Modern periodic law

“Moseley fired electrons at metal plates; the X-ray color shifted systematically as the element’s atomic number increased.”

The modern periodic law states that elemental properties repeat periodically with atomic number Z, not atomic mass. Moseley’s law showed √(frequency of characteristic X-rays) ∝ Z, confirming Z as the ordering principle. In exams, never use atomic weight to locate an element; always use Z.

Memory trick: Moseley → √ν ∝ Z

Atomic radius trend

“A basketball gets smaller as you move from the back row to the front row of a stadium, but gets bigger when you climb to a higher tier.”

Atomic radius decreases across a period because increasing nuclear charge pulls electrons closer, while it increases down a group as each new shell adds distance. Poor shielding by d-electrons causes a slight irregularity, and the lanthanide contraction makes later transition metals smaller than expected. Remember: across → shrink, down → grow; d-block gets a hidden squeeze.

Memory trick: Across shrink, down grow; lanthanide squeezes d-block

Ionization energy trend

“Pulling an electron from oxygen is like tugging a loose thread, easier than pulling the tightly woven half-filled nitrogen.”

Ionization energy rises across a period due to higher effective nuclear charge and falls down a group as electrons are farther from the nucleus. The N-O anomaly arises because oxygen’s 2p⁴ has a paired electron, increasing electron-electron repulsion and lowering IE, whereas nitrogen’s half-filled 2p³ is relatively stable, giving a higher IE. Do not blame the dip on nitrogen.

Memory trick: O’s pair → lower IE; N’s half-filled → higher IE

Electronegativity trend

“A magnet’s north pole pulls iron strongest; similarly fluorine pulls shared electrons hardest.”

Electronegativity increases from left to right as atoms more strongly attract bonding electrons, and it decreases down a group as distance grows. Fluorine is the most electronegative element. Noble gases have no EN value.

Memory trick: EN ↑ right, ↓ down; F tops.

Metallic character

“Sodium in a spark easily gives away its electron, while chlorine in bleach holds onto its electrons tightly.”

Metallic character is the ease of losing electrons; it diminishes across a period and grows down a group, opposite to electronegativity. Transition metals show intermediate behavior, not extreme metals.

Memory trick: Metallic = opposite EN.

Noble gases

“Helium fills a balloon and stays calm because its outer shell is already full, so it won’t react.”

Group-18 elements have complete valence shells, giving them very high ionization energies and negligible tendency to gain or lose electrons, so they are chemically inert. He’s an exception with only 2 electrons but a full 1s shell.

Memory trick: Full shell = inert.

Ionic bond

“When you rub a glass rod with silk, electrons move and the rod becomes positively charged, just like metal losing electrons to form cations.”

Ionic bond forms by complete transfer of valence electrons from a metal to a non-metal, creating oppositely charged ions that attract electrostatically. The bond strength increases with higher charge magnitude and smaller ionic radii. Exam trap: never assume a metal-metal or non-metal-non-metal pair can form an ionic bond.

Memory trick: Metal → lose e⁻, Non-metal → gain e⁻ (L-G)

Covalent bond

“Two hydrogen atoms share their single electrons to make H₂, just as friends share a single umbrella in rain.”

Covalent bond involves sharing of one or more electron pairs between non-metals to attain octet (or duet). Shared electrons belong to both atoms, giving a bond order equal to number of shared pairs. Trap: a double bond is not two separate bonds; it is one σ plus one π pair.

Memory trick: Share = 2e⁻ per bond; double = 4e⁻; triple = 6e⁻

VSEPR theory

“The shape of a water molecule (bent) comes from two bonding pairs and two lone pairs pushing the H atoms apart.”

VSEPR states that electron pairs (bonding and lone) around a central atom arrange themselves to minimize repulsion, determining molecular geometry. Lone pairs occupy more space than bonding pairs, giving smaller bond angles. Trap: ignore lone-pair repulsion and predict tetrahedral for NH₃ (actually trigonal pyramidal).

Memory trick: AXₙEₘ: A=central, X=bond pairs, E=lone pairs → shape

Hybridisation

“The four sp³ hybrids point like the tips of a pyramid-shaped tent, each 109.5° apart.”

sp³ hybrids arise from one s + three p orbitals, giving four equivalent directions toward tetrahedral corners (109.5°). sp² mixes one s + two p → three planar orbitals (≈120°) plus one unhybridised p for π bonds. sp mixes one s + one p → two opposite orbitals (180°) leaving two p for π bonds. Remember: number of hybrids equals σ bonds plus lone pairs, not just drawn bonds.

Memory trick: sp³ → 4, sp² → 3, sp → 2 hybrid orbitals.

Sigma vs pi bond

“A single bond in ethane is like two hands clasping head-on (σ), while the extra bond in ethene is a side-by-side overlap (π) like two fingers sliding past each other.”

σ bond results from end-to-end overlap of orbitals and exists in every bond; it allows free rotation. π bond arises from side-on overlap of parallel p orbitals, present only in double/triple bonds, restricting rotation and being weaker. Trap: count bonds by σ+π, not by bond lines alone; a double bond = 1σ+1π.

Memory trick: Single=1σ; Double=1σ+1π; Triple=1σ+2π

Hydrogen bonding

“Water’s high boiling point is due to each H-bond acting like a tiny Velcro hook between H of one molecule and O of another.”

