Formulae are built from chemical symbols

1. Each element has a chemical symbol of one or two letters, such as H for hydrogen and Mg for magnesium.
2. The first letter of a symbol is always a capital letter, and a second letter is always lower case, as in Ca for calcium and Cl for chlorine.
3. The Periodic Table you are given shows the symbol, name, atomic number and relative atomic mass of every element, so you can look up any symbol you are unsure of.
4. Some symbols come from Latin names, so sodium is Na, potassium is K, iron is Fe, copper is Cu, silver is Ag and lead is Pb.
5. A chemical formula combines symbols to show which elements a substance contains.
   1. A small number written after and below a symbol, called a subscript, gives the number of atoms of that element, and no number means one atom.

Formulae of elements and simple covalent compounds

1. Metals, carbon and the noble gases are written as their symbol alone, such as Fe, C and He.
2. Seven elements exist as diatomic molecules, so their formulae are $H_2$, $N_2$, $O_2$, $F_2$, ${Cl}_2$, ${Br}_2$ and $I_2$.
   1. Hydrogen gas is therefore $H_2$, while H on its own stands for a single hydrogen atom.
3. A simple covalent compound is made of small molecules, and its formula counts every atom in one molecule.
   1. Water is $H_{2}O$, ammonia is ${NH}_3$, methane is ${CH}_4$ and hydrogen chloride is $HCl$.
4. In the names of covalent compounds, the prefixes mono, di and tri mean one, two and three atoms of the element they are attached to.
   1. Carbon monoxide is $CO$, carbon dioxide is ${CO}_2$ and nitrogen dioxide is ${NO}_2$.

Formulae and names of simple ionic compounds

1. An ionic compound usually forms when a metal reacts with a non-metal, and the metal is named first.
2. A name ending in -ide usually means the compound contains only two elements, as in sodium chloride, $NaCl$, and magnesium oxide, $MgO$.
   1. Hydroxide is the exception, because it contains oxygen and hydrogen, as in sodium hydroxide, $NaOH$.
3. A name ending in -ate means the compound also contains oxygen, as in calcium carbonate, ${CaCO}_3$, copper sulfate, ${CuSO}_4$, and sodium nitrate, ${NaNO}_3$.
4. The formula gives the simplest ratio of ions, because the ions form a giant lattice rather than separate molecules.
   1. In ${MgCl}_2$ there are two chloride ions for every magnesium ion.
5. The common acids have the formulae $HCl$ for hydrochloric acid, $H_2{SO}_4$ for sulfuric acid and ${HNO}_3$ for nitric acid.

Balanced equations conserve atoms on both sides

1. In a reaction, atoms are rearranged into new substances, but no atoms are made or destroyed.
   1. The total mass of the products therefore equals the total mass of the reactants, which is the conservation of mass.
2. An equation shows the reactants on the left of the arrow and the products on the right, with a plus sign between substances.
3. To balance an equation, write a balancing number in front of a formula, which multiplies every atom in that formula.
   1. In $2H_{2}O$ there are four hydrogen atoms and two oxygen atoms.
4. Never change the subscripts in a formula, because that would change the substance.
   1. Changing $H_{2}O$ to $H_2{O}_2$ would turn water into hydrogen peroxide.

Four steps from names to a balanced equation

1. Start with a word equation, such as magnesium + oxygen $\to$ magnesium oxide.
2. Replace each name with its correct formula, using the Periodic Table for any symbol you need: $$Mg+O_2\to MgO$$
   1. Oxygen is written as $O_2$, because it is one of the diatomic elements.
3. Balance one element at a time, leaving any element that appears in several formulae until last.
   1. Putting $2$ in front of $MgO$ balances oxygen, and then $2$ in front of $Mg$ balances magnesium: $$2Mg+O_2\to 2MgO$$
4. Finish by recounting every atom on both sides, and use the smallest whole-number balancing numbers that work.