Ionic Compounds (SQA National 5 Chemistry): Revision Note

Forming ions

• Atoms are most stable when they have a full outer shell of electrons, like the Noble Gases in Group 0 (8)

• To achieve this stable electron arrangement, atoms will either lose or gain electrons

• When an atom loses or gains electrons, it becomes a charged particle called an ion

Metals form positive ions

• Metal atoms (from the left of the Periodic Table) have 1, 2 or 3 electrons in their outer shell

• They achieve a stable electron arrangement by losing these outer electrons

• When a neutral atom loses negative electrons, it is left with more positive protons.

  • This results in a positive ion

Example: Magnesium (Mg)

• A magnesium atom has the electron arrangement 2,8,2

• It loses its 2 outer electrons to achieve the stable arrangement 2,8

• It now has 12 protons but only 10 electrons

  • So, it forms the Mg²⁺ ion

Non-metals form negative ions

• Non-metal atoms (from the right of the Periodic Table) have 5, 6 or 7 outer electrons

• They achieve a stable electron arrangement by gaining electrons to complete their outer shell

• When a neutral atom gains extra negative electrons, it is left with more negatove electrons

  • This results in a negative ion

Example: Oxygen (O)

• An oxygen atom has the electron arrangement 2,6

• It gains 2 electrons to achieve the stable arrangement 2,8

• It now has 8 protons but 10 electrons

  • So, it forms the O^2- ion

Writing ion-electron equations

• We can show the process of an atom losing or gaining electrons by writing an ion-electron equation

  • The symbol for an electron is e^-

Losing electrons (metals)

• When an atom loses electrons, the electrons are shown on the product (right) side of the equation

• For example, magnesium:

Mg → Mg²⁺ + 2e^-
(A magnesium atom forms a magnesium ion and two electrons)

Gaining electrons (non-metals)

• When an atom gains electrons, the electrons are shown on the reactant (left) side of the equation

• For example, oxygen

O + 2e^- → O^2-
(An oxygen atom and two electrons form an oxide ion)

Ionic bonding & lattices

Ionic bonding

• Ionic bonding occurs between metal and non-metal atoms

• It involves the transfer of electrons from the metal atom to the non-metal atom

  • The metal atom loses its outer electrons to form a positive ion

  • The non-metal atom gains these electrons to form a negative ion

Ionic lattices

• Ionic bonds are very strong and act in all directions

• This means that a single positive ion doesn't just attract one negative ion; it attracts all the oppositely charged ions around it

• As a result, ionic compounds do not form small, separate molecules

  • Instead, they build up into a giant, repeating, three-dimensional (3D) pattern called an ionic lattice

• An ionic lattice is a regular, crystal structure

  • Each positive ion is surrounded by negative ions

  • Each negative ion is surrounded by positive ions

• This arrangement ensures the electrostatic attraction is maximised, creating a very strong and stable structure

Giant ionic lattice of sodium chloride