Hydrogen bond is a strong dipole-dipole attraction when H is covalently bound to highly electronegative N, O, or F and interacts with a lone pair on another N/O/F. It is directional and significantly raises boiling points. Trap: not every H-X…Y interaction is H-bond; X must be N/O/F and H must be attached to X.

Memory trick: H-bond: H-(N/O/F) … lone pair on N/O/F

Polar molecule

“Water’s bent shape makes its O–H bonds point to different sides, so the molecule pulls water toward a glass rod.”

A molecule is polar when the vector sum of its bond dipoles is non-zero. Polar bonds alone are insufficient; a symmetric geometry (e.g., CO₂) cancels them. In exams, always examine both bond polarity and molecular shape before declaring a molecule non-polar.

Memory trick: Polar = dipole + asymmetry

Boyle's law

“When you pump a bicycle tire, squeezing the air makes the gauge needle jump up.”

At constant temperature, pressure and volume of a fixed amount of gas are inversely related (PV = constant). Halving the volume doubles the pressure. The common trap: temperature must stay unchanged; mixing with Charles’s law gives wrong results.

PV = constant

Memory trick: Boyle: B = volume ↓ → pressure ↑

Charles's law

“A hot-air balloon swells as the burner heats the air, showing V ∝ T at constant pressure.”

At a fixed pressure, an ideal gas’s volume grows linearly with absolute temperature (Kelvin). Hence V1/T1 = V2/T2. The exam trap: forgetting to convert °C to K or assuming pressure changes gives a wrong answer.

V/T = constant

Memory trick: V∝T → V/T constant; keep P same, use K.

Real vs ideal gas

“Liquefying oxygen in a cryogenic tank shows ideal gas law breaks down at low temperature and high pressure.”

Real gases deviate from PV = nRT when molecules are close together; their finite size and intermolecular attractions matter, especially at high pressure or low temperature. The nuance: near the critical region you need van der Waals correction.

Memory trick: Real gas = R(e)al → high P, low T

Enthalpy

“When a match burns, the flame heats the air, but the match’s chemical system loses heat, so ΔH is negative.”

ΔH is the heat exchanged by the system at constant pressure (ΔH = qp). Exothermic: system releases heat → ΔH < 0; Endothermic: system absorbs heat → ΔH > 0. The sign always refers to the system, not the surroundings – a frequent exam trap.

ΔH

Memory trick: ΔH negative = heat out of system; think “H = Hot out.”

Entropy

“Ice melting into water spreads the molecules, increasing disorder – the rise in ΔS you can see.”

Entropy measures the degree of molecular disorder. A spontaneous process at constant T and P must have ΔS positive enough to make ΔG negative. Beware the trap of ignoring the temperature factor; ΔS can be negative yet ΔG still negative if TΔS is small.

ΔS

Memory trick: More ways → higher ΔS

Gibbs free energy

“A battery delivering current converts chemical energy to electrical work, the ΔG of the cell reaction.”

ΔG = ΔH − TΔS predicts spontaneity at constant temperature and pressure; ΔG < 0 means the reaction proceeds spontaneously. A common mistake is using a plus sign before TΔS or neglecting the temperature dependence when ΔH and ΔS have opposite signs.

ΔG = ΔH − TΔS

Memory trick: ΔG = ΔH minus T × ΔS

Hess's law

“Making water directly from H₂ and O₂ or via H₂O₂ gives the same ΔH – like two routes to the same destination.”

Hess’s law follows from enthalpy being a state function: the total ΔH is independent of the reaction path. Use it by adding and reversing known equations, but remember to change the sign when a reaction is reversed and keep equations balanced.

Memory trick: State function → path irrelevant

Dynamic equilibrium

“Like a busy highway where cars enter and exit at the same rate, forward and reverse reactions keep the concentration traffic steady.”

In a reversible reaction the forward and reverse rates become equal. When this happens the concentrations of all species stay constant, although both reactions continue. The exam trap: students often think the concentrations become equal, not just the rates.

Memory trick: Equal rates → steady traffic, not equal amounts.

Le Chatelier's principle

“Adding water to a sponge makes it squeeze out air to keep its shape; a system shifts to oppose any disturbance in the same way.”

If concentration, pressure or temperature of a system at equilibrium is changed, the equilibrium moves to reduce that change. The direction follows the side that consumes the added component. Trap: temperature changes affect the position according to the reaction’s enthalpy, not just concentration.

Memory trick: Add → system consumes; heat acts like reactant or product.

pH scale

“A lemon’s juice (pH≈2) feels sharp, while household soap (pH≈12) feels slippery, showing the ten-fold log scale of acidity.”

pH = −log[H⁺]; a decrease of 1 pH unit means [H⁺] rises tenfold. Below 7 the solution is acidic, above 7 basic, and at 25 °C neutral is 7. Trap: many forget that lower pH = higher [H⁺] and that neutrality shifts with temperature.

pH = −log[H⁺]

Memory trick: pH down → H⁺ up ×10

Strong vs weak acid

“Adding HCl to water makes the solution conduct electricity instantly, unlike acetic acid, which conducts only weakly.”

A strong acid dissociates completely (virtually 100 %) in dilute water, giving a large Ka (≫1). A weak acid ionises only partly, with Ka ≪1, but no fixed Ka cut-off defines weakness. Trap: don’t rely on a single Ka like 10⁻³; weakness is about degree of ionisation, not an arbitrary number.

Memory trick: Strong = full split; weak = holds back