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
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
A frequent error is reversing the across-a-period trend. In reality, atomic size decreases across a period (more nuclear pull) and increases down a group (new shells).
Memory trick: more protons but the same shell → electrons pulled in tighter across a period.
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
The energy to remove the outermost electron; it rises across a period and falls down a group.
Memory trick: removing an electron is hardest from small, tightly-held atoms.
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
A frequent error is ignoring small dips (e.g. B<Be, O<N) caused by sub-shell stability. In reality, the energy to remove the outermost electron; it rises across a period and falls down a group.
Memory trick: removing an electron is hardest from small, tightly-held atoms.
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
An atom's tendency to attract shared electrons; fluorine is the highest.
Memory trick: electronegativity rises toward the top-right (fluorine) of the table.
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
A frequent error is assuming metals have high electronegativity. In reality, an atom's tendency to attract shared electrons; fluorine is the highest.
Memory trick: electronegativity rises toward the top-right (fluorine) of the table.
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
A frequent error is confusing metallic character with electronegativity (they're opposites). In reality, the tendency to lose electrons; it decreases across a period and increases down a group.
Memory trick: metals sit on the left and bottom; non-metals on the top-right.
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
As atomic radius decreases across a period, ionization energy increases — smaller atoms hold their electrons more tightly.
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
Metals lose electrons easily (low ionization energy, form cations); non-metals gain electrons (high electronegativity, form anions).
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
A group is a vertical column (same valence electrons, similar properties); a period is a horizontal row (same number of shells).
“Mendeleev left gaps in his table and correctly predicted elements no one had discovered yet.”
It DECREASES left-to-right as nuclear charge pulls electrons closer.
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
A frequent error is reading the order off the balanced equation's coefficients. In reality, the power to which a reactant's concentration is raised in the experimentally-determined rate law.
Memory trick: order comes from experiment, not stoichiometry (unless the step is elementary).
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
The proportionality constant k in the rate law; it rises sharply with temperature.
Memory trick: k depends on temperature and catalyst, not on how much reactant you have.
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
A frequent error is thinking k depends on concentration. In reality, the proportionality constant k in the rate law; it rises sharply with temperature.
Memory trick: k depends on temperature and catalyst, not on how much reactant you have.
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
A frequent error is thinking a catalyst changes ΔH or the position of equilibrium. In reality, the minimum energy barrier reactants must cross; a catalyst lowers it.
Memory trick: a catalyst speeds BOTH directions equally — it never shifts equilibrium.
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
The time for a reactant to fall to half its amount; for first-order reactions it's constant.
Memory trick: constant half-life is a signature of first-order kinetics.
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
A frequent error is assuming half-life is constant for every order. In reality, the time for a reactant to fall to half its amount; for first-order reactions it's constant.
Memory trick: constant half-life is a signature of first-order kinetics.
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
Arrhenius equation linking rate constant to temperature. Use it when Ea is the activation energy.
k = A·e^(−Ea/RT)
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
Half-life of a first-order reaction. Use it when first-order kinetics only.
t½ = 0.693/k
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
Order is experimental and can be zero or fractional; molecularity is the count of species in an elementary step and is always a whole number.
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
Rate depends on concentration and changes as the reaction proceeds; the rate constant k is fixed at a given temperature.
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
A catalyst speeds a reaction by lowering activation energy but does NOT shift the equilibrium position.
“Food lasts longer in a fridge because cold slows the reactions that spoil it.”
Order is found by experiment; it only matches the coefficients for a genuinely elementary reaction.
“The battery in your phone is a controlled chemical reaction pushing electrons through the circuit.”
A galvanic cell turns a spontaneous reaction into electricity; an electrolytic cell uses electricity to drive a non-spontaneous reaction.
Memory trick: galvanic = battery (gives energy); electrolytic = needs a power source.
“The battery in your phone is a controlled chemical reaction pushing electrons through the circuit.”
A frequent error is swapping which cell is spontaneous. In reality, a galvanic cell turns a spontaneous reaction into electricity; an electrolytic cell uses electricity to drive a non-spontaneous reaction.
Memory trick: galvanic = battery (gives energy); electrolytic = needs a power source.