Mass number is the sum of number of protons and neutrons in nucleus.

Isotopes are atoms of same element (same atomic number) but with different mass numbers (different number of neutrons in their nuclei).

Atomic weight is the mass of an atom relative to the mass of an atom of carbon-12, which has an atomic weight of exactly 12.00000 amu.

Electronic structure of atoms follows the pattern of subshells:

1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s
s-subshell=1orbital; p-subshell=3 orbitals; d-subshell=5 orbitals

Valence electron(s) are farthest away from the nucleus responsible for the chemical properties of the atom. Elements in the same vertical column in the periodic table have the same number of valence electrons. Valence electrons are shown as dots in Lewis symbols of the elements.

When balancing equations, balance elements other than O first, then O, and then H to get same number of atoms of each element on both sides.

Reading balanced equations are mole statements. For example:

2 H₂O(l) + 2 Na(s) 2 NaOH(aq) + H₂(g)
is read as 2 moles liquid water + 2 moles solid sodium 2 moles of NaOH dissolved in water + 1 mole of hydrogen gas.

Converting moles of each species to mass:

6 g of liquid water + 46 g solid sodium

A balanced equation always obeys the Law of Conservation of Mass:

36 g + 46 g = 82 g = 80 g + 2 g;
though the total number of moles on either side of the arrow need not be equal.

One mole of anything is 6.02 x 10²³ (Avogadro's number) of that thing. One mole of an element is 6.02 x 10²³ atoms of that element and is a mass of that element equal to its atomic weight in grams. One mole of a compound is 6.02 x 10²³ molecules of that compound and equals its formula weight (the sum of the atomic weights of all the atoms in the formula) in grams. For example, 1 mole of C = 12 g C = 6.02 x 10²³ atoms; 1 mole CO₂ = 44 g CO₂ = 6.02 x 10²³ molecules of CO₂. And (grams of compound) × (formula weight) = number of moles of compound (number of moles of compound) × (formula weight) = grams of compound.

In reading formulas, the molecular formula of carbon dioxide, CO₂, shows that one mole of carbon (12 g) and two moles of oxygen (2 x 16 g) compose 1 mole of CO2 (44 g of CO₂). Covalent bonds form by sharing electron pairs between atoms, 2 electrons per bond. In compounds, carbon forms 4 bonds; oxygen, 2; nitrogen, 3; and hydrogen, 1. Atoms will lose, gain, or share electrons to achieve 8 electrons in their valence shells.

Lewis formulas show how valence electrons are arranged as bonding or nonbonding pairs. The ionic bond is the attraction between ions of opposite charge in a crystal. Gases expand when heated (Charles' law) and contract when pressure is applied to them (Boyle's law). One mole of any gas occupies 22.4 L at standard temperature and pressure (STP), which is 0° C and 1 atm of pressure.

Acids form H⁺ (aq) in water; bases form OH^- in water. Acids react with bases (neutralization), forming water and a salt. Strong acids and strong bases ionize completely, but only a small fraction of the molecules of weak acids or bases form ions in water, being classed as strong electrolytes and weak electrolytes, respectively. Weak acids and bases exist in an equilibrium in solution. pH is a measure of acidity or basicity of a solution. If pH is less than 7, the solution is acidic; if greater than 7, basic; if exactly 7, neutral. pH = -log[H⁺].

Oxidation is the loss of electron(s) by a species, and reduction is the gain of electron(s). Oxidation and reduction occur simultaneously in redox reactions. In a balanced redox equation, the total number of electrons lost equals the total gained. Voltaic cells (batteries) use redox reactions to cause a flow of electrons. Electrolytic cells are just the opposite and use a flow of electrons to cause a chemical reaction. The charge on one mole of electrons is called the Faraday. One Faraday will reduce one mole of Na⁺ to one mole of sodium atoms, Na.

Chemical equilibrium exists when two opposing changes occur simultaneously at the same rate. For a given reaction, only temperature can change the equilibrium constant, K.

aA(g) + bB(g) cC(g); K=

Le Chatelier's principle states that if a system at equilibrium is disturbed in a way to upset the equilibrium, the system will change in such a way to form a new equilibrium that offsets the disturbance (the disturbance being a change in temperature or change in concentration).

A negative enthalpy change ΔH, a loss of heat energy, and a positive entropy change ΔS, an increase in disorder, are driving forces for chemical and physical changes. They are combined in the equation ΔG = ΔH - TΔS, leaving the free energy change, ΔG, as the ultimate term for predicting spontaneity. If ΔG is negative, the change will proceed as written in the equation. If ΔG is positive, the spontaneous reaction is in the opposite direction.