OCR Gateway Chemistry C4.2

Atoms with an outer shell of 8 electrons have a stable electronic structure. Atoms can be made stable by transferring electrons. This is called ionic bonding.

Metal atoms lose electrons to get a stable electronic structure. Therefore they form a positive ion, as there are fewer negatively charged electrons than positively charged protons.

Non-metal atoms gain electrons to get a stable electronic structure. If an atom gains electrons then a negative ion is formed.

During ionic bonding, the metal atom becomes a positive ion and the non-metal atom becomes a negative ion. These two ions then attract one another.

Dot and cross models are used to describe ionic bonding. Here is an example.

The structure of sodium chloride or magnesium oxide is a giant ionic lattice, in which positive ions have strong electrostatic attraction to negative ions. Substances with this structure are always solids.

Sodium chloride solution can conduct electricity.

Sodium chloride and magnesium oxide conduct electricity when they are molten, i.e. when when their ions can move freely.

The physical properties of sodium chloride and magnesium oxide mean that:

• they have high melting points due to strong attractions between positive and negative ions
• they don't conduct electricity when solid because the ions cannot move
• they conduct electricity when in solution or as a molten liquid as the ions are free to move

The melting point of magnesium oxide is higher than that of sodium chloride because:

• magnesium has a charge of 2+ and oxygen has a charge of 2-, as opposed to sodium (+) and chloride (-), so there are stronger electrostatic attractions between + and - ions
• each magnesium atom donates two electrons to the oxygen atom, which makes a stronger bond than when sodium atoms transfer only one electron to chlorine atoms
• magnesium ions have a very small radius, so magnesium can get much closer to oxygen and the bond is therefore stronger