3.1 - 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.