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
“If you weigh 18 g of water, dividing by its molar mass 18 g/mol shows you have exactly 1 mol of H₂O.”
The number of moles n equals the given mass divided by the substance’s molar mass (n = m ÷ M). This relation is used whenever the mass is known and the molar mass is available from the periodic table. Using the wrong molar mass gives an incorrect n.
n = mass / molar mass
Memory trick: n = m ÷ M
“When you dissolve sugar in a 1-L bottle of water, the molarity tells how many moles of sugar are in that bottle.”
Molarity (M) is the number of moles of solute per litre of solution. Use litre-volume of the whole solution, not just the solvent. A common trap is using the solvent volume, which gives a higher M than the true value.
Molarity = moles / volume(L)
Memory trick: M = n ÷ V(L)
“Glucose’s empirical formula CH₂O is like the recipe’s ingredient ratio, while C₆H₁₂O₆ is the full shopping list.”
The empirical formula shows the simplest whole-number atom ratio; the molecular formula gives the actual number of atoms per molecule. Multiply the empirical formula by n = (molar mass)/(empirical mass). Forgetting to find n leads to wrong molecular formula.
Memory trick: Molecular = Empirical × (Molar/Empirical mass)
“A car’s fuel gauge (litres) is like molarity, changing with temperature, while the weight of fuel (kg) is like molality, staying constant.”
Molarity = moles / litre of solution (volume changes with temperature). Molality = moles / kg of solvent (independent of temperature). In colligative-property questions, using M instead of m gives wrong answer.
Memory trick: M = n/Vsol, m = n/kg solvent
“A single carbon atom weighs 12 u, but a mole of carbon atoms weighs 12 g – like a single grain vs a kilogram of grain.”
Atomic mass (u) is the mass of one atom; molar mass (g / mol) is the mass of Avogadro’s number of atoms. Numerically they are equal but units differ. Mixing units causes calculation errors in stoichiometry.
Memory trick: Atomic u ↔ g/mol same number
“Building a LEGO model without following the instruction sheet (balancing) leaves missing pieces and wrong shape.”
Always balance the chemical equation before any stoichiometric work. The coefficients give the correct mole ratios; an unbalanced equation gives wrong limiting-reactant or product amounts.
Memory trick: Balance first, then count
“Think of an apartment building: the floor number tells you n, while the type of apartment (studio, 1-BHK, 2-BHK) tells you l.”
n (principal quantum number) fixes the energy shell; l (azimuthal) fixes the subshell shape and can take values 0 to n-1. Mixing them gives impossible combos like l=3 for n=2, a frequent NEET/JEE error.
Memory trick: n = floor, l = room type (0-studio,1-1BHK,… up to n-1).
“A weather radar shows a fuzzy cloud where rain is most likely, just like an s-orbital is a spherical cloud of electron probability.”
An orbital is a region where the probability of finding an electron is highest; s-orbitals are spherical, p-orbitals are dumbbell-shaped. Remember: they are probability maps, not fixed paths, a point often missed in diagrams.
Memory trick: s = sphere, p = pair-dumbbell.
“Imagine a GPS satellite’s precise track versus the fuzzy cloud of a weather radar; electrons are like the radar cloud, not the satellite’s orbit.”
Bohr’s circular orbits are outdated; quantum mechanics gives probability clouds. In exams, drawing a neat circle for a p-orbital loses marks; you must sketch the correct shape (s sphere, p dumbbell) not a circle.
Memory trick: Orbit ≠ circle; cloud ≠ track.
“Seating a bus: you fill each row with one passenger first (Hund), then add a second seat (Pauli), and start from the front rows (Aufbau).”
Electrons occupy lowest-energy orbitals first (Aufbau), no more than two per orbital with opposite spins (Pauli), and fill each degenerate set singly before pairing (Hund). A trap: forgetting Hund gives wrong configurations for O, N.
Memory trick: Front-row, one-by-one, then pair.
“In a theater, you never seat two people in the same seat before filling the empty seats in the row; the same rule applies to electrons.”
The mistake is pairing electrons in a subshell before each orbital has one. Hund’s rule requires each orbital of a subshell to get one electron with parallel spin before any pairing. Ignoring this yields wrong term symbols.
Memory trick: Single first, then pair – like filling empty chairs.
“Trying to capture a fast cricket ball with a camera that also tells its exact speed is impossible—position and momentum can’t both be precise.”
Heisenberg’s principle states Δx·Δp ≥ ħ/2; the more precisely you know position, the less precisely you know momentum. Exams often ask which quantity becomes uncertain; remember you cannot specify both simultaneously.
Memory trick: Precise spot → fuzzy speed; precise speed → fuzzy spot.
“When you see the red hydrogen line in a nebula, its wavelength comes from electrons dropping between levels given by E_n = -13.6 Z²/n² eV.”
For any hydrogen-like ion (one electron, nuclear charge Z) the bound-state energy is E_n = –13.6 Z²/n² eV. The negative sign indicates a bound electron; the magnitude grows with Z and shrinks with n². Photon energy emitted = |ΔE| = |E_final – E_initial|, never the signed ΔE. Forgetting the sign or using Z of a neutral atom is a common mistake.
