1.1 Chemical bonding
Elements react together to form compounds by gaining or losing electrons or by sharing electrons.
A compound contains two or more elements which are chemically combined.
When atoms of non-metallic elements join together by sharing electrons it is called COVALENT BONDING
When metallic elements react with non-metallic elements they produce ionic compounds:
- The metal atoms lose electrons to form positive ions
- The atoms of non-metals gain electrons to form negative ions
- The ions have the stable electronic structure of a noble gas
- The oppositely charged ions attract each other in the ionic compound
- This is called IONIC BONDING
1.2 Ionic Bonding
Ionic bonding holds oppositely charged ions together in giant structures. The giant structure of ionic compounds is very regular because the ions are packed neatly together.
Strong electrostatic forces of attraction act in all directions. Each ion in the giant structure or lattice is surrounded by ions with the opposite charge and so is held firmly in place.
Sodium chloride contains equal numbers of sodium ions and chloride ions as shown by its formula NaCl. The sodium ions and chloride ions alternate to form a cubic lattice.
The ratio of ions in the structure of an ionic compound depends on the charges on the ions. For example, calcium ions are Ca²⁺, and chloride ions are Cl⁻, so calcium chloride contains twice as many chloride ions as calcium ions, and its formula is CaCl2.
1.3 Formulae of ionic compounds
Ionic compounds are neutral, as the charges on the ions in an ionic compound always cancel each other out.
The formula of an ionic compound shows the ratio of ions present in the compound.
1.4 Covalent Bonding
A covalent bond is formed when two atoms share a pair of electrons.
The number of covalent bonds an atom forms depends on the number of electrons it needs to achieve a stable electronic structure.
Covalent bonds join atoms together to form molecules.
Many covalently bonded substances consist of small molecules, but some atoms that form several bonds, e.g. carbon, can join together in giant covalent structures, or macromolecules.
The atoms in metals are closely packed together and arranged in regular layers.
- metal atoms pack together and the electrons in the highest energy level delocalise
- they can now move freely between atoms
- this produces a lattice of positive ions in a "sea" of moving electrons
- the delocalised electrons strongly attract the positive ions and hold the giant structure together