Specification Point 1.39:
Write Formulae for Compounds Formed Between the Ions Listed Above

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-

O2   2 . 8 . 7      →      O22-   2 . 8 . 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.