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.
1,200 fundamentals
“Inner electrons act as a screen softening the nucleus's pull.”
The net positive pull felt by an outer electron after inner electrons shield it.
Memory trick: Actual pull after shielding.
“Chlorine is greedy for that extra electron, releasing lots of energy.”
The energy change when a gaseous atom gains an electron; halogens release the most.
Memory trick: Halogens love electrons.
“Salted water boils a touch hotter than pure water.”
Dissolving a solute raises a solvent's boiling point (a colligative property).
Memory trick: Solute raises boiling point.
“Salt melts icy roads by dropping water's freezing point.”
A dissolved solute lowers a solvent's freezing point.
Memory trick: Solute lowers freezing point.
“It's the recipe for mixing a buffer at a target pH.”
Relates a buffer's pH to the acid's pKa and the salt-to-acid ratio.
Memory trick: pH = pKa + log(salt/acid).
“Weak acids dissociate only a little; strong acids fully.”
The fraction of a substance that has ionised in solution.
Memory trick: Fraction ionised.
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
A counting unit for particles — one mole is 6.022×10²³ (Avogadro's number) of anything.
Memory trick: think of a mole like a 'dozen' — just a fixed count of particles.
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
A frequent error is confusing number of moles with number of grams or molecules. In reality, a counting unit for particles — one mole is 6.022×10²³ (Avogadro's number) of anything.
Memory trick: think of a mole like a 'dozen' — just a fixed count of particles.
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
A frequent error is using atomic number instead of atomic mass to find molar mass. In reality, the mass of one mole of a substance in grams, numerically equal to its atomic/molecular mass.
Memory trick: moles = given mass ÷ molar mass.
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
A frequent error is assuming the reactant with the smaller mass is limiting. In reality, the reactant that runs out first and therefore caps how much product forms.
Memory trick: convert each reactant to moles, divide by its coefficient — the smallest wins.
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
Molarity is moles of solute per litre of solution; molality is moles per kilogram of solvent.
Memory trick: molarity changes with temperature (volume expands); molality doesn't.
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
A frequent error is thinking molarity and molality are interchangeable. In reality, molarity is moles of solute per litre of solution; molality is moles per kilogram of solvent.
Memory trick: molarity changes with temperature (volume expands); molality doesn't.
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
Number of moles from a given mass. Use it when you know the substance's molar mass.
n = mass / molar mass
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
Concentration of a solution. Use it when volume measured in litres.
Molarity = moles / volume(L)
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
The empirical formula is the simplest whole-number ratio of atoms (CH₂O); the molecular formula is the actual count (C₆H₁₂O₆).
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
Molarity is per litre of solution (temperature-dependent); molality is per kg of solvent (temperature-independent).
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
Atomic mass is for one atom (in u); molar mass is for one mole of them (in g/mol) — same number, different units.
“A chemist counts atoms by weighing them — the mole is chemistry's 'dozen'.”
Always balance the equation before doing any stoichiometry.
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
A frequent error is confusing the principal quantum number n (shell) with l (subshell). In reality, four numbers (n, l, m, s) that pin down an electron's shell, subshell, orbital orientation and spin.
Memory trick: n = which floor, l = which room type, m = which room, s = spin up/down.
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
Regions around the nucleus where an electron is most likely to be found (s spherical, p dumbbell).
Memory trick: an orbital is a probability map, not a track.
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
A frequent error is drawing fixed circular orbits (Bohr) instead of probability clouds. In reality, regions around the nucleus where an electron is most likely to be found (s spherical, p dumbbell).
Memory trick: an orbital is a probability map, not a track.
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
Aufbau (lowest energy first), Pauli (max two electrons, opposite spins) and Hund (singly fill before pairing).
Memory trick: fill each orbital singly with parallel spins first, then pair up.
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
A frequent error is pairing electrons in a subshell before every orbital has one. In reality, Aufbau (lowest energy first), Pauli (max two electrons, opposite spins) and Hund (singly fill before pairing).
Memory trick: fill each orbital singly with parallel spins first, then pair up.
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
A frequent error is expecting a definite electron path. In reality, you cannot know an electron's exact position and momentum at the same time.
Memory trick: this is why we talk about probability, not orbits.