empirical formula.
Percent composition
The mass percentage of each element in a compound, calculated by dividing the element’s mass contribution per mole by the compound’s total molar mass, then multiplying by 100.
Percent yield
The ratio of actual product obtained to the expected (theoretical) product, expressed as a percentage.
Percent error
The absolute value of the difference between the measured and accepted values, divided by the accepted value, times 100.
Half-life
The time it takes for half of a radioactive sample to decay. After n half-lives, the fraction remaining is (1/2)ⁿ.
Quantum numbers
A set of four numbers (n, l, mₗ, mₛ) that describe the energy, shape, orientation, and spin of an electron in an atom.
Ionic compounds form between a metal (or transition metal) and a non-metal. The metal loses electrons (becomes a cation), the non-metal gains them (becomes an anion). Charges must balance: if one ion is +3 and the other is −2, the formula is X₃Y₂.
Covalent compounds form between two non-metals, which share electron pairs.
Know your polyatomic ions for naming ionic compounds.
Naming covalent compounds:
Use Greek prefixes for the number of atoms: mono-, di-, tri-, tetra-, penta-, etc.
The first element keeps its name (no "mono-" prefix if there is only one). The second element gets the prefix and an "-ide" ending.
Example: N₂O₄ = dinitrogen tetroxide.
Naming acids:
If the anion ends in "-ate," the acid name uses "-ic acid" (e.g. sulfate → sulfuric acid).
If the anion ends in "-ite," the acid name uses "-ous acid" (e.g. sulfite → sulfurous acid).
If the anion ends in "-ide," the acid name uses "hydro-___-ic acid" (e.g. chloride → hydrochloric acid).
VSEPR theory: electron pairs around a central atom repel each other and arrange themselves as far apart as possible. The molecular shape depends on how many of those pairs are bonding pairs vs. lone pairs.
Bonded pairs | Lone pairs | Molecular shape | Electron domain shape | Bond angle | Hybridisation |
|---|---|---|---|---|---|
6 | 0 | Octahedral | Octahedral | 90° | sp³d² |
5 | 0 | Trigonal bipyramidal | Trigonal bipyramidal | 90° / 120° | sp³d |
4 | 0 | Tetrahedral | Tetrahedral | 109.5° | sp³ |
3 | 1 | Trigonal pyramidal | Tetrahedral | 107.3° | sp³ |
3 | 0 | Trigonal planar | Trigonal planar | 120° | sp² |
2 | 2 | Bent (3D) | Tetrahedral | 104.5° | sp³ |
2 | 1 | Bent (2D) | Trigonal planar | <120° | sp² |
2 | 0 | Linear | Linear | 180° | sp |
Hybridisation is the mixing of atomic orbitals to form new hybrid orbitals. Count the total number of electron domains (bonded atoms + lone pairs) around the central atom to determine the hybridisation: 2 domains = sp, 3 = sp², 4 = sp³, 5 = sp³d, 6 = sp³d².
Polarity: a molecule is polar if it has polar bonds and the bond dipoles do not cancel due to an asymmetric shape. Symmetric shapes (linear with identical atoms, trigonal planar, tetrahedral with identical substituents) are non-polar even with polar bonds.
Synthesis (combination): A + B → AB. Two or more substances combine to form a single product.
Decomposition: AB → A + B. A single compound breaks down into two or more simpler substances.
Single replacement: A + BX → AX + B. One element displaces another in a compound. The activity series determines whether the reaction proceeds: if A is more reactive than B, the reaction occurs. If A is less reactive, no reaction (NR).
Double replacement: AX + BY → AY + BX. Two compounds exchange ions. A and B are cations; X and Y are anions.
Solubility rules to know: anything in Group 1, anything with ammonium, and anything with nitrates, chlorates, or perchlorates is always soluble. Halides paired with zinc, silver, or mercury are insoluble. Most hydroxides are insoluble, except those with alkali metals or barium.
Combustion: a substance reacts with O₂, releasing energy as heat and light. Hydrocarbon combustion typically produces CO₂ and H₂O.
