Alkaline earth metals

“Calcium builds your bones; magnesium sits at the heart of chlorophyll.”

Group-2 metals are harder and less reactive than group-1 and form +2 ions.

Memory trick: Form 2+ ions.

Inert pair effect

“Lead prefers +2 over +4, which is why lead(IV) is a strong oxidiser.”

Down groups 13–15 the lower oxidation state grows more stable as the s-electrons resist bonding.

Memory trick: Heavier element → lower state favoured.

Nitrogen's inertness

“78% of every breath is inert nitrogen doing nothing but padding the oxygen.”

N₂ is very unreactive due to a strong triple bond, so it dilutes the air safely.

N≡N

Memory trick: Triple bond → hard to break.

Oxides: acidic, basic, amphoteric

“The same oxide can act as acid or base depending on its partner.”

Non-metal oxides are acidic, metal oxides basic, and some (Al₂O₃, ZnO) are amphoteric.

Memory trick: Metal oxide basic, non-metal acidic.

Halogens

“Chlorine disinfects your drinking water by its fierce reactivity.”

Group-17 elements are reactive non-metals; reactivity decreases down the group (F > Cl > Br > I).

Memory trick: F is the most reactive.

Transition metals

“Iron rusts red, copper turns green — d-electrons give them their colours.”

d-block metals show variable oxidation states, coloured ions, and catalytic and magnetic behaviour.

Memory trick: d-electrons → colour + catalysis.

Lanthanide contraction

“It's why zirconium and hafnium are almost impossible to tell apart.”

A steady size decrease across the lanthanides makes later elements unexpectedly small.

Memory trick: 4f electrons shield poorly.

Coordination number

“Haemoglobin's iron has a coordination number that lets it grab oxygen.”

The number of ligand donor atoms directly bonded to the central metal ion.

Memory trick: Count the metal–ligand bonds.

Ligand

“Ligands are the metal's chemical 'handshake' partners.”

An ion or molecule that donates an electron pair to a central metal.

Memory trick: Electron-pair donor.

Chelate

“EDTA's multi-grip makes it a powerful metal remover in medicine and cleaning.”

A ligand that grips a metal at two or more points, forming an extra-stable ring.

Memory trick: Multi-point grip = stable.

Types of solids

“Diamond is crystalline; window glass is a frozen liquid, amorphous.”

Solids are crystalline (ordered) or amorphous (disordered like glass).

Memory trick: Ordered vs disordered.

Unit cell

“A crystal is just one tiny brick copied billions of times.”

The smallest repeating unit that builds a whole crystal lattice.

Memory trick: Repeat unit of a crystal.

Packing efficiency

“Oranges stacked in a grocer's pyramid mimic close packing.”

How much of a crystal's space is filled: fcc/hcp ~74%, bcc ~68%, simple cubic ~52%.

Memory trick: Close packing ≈ 74%.

Crystal defects

“A pinch of impurity turns pure silicon into a useful semiconductor.”

Real crystals have point defects; doping semiconductors deliberately adds them.

Memory trick: Imperfections can be useful.

Raoult's law

“Salt on icy roads works by lowering water's freezing point via this idea.”

The vapour pressure of a solvent falls in proportion to the mole fraction of solute dissolved.

Memory trick: More solute → lower vapour pressure.

Colligative properties

“Antifreeze protects a car radiator by depressing the freezing point.”

Properties depending on the number of solute particles, not their identity (freezing-point depression, boiling-point elevation).

Memory trick: Count particles, not type.

Henry's law

“A fizzy bottle hisses and bubbles the moment you release the pressure.”

The solubility of a gas is proportional to its partial pressure above the liquid.

Memory trick: More pressure → more dissolved gas.

Molality

“For precise low-temperature work, chemists prefer molality.”

Moles of solute per kilogram of solvent — unlike molarity, it doesn't change with temperature.

m = mol/kg

Memory trick: Per kg of solvent.

Adsorption vs absorption

“A sponge absorbs water; charcoal adsorbs impurities on its surface.”

Adsorption is sticking to a surface; absorption is soaking through the bulk.

Memory trick: Surface vs throughout.

Colloids and the Tyndall effect

“Sunbeams through mist or a projector's dusty light are Tyndall in action.”

Colloids scatter light, making a visible beam (the Tyndall effect).

Memory trick: Colloids scatter light.

Roasting and calcination

“The first step of turning rock into metal is driving off the unwanted parts.”

Roasting heats an ore in air (for sulphides); calcination heats it without air (for carbonates).

Memory trick: Air = roasting; no air = calcination.

Bond order

“A triple bond (order 3) is shorter and stronger than a single bond.”

Half the difference between bonding and antibonding electrons; higher bond order means stronger, shorter bonds.

Memory trick: Higher order = shorter, stronger.

Dipole moment

“Water's non-zero dipole is why it dissolves salt so well.”

A measure of molecular polarity; it is a vector that can cancel by symmetry.

Memory trick: Symmetry can zero it out.

Bond enthalpy

“Breaking bonds costs energy; forming them releases it — the balance sets ΔH.”

The energy needed to break one mole of a particular bond in the gas phase.

Memory trick: Break costs, form releases.