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
“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.
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
Energy of an electron in a hydrogen-like atom. Use it when single-electron (H-like) species.
E_n = −13.6 Z²/n² eV
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
An orbit is a fixed Bohr path (outdated); an orbital is a 3-D region of high probability for finding an electron.
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
A shell is a principal energy level (n); a subshell (s, p, d, f) is a division within a shell.
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
Aufbau decides the ORDER subshells fill (low to high energy); Hund decides HOW electrons spread within a subshell (singly, same spin).
“Fireworks glow in colours because excited electrons drop back and release exact packets of light.”
Chromium is 3d⁵ 4s¹ and copper 3d¹⁰ 4s¹ — half/fully-filled d subshells are extra stable.
“Water is bent, not straight — and that single fact is why life exists.”
Atoms tend to gain, lose or share electrons to reach eight in their outer shell.
Memory trick: the octet rule is a guide, not a law — watch for electron-deficient and expanded-octet molecules.
“Water is bent, not straight — and that single fact is why life exists.”
A frequent error is forcing an octet on exceptions like BeCl₂, BF₃ or SF₆. In reality, atoms tend to gain, lose or share electrons to reach eight in their outer shell.
Memory trick: the octet rule is a guide, not a law — watch for electron-deficient and expanded-octet molecules.
“Water is bent, not straight — and that single fact is why life exists.”
A frequent error is counting only bonding pairs and ignoring lone pairs. In reality, electron pairs around a central atom arrange to be as far apart as possible, fixing the molecule's shape.
Memory trick: lone pairs take more room and squeeze bond angles smaller.
“Water is bent, not straight — and that single fact is why life exists.”
Atomic orbitals mix to form equivalent hybrid orbitals (sp, sp², sp³) that set the geometry.
Memory trick: sp³ → tetrahedral, sp² → trigonal planar, sp → linear.
“Water is bent, not straight — and that single fact is why life exists.”
A frequent error is predicting geometry without counting lone pairs in the hybridization. In reality, atomic orbitals mix to form equivalent hybrid orbitals (sp, sp², sp³) that set the geometry.
Memory trick: sp³ → tetrahedral, sp² → trigonal planar, sp → linear.
“Water is bent, not straight — and that single fact is why life exists.”
A molecule is polar only if bond dipoles don't cancel, which depends on both electronegativity and shape.
Memory trick: symmetry can cancel dipoles — shape matters as much as bonds.
“Water is bent, not straight — and that single fact is why life exists.”
A frequent error is calling any molecule with polar bonds a polar molecule. In reality, a molecule is polar only if bond dipoles don't cancel, which depends on both electronegativity and shape.
Memory trick: symmetry can cancel dipoles — shape matters as much as bonds.