Neutralisation: Acid + Base → Salt + Water. A salt is the cation of the base combined with the anion of the acid. The net ionic equation for every neutralisation is: OH⁻ + H⁺ → H₂O.
LEO: Losing Electrons is Oxidation.
GER: Gaining Electrons is Reduction.
In a single-replacement reaction, the element that displaces is oxidised (loses electrons to form a compound), and the displaced ion is reduced (gains electrons to become a free element).
1 mol = 6.02 × 10²³ particles (Avogadro’s number).
1 mol of any ideal gas at STP = 22.4 L.
Molar mass = sum of the atomic masses of every atom in the formula (units: g/mol).
Empirical vs. molecular formulas:
The empirical formula is the simplest whole-number ratio of elements. CH₂O is an empirical formula.
The molecular formula shows the actual number of atoms per molecule. C₆H₂₂O₆ (glucose) is a molecular formula whose empirical formula is CH₂O.
To find the molecular formula: divide the compound’s molar mass by the empirical formula’s molar mass. Multiply every subscript in the empirical formula by that integer.
Every empirical formula is technically a molecular formula, but not every molecular formula is an empirical formula. They can be the same (e.g. H₂O).
Percent composition: (mass of element in one mole / molar mass of compound) × 100.
Percent yield: |(actual value / expected value)| × 100.
Percent error: |(actual value – expected value) / expected value| × 100.
Type | Equation pattern | Particle emitted / absorbed |
|---|---|---|
Alpha emission | Parent → Daughter + ⁴₂He | Alpha particle (helium nucleus) |
Beta emission | Parent → Daughter + ⁰₋₁e | Beta particle (electron) |
Positron emission | Parent → Daughter + ⁰₁e | Positron |
Gamma emission | Parent → Daughter + ⁰₀γ | Gamma ray (no mass, no charge) |
Electron capture | Parent + ⁰₋₁e → Daughter | Electron absorbed by nucleus |
Alpha bombardment | Parent + ⁴₂He → Daughter | Alpha particle absorbed |
Fission | Parent + ¹₀n → fragments + neutrons | Neutron splits a heavy nucleus |
Induced transmutation | Parent + ⁴₂He → Daughter + ¹₁p | Proton emitted |
Half-life: after each half-life, half of the remaining sample has decayed. After n half-lives, the fraction remaining = (1/2)ⁿ.
Wavelength (λ) and frequency (ν) are inversely proportional.
c = λν (speed of light = wavelength × frequency). c = 3.0 × 10⁸ m/s.
E = hν (energy = Planck’s constant × frequency). h = 6.6262 × 10⁻³⁴ J·s.
Energy increases with increasing frequency (and decreasing wavelength).
De Broglie relation: λ = h / mv, where m is mass and v is velocity.
Symbol | Name | Possible values | What it tells you |
|---|---|---|---|
n | Principal quantum number | 1, 2, 3, 4, … | Energy level (shell) |
l | Angular momentum quantum number | 0, 1, 2, … (n – 1) | Shape of the orbital (0 = s, 1 = p, 2 = d, 3 = f) |
mₗ | Magnetic quantum number | –l, …, 0, …, +l | Orientation of the orbital |
mₛ | Spin quantum number | +½, –½ | Spin direction of the electron |
Max electrons in a shell: 2n².
Max electrons in a subshell: 4l + 2.
c = λν
E = hν
λ = h / mv (De Broglie)
c = 3.0 × 10⁸ m/s
h = 6.6262 × 10⁻³⁴ J·s
Avogadro’s number = 6.02 × 10²³
1 mol gas at STP = 22.4 L
Max electrons in shell = 2n²
Max electrons in subshell = 4l + 2
Percent composition = (element mass per mol / compound molar mass) × 100
Percent yield = |(actual / expected)| × 100
Percent error = |(actual – expected) / expected| × 100
Molecular formula multiplier = molar mass / empirical molar mass
VSEPR geometry explains why water is bent (two lone pairs on oxygen push the hydrogens closer together), which in turn explains water’s polarity and its role as a solvent for ionic compounds. The activity series is the principle behind electrochemical cells and batteries. Half-life calculations are used in carbon-14 dating to estimate the age of archaeological artefacts, and in nuclear medicine to determine safe dosing of radioactive tracers.