E_n = −13.6 Z²/n² eV
Memory trick: E_n = –13.6·Z²÷n²: negative, 13.6 for H, square Z, divide n²
“Bohr’s planet-like orbit is like a marble on a track, while an orbital is a fuzzy cloud where the marble could be anywhere.”
An orbit is the fixed circular path from Bohr’s outdated model; an orbital is the three-dimensional region of high probability from quantum mechanics. Modern chemistry and physics use only orbitals. Trap: never use ‘orbit’ for electron placement in JEE/NEET problems.
Memory trick: Orbit = track; Orbital = cloud
“Think of a building: each floor is a shell (n=1,2,3…) and each room on a floor is a subshell (s, p, d, f, …).”
A shell is defined by the principal quantum number n, setting the main energy level. A subshell is set by the azimuthal quantum number ℓ, which can be any integer from 0 to n-1; letters s, p, d, f correspond to ℓ=0,1,2,3, but for n≥5 higher-ℓ subshells (g, h…) exist. The common mistake is to think ℓ never exceeds 3.
Memory trick: ℓ = 0…n-1; s=0, p=1, d=2, f=3, g=4…
“Electrons fill orbitals like cars taking the nearest empty parking spot: the spot with the smallest (n+ℓ) gets occupied first.”
Aufbau orders orbital filling by increasing (n+ℓ); when two subshells share the same (n+ℓ), the one with lower n fills first (e.g., 3d before 4p). Hund’s rule then says electrons in a subshell occupy separate orbitals with parallel spins before pairing. Forgetting the tie-breaker is a frequent exam trap.
Memory trick: Aufbau: low (n+ℓ) first; tie → lower n.
“Chromium’s extra stability is like a soccer team with five defenders (3d⁵) and one goalkeeper (4s¹) – half-filled defense is strongest.”
Cr has the ground state [Ar] 3d⁵ 4s¹ and Cu [Ar] 3d¹⁰ 4s¹ because half-filled (Cr) or fully-filled (Cu) d subshells lower the energy. The usual writing 3d⁴ 4s² or 3d⁹ 4s² is incorrect. Trap: always check for extra stability of d⁵ or d¹⁰ before assigning electrons.
Memory trick: Cr: 3d⁵4s¹ (half), Cu: 3d¹⁰4s¹ (full)
“When sodium metal reacts with chlorine, Na gives up one electron to become Na⁺, achieving a neon-like octet.”
Atoms aim for a full valence shell of eight electrons (the octet). They lose, gain or share electrons to reach the nearest noble-gas configuration. In JEE/NEET, remember period-2 elements cannot have expanded octets and electron-deficient molecules (e.g., BF₃) break the rule.
Memory trick: Lose-Gain-Share → reach 8
“BeCl₂ is like a two-handed glove; it’s stable with only four valence electrons, not a full octet.”
Elements such as Be, B, Al form electron-deficient compounds with less than eight electrons around the central atom. Sulphur in SF₆ can use d-orbitals to hold more than eight electrons, giving an expanded octet. The exam trap is to force an octet on every molecule—first check the element’s period and possible d-orbital use.
Memory trick: Be, B, Al → ≤8; S, P, Cl → can expand.
“In a crowded elevator, lone pairs are the bulky passengers that push bonding pairs closer together.”
VSEPR states that all electron pairs (bonding and lone) arrange to maximize separation, determining molecular shape. Lone pairs occupy more space than bonding pairs, so they compress adjacent bond angles. Ignoring lone pairs leads to wrong predictions, e.g., calling H₂O linear instead of bent.
Memory trick: Lone > Bond → smaller angles.
“A sp³ hybrid is like mixing red, green, blue, and yellow paints to get a uniform shade for a tetrahedral sculpture.”
Hybridization mixes the central atom’s s and p orbitals to form equivalent hybrid orbitals (sp, sp², sp³) that dictate geometry. The number of hybrids equals the steric number (σ bonds + lone pairs). Exams match sp³ → tetrahedral, sp² → trigonal planar, sp → linear.
Memory trick: Hybrid count = σ + LP; sp³ =4, sp² =3, sp =2.
“Counting only bonds for NH₃ is like counting chairs without noting the empty seat; the lone pair forces sp³ hybridization.”
When assigning hybridization, include both σ bonds and lone pairs in the steric number. Ignoring lone pairs gives wrong hybrids, e.g., NH₃ is sp³ (3 bonds + 1 lone pair) not sp². The common pitfall is equating hybrid type with number of bonds alone.
Memory trick: Hybrid = bonds + lone pairs.
“CO₂ is like two equally strong magnets placed back-to-back; their forces cancel, making the molecule non-polar.”
A molecule is polar only if the vector sum of its bond dipoles is non-zero. Both electronegativity differences and geometry matter; symmetric shapes (CO₂, BF₃) cancel dipoles despite polar bonds. The trap is to label any molecule with polar bonds as polar without checking symmetry.
Memory trick: Polar bonds + asymmetry = polar molecule.
“Like a seesaw, if the heavy side (strong bond dipole) isn’t balanced by the shape, the molecule tips and stays polar.”
A molecule is polar only when the vector sum of its bond dipoles is non-zero. Both electronegativity difference and molecular geometry decide this. The common trap is to label any molecule with polar bonds as polar, ignoring symmetry.
Memory trick: Polar = Polar bonds + Asymmetry (P + A)