Fundamentals
The core facts every aspirant should own — each a titled nugget with a real-world story, the concept in plain words, and a memory trick. Works even when the internet doesn't.
300 fundamentals
“Inflating a bike tire shows Kp as the pressure gauge reading, while the amount of air per volume inside is Kc.”
Kc uses molar concentrations, Kp uses partial pressures. For a gas reaction Δn = moles(gas products) – moles(gas reactants); Kp = Kc·(RT)^Δn. A frequent slip is using Kc when the problem asks for Kp (or vice-versa) or forgetting the Δn exponent.
Memory trick: Kp = Kc·(RT)^Δn
“Your stomach’s HCl fully dissociates, but the acetic acid in vinegar only partly does, so vinegar tastes less corrosive.”
A strong acid (e.g., HCl) ionises completely in water, giving a very large Ka. A weak acid (e.g., CH₃COOH) establishes the equilibrium HA ⇌ H⁺+A⁻, so only a small fraction dissociates and Ka is small. Remember to treat weak acids with the equilibrium expression, not as fully ionised.
Memory trick: Strong = 100% ionisation; Weak = HA ⇌ H⁺+A⁻ equilibrium
“When you heat a hand-warm pack (exothermic), it cools down because the reaction shifts left; heating a cold-pack (endothermic) makes it get colder as it shifts right.”
Le Chatelier’s principle: Adding heat to an exothermic reaction (ΔH < 0) drives the equilibrium toward the reactants, so the system feels cooler. Adding heat to an endothermic reaction (ΔH > 0) pushes it toward the products, making the system absorb heat. A common mistake is forgetting the sign of ΔH and applying the heat rule backward.
Memory trick: Heat ↔ endo forward, exo backward
“A playground seesaw stays still when the kids’ pushes (rates) are equal, even if one sits higher than the other (different concentrations).”
Chemical equilibrium is reached when the forward and reverse reaction rates are equal, so concentrations stop changing. Equality of concentrations is not required; it occurs only for special cases (e.g., 1:1 reaction with K=1). Trap: assuming [reactant]=[product] gives wrong K unless K=1 by stoichiometry and standard states.
Memory trick: Equal rates → steady, not equal amounts.
“A rusting iron nail loses electrons (oxidation) while atmospheric O₂ gains them (reduction).”
Oxidation = electron loss; reduction = electron gain. Electrons flow from the oxidised half-reaction to the reduced half-reaction. A typical mistake is swapping the half-reactions, leading to sign errors in balancing.
Memory trick: Electrons: loss → gain
“Chemists assign oxidation numbers like a ledger to track electron flow in Fe₂O₃ formation.”
Oxidation number is a bookkeeping charge assigned by rules to each atom to monitor electron transfer; it is not the real charge on the atom. Free elements are 0. The trap is treating the oxidation number as a physical charge rather than a formal tool.
Memory trick: Ox no = bookkeeping charge
“In hydrogen peroxide each O is –1, while in potassium superoxide O is –½, showing oxidation numbers can be fractional.”
Free elements have oxidation number 0. Oxygen is usually –2, but in peroxides (e.g., H₂O₂) it is –1 and in superoxides (e.g., KO₂) it is –½. Fractional values also appear as average states in mixed-valence compounds (e.g., Fe₃O₄). The exam trap: assuming oxidation numbers are always integers or forgetting the –½ case.
Memory trick: O: –2, peroxides –1, superoxides –½; free element =0.
“In a rusting nail, the oxygen in air is the oxidising agent—it pulls electrons from iron, just like a magnet pulling metal.”
The oxidising agent gains electrons and is therefore reduced; the reducing agent loses electrons and is oxidised. They are named by the effect they have on the other species, not by their own change. Exam-makers often swap the agents when writing half-reactions.
Memory trick: Oxidiser = electron receiver; Reductor = electron donor
“When copper dissolves in nitric acid, Cu is oxidised, but HNO₃ is the oxidising agent pulling electrons away.”
The substance that gets oxidised is not the oxidising agent. The oxidising agent is the species that gets reduced (gains electrons). Mislabeling leads to wrong half-reaction assignment and incorrect balancing of redox equations.
Memory trick: Agent = opposite change
“A battery’s anode loses electrons (oxidation) while the cathode gains them (reduction), just like a drain and a faucet.”
Oxidation = loss of electrons, so the oxidation number of the element rises. Reduction = gain of electrons, so its oxidation number falls. A frequent mistake is to reverse the sign of the oxidation-number change when dealing with polyatomic ions.
Memory trick: Oxi = lose e⁻; Red = gain e⁻
“Bleach (ClO⁻) accepts electrons from stains, acting as the oxidising agent, like a sponge soaking up water.”
An oxidising agent accepts electrons and is itself reduced; a reducing agent donates electrons and is itself oxidised. The trap is to think the agent’s own oxidation number stays unchanged, which is false in most redox processes.
Memory trick: Oxidiser takes, Reducer gives
“In the reaction between H₂ and F₂, F₂ is the oxidising agent—it gets reduced to F⁻, just as a stronger magnet pulls the weaker one.”
