2.1 - Chemical Formulae

Chemical symbols and formulae

• Every element has its own chemical symbol - a shorthand of one or two letters standing for one atom of that element.
• The first letter is always a capital and the second is lower case: Co is cobalt, but CO is carbon monoxide (one carbon atom joined to one oxygen atom).
• Most symbols come from the element's English name (e.g. H for hydrogen, C for carbon, Ca for calcium).
• Some come from older Latin names:
  • Na (sodium), K (potassium), Fe (iron), Cu (copper), Pb (lead)

Diatomic elements

• Seven elements exist as molecules of two atoms joined together and are described as diatomic:
  • Hydrogen (H₂), nitrogen (N₂), oxygen (O₂), fluorine (F₂), chlorine (Cl₂), bromine (Br₂), iodine (I₂)
• Oxygen gas is always written O₂, never O. It's still an element because it contains only one type of atom.

Reading a formula

• A chemical formula shows which elements a substance contains, and how many atoms of each.
• Numbers are written as subscripts and apply only to the symbol immediately in front of them (a symbol with no subscript means one atom):
  • H₂O = two hydrogen atoms, one oxygen atom
  • CO₂ = one carbon atom, two oxygen atoms
  • H₂SO₄ = two hydrogen atoms, one sulfur atom, four oxygen atoms (seven atoms in total)
• For a substance made of separate molecules, this is called the molecular formula.

Brackets in formulae

Brackets are used when a group of atoms is repeated. The subscript after the bracket multiplies everything inside it:

• Ca(OH)₂ = one calcium, two oxygen, two hydrogen atoms
• Mg(NO₃)₂ = one magnesium, two nitrogen, six oxygen atoms

Formulas from diagrams

• In a molecular diagram, each circle represents an atom, identified by a key.
• You can write the formula by counting the atoms of each element:
  • In a molecular substance, the formula describes one molecule.
  • In a giant network, the formula gives the simplest whole-number ratio of the atoms or ions.

Naming a compound and writing its formula by valency

What the ending tells you

• -ide - The compound usually contains just the two elements named. The metal comes first and keeps its name; the non-metal takes the -ide ending (e.g. sodium chloride, NaCl; magnesium oxide, MgO).
  • Hydroxide (OH^-) is an exception containing both oxygen and hydrogen.
• -ate (or -ite) - Means oxygen is present as well as the elements named (e.g. calcium carbonate, CaCO₃; copper sulfate, CuSO₄).

Prefixes that count atoms

• mono- - One (carbon monoxide, CO)
• di- - two (carbon dioxide, CO₂)
• tri- - Three (sulfur trioxide, SO₃)

Valency

• The valency of an element is its combining power: the number of chemical bonds an atom can form.
• For main-group elements, valency is found from the periodic table:

For transition metals with variable valency, the valency is given as a Roman numeral in brackets in the name (e.g. iron in iron(II) oxide has a valency of 2; in iron(III) oxide it has a valency of 3).

The S.V.S.D.F. method

1. Symbols - Write the symbol of each element.
2. Valency - Write each element's valency below its symbol.
3. Swap - Swap the valency numbers.
4. Divide - Divide by the highest common factor to get the simplest ratio.
5. Formula - Write the final formula, omitting any subscript that is 1.

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Worked example - Formula of silicon dioxide

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Group ions

• Group ions contain several atoms acting as a single unit (e.g. sulfate, SO₄^2-).
• The charge on a group ion equals its valency (e.g. sulfate has a charge of 2−, so its valency is 2).
• When more than one group ion is needed in the formula, enclose the ion in brackets: iron(III) sulfate is Fe₂(SO₄)₃.

Showing the charges

• In a formula, charges are written as superscripts and numbers of atoms or ions as subscripts.
• An ionic formula gives the simplest ratio of the ions, and can show the charge on each ion when a question asks for it: magnesium chloride is Mg²⁺(Cl^-)₂ and iron(III) sulfate is (Fe³⁺)₂(SO₄^2-)₃.
• The brackets go round the whole ion, charge included, with the number of ions outside them.