Modern periodic law

“Mendeleev's table was so powerful it predicted elements not yet discovered.”

Element properties are a periodic function of atomic number.

Memory trick: Order by proton count.

Atomic radius trend

“More protons pull the same shell inward across a period, shrinking the atom.”

Atomic size decreases across a period and increases down a group.

Memory trick: Smaller top-right, larger bottom-left.

Ionization energy trend

“Tiny, tightly-held atoms guard their electrons most fiercely.”

Energy to remove the outer electron rises across a period and falls down a group.

Memory trick: Hardest to ionise: top-right (except noble gases).

Electronegativity trend

“Fluorine is the greediest atom in chemistry.”

The tendency to attract shared electrons rises across a period; fluorine is highest.

Memory trick: Rises toward fluorine.

Metallic character

“Metals crowd the left and bottom; non-metals the top-right.”

The tendency to lose electrons; increases down a group, decreases across a period.

Memory trick: Opposite of electronegativity.

Noble gases

“Helium in party balloons and neon in signs are famously aloof — they rarely react.”

Group-18 elements have full outer shells, making them extremely unreactive.

Memory trick: Full octet = content, unreactive.

Ionic bond

“Table salt is a lattice of Na⁺ and Cl⁻ locked by ionic attraction.”

Electrons transfer from metal to non-metal, forming oppositely charged ions that attract.

Memory trick: Metal + non-metal → transfer.

Covalent bond

“Two hydrogen atoms share electrons to become a stable H₂ molecule.”

Non-metals share electron pairs to complete their octets.

Memory trick: Non-metals share.

VSEPR theory

“Lone pairs are pushy — they squeeze bond angles smaller (water is bent, not straight).”

Electron pairs around a central atom spread out to minimise repulsion, setting the shape.

Memory trick: Count pairs → predict shape.

Hybridisation

“Carbon's sp³ hybridisation is why diamond is a rigid 3-D network.”

Atomic orbitals mix into equal hybrids: sp³ (tetrahedral), sp² (planar), sp (linear).

Memory trick: sp³=4, sp²=3, sp=2 regions.

Sigma vs pi bond

“A double bond is one strong sigma plus one weaker pi — which is the reactive one.”

A sigma bond forms by head-on overlap (strong); a pi bond by sideways p-orbital overlap (weaker).

Memory trick: Single = 1σ; double = 1σ+1π.

Hydrogen bonding

“Hydrogen bonds give water its high boiling point and hold the DNA double helix together.”

A strong dipole attraction when H bonds to N, O or F.

Memory trick: H with N/O/F → extra sticky.

Polar molecule

“CO₂ has polar bonds but is non-polar; its linear shape cancels the dipoles.”

A molecule is polar only if bond dipoles don't cancel — shape matters as much as bonds.

Memory trick: Symmetry can cancel polarity.

Boyle's law

“Squeeze a sealed syringe and the trapped air pushes back harder.”

At constant temperature, a gas's pressure is inversely proportional to its volume.

PV = constant

Memory trick: Squeeze → pressure up.

Charles's law

“A balloon shrinks in the freezer and swells in the sun.”

At constant pressure, a gas's volume is proportional to its absolute temperature.

V/T = constant

Memory trick: Hotter gas → bigger volume.

Real vs ideal gas

“Ideal-gas rules bend just when a gas is about to liquefy.”

Real gases deviate from ideal behaviour at high pressure and low temperature, where molecular size and attractions matter.

Memory trick: Ideal fails: cold + squeezed.

Enthalpy

“Hand-warmers release heat (exothermic); instant cold-packs absorb it (endothermic).”

Heat content at constant pressure; ΔH is negative for exothermic, positive for endothermic reactions.

ΔH

Memory trick: Released → ΔH negative.

Entropy

“A shuffled deck won't sort itself — disorder is the universe's default.”

A measure of molecular disorder; it tends to increase in spontaneous processes.

ΔS

Memory trick: More disorder = more entropy.

Gibbs free energy

“Some endothermic reactions still happen because a big entropy gain wins.”

ΔG = ΔH − TΔS decides spontaneity; negative ΔG means spontaneous.

ΔG = ΔH − TΔS

Memory trick: ΔG < 0 → spontaneous.

Hess's law

“Enthalpy is like altitude — only start and end points matter, not the path.”

The total enthalpy change of a reaction is the same regardless of route.

Memory trick: ΔH is a state function.

Dynamic equilibrium

“Equilibrium is a busy escalator that looks still — both directions run at once.”

Forward and reverse reactions occur at equal rates, so concentrations stay constant.

Memory trick: Equal rates, not equal amounts.

Le Chatelier's principle

“Add more reactant and the reaction pushes forward to use it up.”

A disturbed equilibrium shifts to oppose the change.

Memory trick: System fights the change.

pH scale

“Lemon juice (~2) is a million times more acidic than pure water (7).”

pH = −log[H⁺]; below 7 acidic, above 7 basic, 7 neutral at 25°C.

pH = −log[H⁺]

Memory trick: Each pH step = 10×.

Strong vs weak acid

“Stomach acid is strong; the acetic acid in vinegar is weak.”

A strong acid ionises completely (HCl); a weak acid only partly (CH₃COOH).

Memory trick: Strong = fully ionised.