Names are based on what the species does to the other: an oxidising agent causes oxidation of another, so it is reduced; a reducing agent causes reduction of another, so it is oxidised. Forgetting this opposite change gives wrong agent identification in equations.
Memory trick: Agent’s change = opposite of its name
“When you move from sodium to chlorine in the same row, the atoms shrink like a hand squeezing a balloon.”
Atomic radius decreases across a period due to increasing Z_eff pulling electrons closer, and increases down a group as new electron shells are added. The measurement is from nucleus to outermost electron. Trap: students sometimes reverse the trend or forget the role of added shells.
Memory trick: Across: pull tighter; Down: add shells
“Like pulling a tightly coiled spring from a tiny atom, you need more force as you move right across a period.”
Ionization energy (IE) is the energy needed to remove the outermost electron. IE rises across a period because atomic radius shrinks and nuclear charge increases, and it falls down a group as electrons are farther from the nucleus. Exam trap: the two regular dips—IE of B is lower than Be, and IE of O is lower than N—must be remembered.
Memory trick: IE ↑ across period, ↓ down group; B<Be (p-electron), O<N (pairing).
“When you lift a ball from a shallow bowl (p-orbital) it’s easier than from a deep well (s-orbital).”
The B-Be dip occurs because the electron removed from B is taken from a 2p orbital, which lies higher in energy and is less tightly bound than the 2s electron removed from Be, so less IE is required. The O-N dip is due to two electrons pairing in O’s 2p orbital; the added repulsion raises the energy of the outer electron, making its removal easier. These are the standard reasons for the only regular IE exceptions.
Memory trick: B dip: remove p-electron; O dip: paired p-electrons increase repulsion.
“In a water molecule, oxygen drags the shared electrons toward itself, making H–O bonds polar.”
Electronegativity is the atom’s ability to attract bonding electrons. It rises across a period because nuclear charge increases while shielding stays almost constant, and it falls down a group as added electron shells weaken the pull. Fluorine tops the scale; the trend is opposite to metallic character, a frequent exam pitfall.
Memory trick: EN ↑ up-right, ↓ down-left on the periodic table.
“A copper wire conducts electricity, but it doesn’t hog electrons like chlorine does in a salt crystal.”
Metals have low electronegativity; they readily give up electrons, whereas non-metals like chlorine have high EN and attract electrons. The trend is opposite to metallic character. Trap: avoid memorising isolated EN numbers—remember the left-right direction on the table.
Memory trick: Metals left → low EN; non-metals right → high EN.
“When sodium metal drops into water, it instantly gives up its outer electron, forming Na⁺ and H₂ gas.”
Metallic character measures how readily an element loses electrons to form cations. It decreases across a period as ionisation energy rises, and increases down a group because larger size and shielding make electron loss easier. Metals are left-bottom, non-metals right-top.
Memory trick: Metallic = lose → left/down; EN = gain → right/up.
“Pulling a nail out of a thin board (small radius) needs more force than from a thick plank (large radius).”
Across a period, atomic radius shrinks while ionization energy rises because electrons are held tighter by a stronger nuclear pull. The two properties vary oppositely. Trap: remember that ionization energy does NOT keep increasing down a group—it generally falls.
Memory trick: Small atom → big IE; big atom → small IE.
“When iron rusts, it gives away electrons to oxygen, while chlorine in bleach grabs electrons to become Cl⁻.”
Metals lose electrons easily (low ionization energy) forming cations; non-metals gain electrons (high electronegativity) forming anions. This distinction underlies oxide and halide formation. Trap: not every solid is a metal—metalloids show intermediate behavior.
Memory trick: Metal = lose, low IE; Non-metal = gain, high EN.
“In a multi-storey building, each floor (period) adds a new layer of rooms, while a column of rooms (group) shares the same layout on every floor.”
A group is a vertical column of elements with the same valence-electron configuration, giving similar chemical properties. A period is a horizontal row where elements have the same principal quantum number (same number of electron shells). Trap: don’t confuse period number with group number.
Memory trick: Group = column, same valence; Period = row, same shells.
“Like a row of magnets, adding protons pulls electrons tighter, so the atom shrinks across a period.”
Atomic radius decreases left-to-right in a period because effective nuclear charge (Z_eff) rises while inner-shell shielding stays almost unchanged. The stronger pull contracts the electron cloud. Exam trap: don’t mix this with the opposite trend down a group.
Memory trick: Period trend: Z_eff ↑ → radius ↓
“A car’s speed rises with how hard you press the accelerator, not with the number of cylinders shown on its blueprint.”
The reaction order is the exponent of each concentration in the experimentally determined rate law. It is obtained from rate data, not from stoichiometric coefficients, except for an elementary step where they coincide. A common trap is to write the overall order as the sum of coefficients of the balanced equation.
Memory trick: Order = experiment, not equation (unless elementary).
“A kettle boils faster on a high flame; the speed constant k climbs sharply with temperature, not with the amount of water.”
k is the proportionality constant in the rate law, reflecting how fast a reaction proceeds at a given temperature. It increases exponentially with temperature (Arrhenius) and changes with a catalyst, but never depends on reactant concentrations. The exam trap is to insert concentration terms into k.
Memory trick: k ∝ e^(−Ea/RT), not [A].