Formulae and Equations

Each element is represented by its own chemical symbol. A chemical symbol:

• consists of one or two letters
• always starts with a capital letter, with any other letter in lower case

For example, the symbol for mercury is Hg.

The periodic table shows the names and symbols of the elements. Elements are arranged in groups that share similar characteristics. Groups are numbered from 1 to 7, and then group 0.

Metals are found on the left, and non-metals are found on the right.

A chemical formula is used to represent an element or compound in balanced  chemical equations. Formulae differ, depending on the element they represent.

The formulae for most elements is just their symbol, however there is a group of non-metal elements that exist as diatomic. Diatomic means it is a molecule of two atoms joined together. To show this, their formulae have a subscript '2'.

These elements are:

• Hydrogen
• Nitrogen
• Fluorine
• Oxygen
• Iodine
• Chlorine                    A mnemonic can be used to remember these:
• Bromine                                Have No Fear Of Ice Cold Beer

A substance formed by the chemical union of two or more elements is called a compound.

The chemical formula for a compound shows:

• the symbols for each element in the compound
• the number of atoms of each element in each unit of the compound

A bond formed between two atoms, where electrons are shared, is called a covalent bond. Covalent compounds usually form between non-metal elements and are held together by covalent bonds. All compounds that exist as molecules are covalent.

Here are some examples:

• Ammonia = NH₃
• Carbon dioxide = CO₂
• Methane = CH₄
• Sulphur dioxide = SO₂
• Water = H₂O

The subscript numbers in formulae show how many atoms of that element appear in the molecule. So, the formula NH₃ shows that ammonia contains one nitrogen and three hydrogen atoms.

A bond formed between two atoms where an electron is transferred from one atom to another is called an ionic bond.

Ionic compounds are made up of atoms joined together by ionic bonds.

They usually (but not always) contain at least one metal element and one non-metal element.

An ion is a charged particle formed when an atom, or a group of atoms, loses or gains electrons. The number and sign of its electrical charges are shown in superscript text.

Names and formulae of some common ions:

• Sodium =                Na⁺ =          One positive
  
• Magnesium =          Mg²⁺ =        Two positive
• Chloride =               Cl^- =            One negative
• Oxide =                   O^2- =           Two negative

The formula of an ionic compound can be predicted by using the formulae of its ions. The number of ions in a formula must give an equal number of positive and negative charges.

Some examples:

• Sodium chloride = NaCl = One positive, one negative
• Sodium oxide = Na₂O = Two positive, two negative
• Magnesium oxide = MgO = Two positive, two negative
• Magnesium chloride = MgCl₂ = Two positive, two negative

A word equation models a chemical reaction using the names of the substances involved.

Word equations do not show any chemical symbols or formulae.

Reactants are the substances that react together in a chemical reaction. In a chemical reaction, the atoms or ions in reactants seperate from one another. They join back together in a different way to form products.

Word equations always take this form:

reactants → products

A + sign seperates two or more reactants, or two or more products.

Potassium hydroxide reacts with sulfuric acid. Potassium sulfate and water are formed in the reaction. This means that:

• the reactants are potassium hydroxide and sulfuric acid
• the products are potassium sulfate and water
• the word equation is: potassium hydroxide + sulfuric acid --> potassium sulfate + water

There can be different numbers of reactants and products. For example:

• sodium + chlorine --> sodium chloride
• calcium carbonate --> calcium oxide + carbon dioxide

A balanced chemical equation models a chemical reaction using the formulae of the reactants and products. It shows the number of units of each substance involved.

It makes sure the number of units of each element present is the same on both sides of the arrow.

Balanced chemical equations include state symbols in brackets after each formula. They show the physical state of each substance. 

• (s) = solid
• (l) = liquid
• (g) = gas
• (aq) = aqueous solution

An aqueous solution forms when a substance dissolves in water.

State symbols are useful because they show what a substance is like. For example:

• H₂O(l) is liquid water, but H₂O(g) is steam
• HCl(g) is hydrogen chloride gas, but HCl (aq) is hydrochloric acid

If an equation is written by just replacing names with formulae, it may not be balanced. The number of atoms of each element on the left must be the same as they are on the right.

To balance an unbalanced equation, numbers need to be added to the left of one or more formulae.

Balanced chemical equations only show formulae, not names. A balancing number multiplies all the atoms in the substance next to it.

Remember to add state symbols once you have balanced your equation, if you are asked to.

A balanced ionic equation shows the reacting ions in a chemical reaction.

These equations can be used to model what happens in precipitation reactions.

A precipitation reaction is a reaction where an insoluble solid is formed when certain solutions are mixed.

In a typical precipitation reaction, two soluble reactants form an insoluble product and a soluble product.

Some ions may remain exactly the same on both sides of the equation, and do not form a precipitate. Ions that remain essentially unchanged during a reaction are called spectator ions. This means you can ignore them when writing the ionic equation. 

In a balanced ionic equation:

• the number of positive and negative charges is the same
• the numbers of atoms of each element on the left and right are the same