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 12-g block of pure carbon-12 contains exactly 6.022×10²³ carbon atoms.”
Avogadro’s number N_A = 6.022×10²³ particles per mole. One mole of any substance holds this many entities, tying the macroscopic gram scale to the atomic scale. Use N_A for mole-to-particle conversions; do not round to 6×10²³ in precise stoichiometry.
N_A = 6.022×10²³
Memory trick: 12 g C-12 = N_A atoms
“Your kitchen scale reading 44 g of CO₂ equals one mole because its molar mass (44 g mol⁻¹) matches the molecular weight.”
Molar mass (M, g mol⁻¹) is the mass of one mole and numerically equals the atomic/molecular mass in amu. Use n = m/M to find moles. Trap: mixing up units – treat M as g mol⁻¹, not as amu, before using the formula.
n = m/M
Memory trick: M (g/mol) = atomic mass (u) × 1 g/u
“Glucose’s sweet formula C₆H₁₂O₆ is six times its simplest ratio CH₂O, like a LEGO set built from the basic CH₂O block.”
Empirical formula gives the lowest whole-number atom ratio; molecular formula = integer multiple of empirical. Find the factor k = (molar mass)/(empirical mass). Trap: assuming empirical = molecular without checking the molar mass.
Memory trick: Molecular = Empirical × (M_molar ÷ M_empirical)
“A soda bottle containing 0.5 mol of CO₂ in 0.25 L gives a Molarity of 2 M, like a concentrated juice concentrate.”
Molarity M = n/V (mol L⁻¹) uses total solution volume, not just solvent. Temperature changes V, so M changes. Trap: using solvent volume or forgetting to convert mL to L.
M = n/V
Memory trick: M = moles ÷ litres of solution
“In a firecracker, the limited amount of black-powder determines the maximum bang, no matter how much fuse you add.”
Convert each reactant to moles and compare with stoichiometric coefficients; the reactant giving the smallest possible product amount is the limiting reagent. Trap: ignoring the coefficient when judging which is limiting.
Memory trick: Limiting = smallest (moles ÷ coefficient)
“Like a positively charged balloon (protons) pulling tiny negatively charged confetti (electrons) around it, while neutrons sit quietly inside.”
Atoms consist of a nucleus containing positively charged protons and neutral neutrons, surrounded by negatively charged electrons in shells. The opposite charge of protons and electrons holds the atom together, while neutrons add mass without affecting charge. Exam trap: mixing up neutron charge as negative.
Memory trick: P+ N0, E− : Proton positive, Neutron zero, Electron negative.
“A school class roll number (Z) tells who you are, while the total number of students plus teachers (A) tells the class size.”
Atomic number Z equals the number of protons and uniquely identifies an element. Mass number A is the sum of protons and neutrons (A = Z + N) and approximates the atom’s mass in atomic mass units. Trap: treating A as the exact atomic mass or forgetting it is an integer count.
A = Z + N
Memory trick: Z = ID, A = heads (protons+neutrons).
“Identical twins (same protons) wearing different weight belts (neutrons) are still the same person.”
Isotopes are atoms with the same Z (same element) but different numbers of neutrons, giving different mass numbers. Chemical behavior remains nearly identical because electrons see the same nuclear charge, while nuclear stability can vary. Trap: assuming isotopes change chemical reactivity.
Memory trick: Same Z, different N → isotopes.
“An apartment address: building (n), floor (l), door (m), and whether the light is on (s) uniquely locates an electron.”
Four quantum numbers (n, l, m, s) uniquely label an electron: n sets the energy shell, l the subshell shape, m the orbital orientation, and s the spin (+½ or –½). Together they obey specific ranges (l < n, m = –l…+l). Trap: assigning m values outside –l to +l or forgetting spin can cause invalid configurations.
Memory trick: n-l-m-s = building-floor-door-switch.
“When filling a parking lot, cars park in the nearest empty spot before moving to farther rows.”
The Aufbau principle orders electron filling by increasing (n+ℓ) value; if two orbitals share the same (n+ℓ), the one with lower n fills first. This explains why 4s fills before 3d. Trap: ignoring the (n+ℓ) rule leads to wrong configurations like placing 3d before 4s.
Memory trick: Fill by smallest (n+ℓ), then lower n.
“A restroom stall can hold at most two people, and they must wear opposite-colored socks (spins).”
Pauli’s exclusion principle states that no two electrons in an atom can have identical sets of all four quantum numbers; thus an orbital can accommodate at most two electrons, which must have opposite spins (+½, –½). This limits electron pairing and shapes the periodic table. Trap: assuming two electrons can share the same spin in one orbital.
Memory trick: One orbital = 2 electrons, opposite spins.
“When you seat people at a round table, you first give each a seat before anyone sits next to another.”
Electrons fill degenerate orbitals singly with parallel spins to maximize total spin (exchange energy). This lowers electron repulsion. In JEE/NEET, pairing starts only after every orbital of the subshell has one electron; missing this gives wrong configurations.
Memory trick: Hund → one each before pair, same spin
“Moseley fired electrons at metal plates; the X-ray color shifted systematically as the element’s atomic number increased.”
