Revision of Atomic Structure

The ions are held together by the strong electrostatic attraction between opposite charges. In the solid they form giant regular structures called lattices.

Greater charges generally means stronger forces, so MgO (Mg2+ O2-) has a higher melting point than NaCl.

Properties of ionic compounds:

        Usually solid at room temperature with high melting points and boiling points.

        Usually hard

        Usually soluble in water

Do not conduct electricity when solid, but can when molten or in aqueous solution because the ions are free to move.

---

Covalent Bonding

Sharing of electrons between atoms.

Usually between non-metal atoms.

Atoms overlap outer shells to create stable full shells.

e.g. hydrogen H2

Shared pair of electrons is known as a single covalent bond and is represented by a single line as above for H2. Covalent bonds are very strong and a lot of energy is required to break them (bond energy – can be measured or calculated).

Atoms of a particular element usually form the same total number of bonds, known as valency – this is dependent on the number of electrons in the atom’s outer shell e.g. carbon forms 4 bonds such as in methane:

* ionic (valency of an ion is equal to its charge)

(NB lower down in the periodic table some atoms can have more than one possible valency e.g. sulfur can form 2, 4 or 6 bonds in its compounds)

Multiple covalent bonds can form between the same atoms e.g. O2 (see next page).

Most covalent compounds form simple molecular structures. The attractive forces between the molecules are weak (‘van der Waals’ forces). Forces between atoms within molecules are strong (bonds).

Properties of simple molecular compounds:

        Gases, liquids or solids with low melting points and boiling points

        Generally do not conduct electricity when molten or in solution

Generally do not dissolve easily in water.  Some molecules react with water to form other compounds or ions, however.

Some atoms that form covalent bonds form giant covalent structures e.g. diamond and graphite are two forms (called allotropes) of elemental carbon which have different giant structures:

Giant covalent structures are stronger with high melting points because the whole structure is held together by strong covalent bonds.

Metallic Bonding

Metals consist of positively charged ions held together by a delocalised ‘sea’ of electrons (negatively charged).

Properties of metals:

        Usually high melting and boiling points

        Conduct electricity

        Malleable and ductile

        High densities