Chemical Symbols and Formulae

Chemical symbols represent individual elements e.g., H for Hydrogen and O for Oxygen.
A chemical formula shows the elements in a compound and the ratio of atoms, e.g., H2O for water, which has two hydrogen atoms and one oxygen atom.
Subscripts in a chemical formula indicate the number of atoms of each element in a compound.
If there is no subscript after a symbol, it implies that there is only one atom of that element.

Balancing Chemical Equations

A chemical equation shows a chemical reaction, with reactants on the left and products on the right.
Atom conservation demands that the number of atoms of each element in the reactants must equal the number of atoms of each element in the products.
Balancing a chemical equation involves adjusting the number of molecules of reactants and products while retaining their proportions.
Number coefficients are put in front of reactants or products to balance the equation.

Ionic compounds consist of cations (positive ions) and anions (negative ions), written with cations first. The overall charge is zero.
In an ionic formula, the charges on ions are balanced so the total positive charge equals the total negative charge.
Covalent compounds consist of nonmetal atoms bonded by sharing electrons. The atoms and numbers are written in the order in which they are bonded.

The group number of an element in the periodic table often represents the number of valence electrons.
The number of missing or extra electrons in the valence shell helps predict the charge of the ion an element may form.
By considering the charges of ions, one can predict the ratio of ions in the compound they may form, thereby deducing the empirical formula.

The relative formula mass of a substance is obtained by adding together the relative atomic masses of all the atoms in its formula.
It is used to determine the amounts of reactants and products involved in a chemical reaction.

State symbols denote the physical states of the reactants and products in a chemical reaction under normal conditions - (s) for solid, (l) for liquid, (g) for gas and (aq) for aqueous.