The modern periodic law states that elemental properties repeat periodically with atomic number Z, not atomic mass. Moseley’s law showed √(frequency of characteristic X-rays) ∝ Z, confirming Z as the ordering principle. In exams, never use atomic weight to locate an element; always use Z.
Memory trick: Moseley → √ν ∝ Z
“A basketball gets smaller as you move from the back row to the front row of a stadium, but gets bigger when you climb to a higher tier.”
Atomic radius decreases across a period because increasing nuclear charge pulls electrons closer, while it increases down a group as each new shell adds distance. Poor shielding by d-electrons causes a slight irregularity, and the lanthanide contraction makes later transition metals smaller than expected. Remember: across → shrink, down → grow; d-block gets a hidden squeeze.
Memory trick: Across shrink, down grow; lanthanide squeezes d-block
“Pulling an electron from oxygen is like tugging a loose thread, easier than pulling the tightly woven half-filled nitrogen.”
Ionization energy rises across a period due to higher effective nuclear charge and falls down a group as electrons are farther from the nucleus. The N-O anomaly arises because oxygen’s 2p⁴ has a paired electron, increasing electron-electron repulsion and lowering IE, whereas nitrogen’s half-filled 2p³ is relatively stable, giving a higher IE. Do not blame the dip on nitrogen.
Memory trick: O’s pair → lower IE; N’s half-filled → higher IE
“A magnet’s north pole pulls iron strongest; similarly fluorine pulls shared electrons hardest.”
Electronegativity increases from left to right as atoms more strongly attract bonding electrons, and it decreases down a group as distance grows. Fluorine is the most electronegative element. Noble gases have no EN value.
Memory trick: EN ↑ right, ↓ down; F tops.
“Sodium in a spark easily gives away its electron, while chlorine in bleach holds onto its electrons tightly.”
Metallic character is the ease of losing electrons; it diminishes across a period and grows down a group, opposite to electronegativity. Transition metals show intermediate behavior, not extreme metals.
Memory trick: Metallic = opposite EN.
“Helium fills a balloon and stays calm because its outer shell is already full, so it won’t react.”
Group-18 elements have complete valence shells, giving them very high ionization energies and negligible tendency to gain or lose electrons, so they are chemically inert. He’s an exception with only 2 electrons but a full 1s shell.
Memory trick: Full shell = inert.
“When you rub a glass rod with silk, electrons move and the rod becomes positively charged, just like metal losing electrons to form cations.”
Ionic bond forms by complete transfer of valence electrons from a metal to a non-metal, creating oppositely charged ions that attract electrostatically. The bond strength increases with higher charge magnitude and smaller ionic radii. Exam trap: never assume a metal-metal or non-metal-non-metal pair can form an ionic bond.
Memory trick: Metal → lose e⁻, Non-metal → gain e⁻ (L-G)
“Two hydrogen atoms share their single electrons to make H₂, just as friends share a single umbrella in rain.”
Covalent bond involves sharing of one or more electron pairs between non-metals to attain octet (or duet). Shared electrons belong to both atoms, giving a bond order equal to number of shared pairs. Trap: a double bond is not two separate bonds; it is one σ plus one π pair.
Memory trick: Share = 2e⁻ per bond; double = 4e⁻; triple = 6e⁻
“The shape of a water molecule (bent) comes from two bonding pairs and two lone pairs pushing the H atoms apart.”
VSEPR states that electron pairs (bonding and lone) around a central atom arrange themselves to minimize repulsion, determining molecular geometry. Lone pairs occupy more space than bonding pairs, giving smaller bond angles. Trap: ignore lone-pair repulsion and predict tetrahedral for NH₃ (actually trigonal pyramidal).
Memory trick: AXₙEₘ: A=central, X=bond pairs, E=lone pairs → shape
“The four sp³ hybrids point like the tips of a pyramid-shaped tent, each 109.5° apart.”
sp³ hybrids arise from one s + three p orbitals, giving four equivalent directions toward tetrahedral corners (109.5°). sp² mixes one s + two p → three planar orbitals (≈120°) plus one unhybridised p for π bonds. sp mixes one s + one p → two opposite orbitals (180°) leaving two p for π bonds. Remember: number of hybrids equals σ bonds plus lone pairs, not just drawn bonds.
Memory trick: sp³ → 4, sp² → 3, sp → 2 hybrid orbitals.
“A single bond in ethane is like two hands clasping head-on (σ), while the extra bond in ethene is a side-by-side overlap (π) like two fingers sliding past each other.”
σ bond results from end-to-end overlap of orbitals and exists in every bond; it allows free rotation. π bond arises from side-on overlap of parallel p orbitals, present only in double/triple bonds, restricting rotation and being weaker. Trap: count bonds by σ+π, not by bond lines alone; a double bond = 1σ+1π.
Memory trick: Single=1σ; Double=1σ+1π; Triple=1σ+2π
“Water’s high boiling point is due to each H-bond acting like a tiny Velcro hook between H of one molecule and O of another.”
Hydrogen bond is a strong dipole-dipole attraction when H is covalently bound to highly electronegative N, O, or F and interacts with a lone pair on another N/O/F. It is directional and significantly raises boiling points. Trap: not every H-X…Y interaction is H-bond; X must be N/O/F and H must be attached to X.
Memory trick: H-bond: H-(N/O/F) … lone pair on N/O/F