Students often think all combustion reactions involve hydrocarbons. Combustion simply means a substance reacts with oxygen and releases energy. Metals can combust too (e.g. magnesium burning in air).
Students often mix up empirical and molecular formulas. Remember: the empirical formula is the simplest ratio, and the molecular formula is always a whole-number multiple of it.
Students often forget that a bent molecule is polar even if the individual bonds look symmetric on paper. The lone pairs create an asymmetric electron distribution.
Students often assume that single-replacement reactions always occur. They only proceed if the replacing element is more reactive (higher on the activity series) than the element being replaced.
⚠️ The VSEPR table is heavily tested. Be able to determine molecular shape, electron domain shape, bond angle, and hybridisation from a Lewis structure.
⚠️ Naming ionic compounds, covalent compounds, and acids: expect several naming questions. Know your polyatomic ions.
⚠️ Reaction type identification: given a balanced equation, classify it as synthesis, decomposition, single replacement, double replacement, or combustion.
⚠️ Mole conversions (grams ↔ moles ↔ particles ↔ litres at STP) are fundamental and will appear in multiple problems.
⚠️ Half-life calculations: practice dividing by 2 repeatedly. Know how to find the amount remaining after n half-lives.
⚠️ Quantum numbers: be able to identify valid vs. invalid sets of quantum numbers, and state which subshell a set describes.
True or False: CO₂ is a polar molecule.
Fill in the blank: A single-replacement reaction follows the pattern A + BX → ___.
True or False: The empirical formula of C₆H₂₂O₆ is CH₂O.
Fill in the blank: After 3 half-lives, the fraction of a radioactive sample remaining is ___.
True or False: A quantum number set of n = 2, l = 2, mₗ = 0, mₛ = +½ is valid.
Answers: 1. False (CO₂ is linear and symmetric, so the dipoles cancel). 2. AX + B. 3. True. 4. 1/8 (i.e. (1/2)³). 5. False (l cannot equal n; for n = 2, l can only be 0 or 1).
Q: What is the molecular shape of a molecule with 3 bonded pairs and 1 lone pair on the central atom?
A: Trigonal pyramidal. The electron domain shape is tetrahedral, but the lone pair compresses the bond angles to about 107.3°.
Q: Classify this reaction: 2 Mg + O₂ → 2 MgO.
A: Synthesis (combination). Two reactants combine to form a single product.
Q: A compound has a molar mass of 180 g/mol and an empirical formula of CH₂O (empirical molar mass = 30 g/mol). What is its molecular formula?
A: 180 / 30 = 6. Multiply each subscript by 6: C₆H₁₂O₆.
Q: A radioactive isotope has a half-life of 10 years. If you start with 80 g, how much remains after 30 years?
A: 30 years = 3 half-lives. 80 → 40 → 20 → 10 g remaining.
Q: Calculate the energy of a photon with a frequency of 5.0 × 10¹⁴ Hz.
A: E = hν = (6.626 × 10⁻³⁴ J·s)(5.0 × 10¹⁴ s⁻¹) = 3.3 × 10⁻¹⁹ J.
Q: Is a set of quantum numbers n = 3, l = 1, mₗ = –2, mₛ = +½ valid? Explain.
A: Invalid. For l = 1, mₗ can only be –1, 0, or +1. A value of –2 is outside the allowed range.
Bonding and molecular geometry are the foundation for understanding intermolecular forces, which in turn explain boiling points, solubility, and phase behaviour (topics that appear later in general chemistry). Reaction types and stoichiometry feed directly into equilibrium, acid-base chemistry, and thermochemistry. The quantum numbers and light/energy equations reappear in spectroscopy and when discussing electron transitions between energy levels.
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