Chemical Symbols & Formulae (WJEC GCSE Chemistry)

Elements

• Elements are often represented using their chemical symbol from the Periodic Table

Chemical symbols of elements table

• Seven elements exist as diatomic molecules, which means that they are molecules made of two atoms:
  1. Hydrogen, H₂ 
  2. Nitrogen, N₂ 
  3. Oxygen, O₂ 
  4. Fluorine, F₂ 
  5. Chlorine, Cl₂ 
  6. Bromine, Br₂ 
  7. Iodine, I₂ 

Periodic Table identifying the 7 diatomic elements

The highlighted elements all exist as diatomic molecules with the formula X₂

• These 7 elements are also classed as simple molecules

Simple molecules 

• Simple molecules are substances that are made from two or more atoms chemically joined together
  • The atoms in simple molecules are non-metals, e.g. carbon, hydrogen, nitrogen
• Simple molecules can be elements or compounds and are represented using chemical formulae
• The chemical formula of a simple molecule shows:
  • The atoms involved, given by the chemical symbol
    AND
  • The number of atoms, given by the subscript (little) number after a chemical symbol

Examples of simple molecule chemical formulae

• Elements and compounds can exist as simple molecules
  • It is important to note that simple molecules are covalent substances
  • In simple terms, this means that they do not contain metal atoms
• Simple molecules are often represented in diagrams
• These diagrams show:
  • The number of atoms involved 
  • How the atoms are arranged

• There are 2 main styles of diagram
  • Using the chemical symbols from the Periodic Table

Representing simple molecules with chemical symbols

When simple molecules are represented using the chemical symbols of the atoms, there is no need for a key 

• • Using different coloured atoms with a key

Representing simple molecules with a key

When simple molecules are represented using coloured atoms, a key is used to show the colours for each type of atom

• Using the water, carbon dioxide and methane examples, it is important to know that these representations also show the arrangement of the atoms in the molecule
• Water, H₂O
  • The hydrogen atoms are on either side of the oxygen atom
  • It does not have two hydrogen atoms and an oxygen atom all joined together in a  row
• Carbon dioxide, CO₂ 
  • The oxygen atoms are on either side of the carbon atom
  • It does not have one carbon atom and two oxygen atoms all joined together in a row
• Methane, CH₄ 
  • The four hydrogen atoms surround the central carbon atom
  • It does not have the carbon atom and four hydrogen atoms all joined together in a row

• Metals and non-metals react together to form ionic compounds
  • Ionic compounds are not simple molecules 
  • Remember: Simple molecules are formed when non-metals react together to form compounds
• Ionic compounds involve the metal losing electrons and the non-metal gaining electrons to form ions

• Some ions that you will be expected to be able to use, because they are stated in the exam specification, include:
  • Hydrogen ions, H⁺ - sometimes referred to as protons
  • Group 1 ions, e.g. Li⁺, Na⁺, K⁺ 
  • Group 7 ions, F^–, Cl^–, Br^– 
  • Copper(II) ions, Cu²⁺ 
  • Iron(II) ions, Fe²⁺ 
  • Iron(III) ions, Fe³⁺ 
• There are some polyatomic (containing more than one atom) ions stated in the exam specification: 
  • Carbonate ions, CO₃^2– 
  • Sulfate ions, SO₄^2– 
  • Hydroxide ions, OH^– 
  • Ammonium ions, NH₄⁺ 

Determining the formulae of ionic compounds

• Ionic compounds typically have no overall charge
  • This means that the size of any positively charged ion is cancelled by the size of any negatively charged ion
  • Careful: This should not be confused with an atom having no overall charge

Direct comparison

• The formula of an ionic compound can be determined by directly comparing the charges of the ions:
  • For example, iron(II) sulfate
    • The iron(II) ion is Fe²⁺, which means that it has a 2+ or +2 charge
    • The sulfate ion is SO₄^2–, which means that it has a 2– or –2 charge
    • The charges cancel each other out
      • Mathematically, (+2) + (–2) = 0
    • This means that one SO₄^2– ion is needed to cancel the +2 charge on Fe²⁺ 
    • Therefore, the formula of iron(II) sulfate is FeSO₄ 

The swap-and-drop method

• When the ions in the ionic compound have different charges, it can be easier to use the swap-and-drop method
  • Careful: If you use this method with ions that have the same charge, then you must give the simplest whole number ratio to get the correct answer
• For example, copper(II) chloride:
  • The copper(II) ion is Cu²⁺, which means that it has a 2+ or +2 charge
  • The chloride ion is Cl^–, which means that it has a 1– or –1 charge
  • The size of the charge on the copper(II) ion indicates the number of chloride ions needed, and the size of the charge on the chloride ion indicates the number of copper(II) ions needed

Determining the formula of copper(II) chloride

The charges swap from element to element and drop down. The positive and negative signs are removed and there is no need for the number 1.

• • • This gives the overall formula of copper(II) chloride as CuCl₂