#### Writing Formulae

Ionic Compounds: Formed when atoms of metals transfer electrons to atoms of non-metals to form compounds made up of ions.

Formulae for compounds formed between ions

• Metals: They will lose electrons to another atom and become positively charged ions
• Non-Metals: They will gain electrons from another atom to become negatively charged ions

EXAMPLE 1:

Sodium Chloride, NaCl, has a typical ionic bond

Sodium Atom

• Sodium is a group 1 metal so will lose one outer electron to another atom to have a full outer shell of electrons
• They will lose an electron to form a positive ion with the charge 1+

Na   2 . 8 . 1      →     Na+   2 . 8

Chlorine Atom

• Chlorine is a Group 7 non-metal so will need to gain an electron to have a full outer shell of electrons
• one electron will be transferred from the outer shell of the sodium atom to the outer shell of the chlorine atom
• Chlorine atom will gain an electron to form a negative ion with charge 1-

Cl   2 . 8 . 7      →      Cl   2 . 8 . 8

Formula of Ionic Compound:    NaCl

EXAMPLE 2:

Magnesium Oxide, MgO, has a typical ionic bond

MAGNESIUM ATOM

• Magnesium is a group 2 metal so will lose two outer electrons to another atom to have a full outer shell of electrons
• They will lose two electrons to form a positive ion with the charge 2+

Mg   2 . 8 . 2      →    Mg2+   2 . 8

OXYGEN ATOM

• Oxygen is a group 6 non-metal so will need to gain two electrons to have a full outer shell of electrons
• Two electrons will be transferred from the outer shell of the Magnesium Atom to the outer shell of the oxygen atom
• Oxygen atom will gain two electrons to form a negative ion with charge 2-

O   2 . 6      →      O2-   2 . 8

Formula of Ionic Compound:    MgO

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### Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.