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
“It's such a huge number that a mole of rice grains would cover Earth metres deep.”
One mole of any substance contains 6.022×10²³ particles.
N_A = 6.022×10²³
Memory trick: Mole = chemistry's 'dozen'.
“18 g of water is exactly one mole — about a mouthful.”
The mass of one mole of a substance in grams, equal to its atomic/molecular mass.
n = m/M
Memory trick: moles = mass ÷ molar mass.
“Glucose and formaldehyde share the empirical formula CH₂O but are wildly different molecules.”
Empirical is the simplest whole-number atom ratio (CH₂O); molecular is the actual count (C₆H₁₂O₆).
Memory trick: Molecular = whole-number multiple of empirical.
“A doctor's saline drip is dosed precisely in molarity.”
Concentration as moles of solute per litre of solution.
M = n/V
Memory trick: Molarity = moles ÷ litres.
“In a sandwich with 10 slices of bread but 4 fillings, the filling is the limiting reagent.”
The reactant that runs out first and caps how much product forms.
Memory trick: Convert to moles, then compare.
“The nucleus is a fly in a stadium — atoms are almost entirely empty space.”
Atoms contain protons (+), neutrons (0) in the nucleus and electrons (−) around it.
Memory trick: Protons define the element.
“Change the neutrons and you get an isotope; change the protons and you get a new element.”
Atomic number = protons; mass number = protons + neutrons.
A = Z + N
Memory trick: Z = protons; A = protons + neutrons.
“Carbon-12 and carbon-14 are chemically identical twins with different weights.”
Atoms of the same element with different numbers of neutrons.
Memory trick: Same protons, different neutrons.
“They're the unique 'postal address' of every electron in an atom.”
Four numbers (n, l, m, s) fix an electron's shell, subshell, orbital and spin.
Memory trick: n=floor, l=room type, m=room, s=spin.
“Like people filling a cinema from the front rows, electrons take the cheapest seats first.”
Electrons fill the lowest-energy orbitals first.
Memory trick: Fill low energy before high.
“Two electrons can share a room only if they spin opposite ways.”
No two electrons in an atom can have all four quantum numbers identical; an orbital holds at most two, with opposite spins.
Memory trick: Max 2 per orbital, opposite spins.
“Passengers take empty double-seats alone before doubling up.”
Electrons singly occupy each orbital of a subshell before pairing, with parallel spins.
Memory trick: Fill singly first, same spin.
“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.
“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.
“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).
“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.
“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.
“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.
“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.
“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.
“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.
“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.
“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 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.