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
“Every power plant on Earth spins a magnet to make your electricity — Faraday's idea, scaled up.”
It is always perpendicular to velocity, so it can never change a charge's speed or energy.
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
A frequent error is thinking very bright low-frequency light can eject electrons. In reality, light ejects electrons from a metal only if its frequency exceeds a threshold, regardless of intensity.
Memory trick: frequency decides IF electrons come out; intensity only decides HOW MANY.
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
A frequent error is treating light purely as a wave when explaining the photoelectric effect. In reality, light comes in packets of energy E = hf = hc/λ.
Memory trick: higher frequency (bluer light) means more energetic photons.
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
A frequent error is using a positive energy for a bound electron — bound states are negative. In reality, electrons occupy quantised orbits; for hydrogen E_n = −13.6/n² eV.
Memory trick: the electron is most tightly bound (most negative) in n=1.
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
A nucleus weighs less than its parts; the missing mass becomes binding energy via E = Δmc².
Memory trick: fusion and fission both release the difference in binding energy.
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
A frequent error is ignoring the mass defect when computing nuclear energy. In reality, a nucleus weighs less than its parts; the missing mass becomes binding energy via E = Δmc².
Memory trick: fusion and fission both release the difference in binding energy.
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
A frequent error is expecting everyday objects to show wave behaviour (their λ is unimaginably small). In reality, every particle has a wavelength λ = h/p, significant only for tiny masses.
Memory trick: matter waves matter only at the electron/atom scale.
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
Energy of a photon. Use it when electromagnetic radiation of frequency f.
E = hf = hc/λ
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
Energy of the nth hydrogen level. Use it when hydrogen atom (Bohr model).
E_n = −13.6/n² eV
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
Energy equivalent of a mass defect. Use it when nuclear reactions.
E = Δmc²
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
De Broglie wavelength of a particle. Use it when particle with momentum p.
λ = h/p
“The solar panel and the smoke detector both run on Einstein-era quantum physics.”
Fission splits a heavy nucleus into lighter ones (reactors); fusion joins light nuclei into a heavier one (the Sun) — both release energy.
“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.