Ionic bonding: the electrostatic attraction between oppositely charged ions.
Ionic boning is usually found in compounds made of a metal and a non-metal. The metal loses electrons and forms a positive ion, the non-metal gains electrons and forms a negative ion. Dot and cross diagrams can be drawn for ionic compounds, showing the simplest whole number ratio - e.g. NaCl would show one Na⁺ ion and one Cl⁻ ion, and MgCl₂ would show one Mg²⁺ ion and two Cl⁻ ions.
The arrangement of ions in an ionic solid is described as a giant ionic lattice, as the ions all fit together in a regular pattern:
This structure gives ionic substances many of its properties:
- High melting and boiling points: because the electrostatic attraction between the +ve and -ve charges is very strong. Also, the larger the charges of the ions are, the stronger those attractions are the higher the melting point/boiling point will be.
- Conduct electricity only when molten or dissolved: because the +ve/-ve ions are fixed in place in the lattice when solid, there are no mobile charge carriers, and thus nothing to conduct electricity. However when molten or in solution, the ions can move and carry charge, thus it conducts electricity.
- Soluble in water: because the δ- charge on the oxygen and the δ+ on the hydrogen of the water molecule are attracted to the + and - charges of the ions, and this attraction is strong enough to pull the ionic lattice apart.
- Brittle: when a force is applied to an ionic substance, the lattice is deformed and ions with like charges will become aligned. Like charges repel, and this repulsion is enough to split the crystalline structure
Covalent bond: a bond formed by a shared pair of electrons
A group of atoms bonded covalently is called a molecule, and these can also be shown through dot and cross diagrams. Because the electrons are shared, not taken, they look different to ionic dot and cross diagrams.
Rules for forming covalent bonds:
- Unpaired electrons pair up
- The maximum number of electrons that can pair up is equal to the number of electrons in the outer shell.
There are two types of pairs of electrons, ones that are being used in a bond, known as bonding pairs, and ones that aren’t shared, known as a lone pair.
Lone pair: a pair of electrons in the outer shell not used in bonding.
Mostly, covalent bonds are formed by each bonding atom donating one electron. In some cases, one atom donates both electrons to the bond. This is known as a dative covalent or coordinate bond.
Dative covalent (coordinate) bond: a bond formed by a shared pair of electrons which has been donated by one of the bonding atoms only. It can be written as A—->B, with the arrow indicating the direction in which the electron pair has been donated.
Some substances may be a mix of covalent and ionic bonding, like MgCO₃:
There are two kinds of covalent structure: simple molecular and giant covalent lattice.
Simple molecular: a 3 dimensional structure of molecules, held together by weak intermolecular forces
- Simple molecular structures have low boiling points, because the intermolecular forces are van der Waals’ forces and relatively weak, so little energy is needed to overcome them.
- They don’t conduct electricity because there are no charged particles that are free to move
- They are soluble in non-polar solvents, because van der Waals’ forces form between the simple molecular structure and the non-polar solvent.
Giant covalent lattice: a 3 dimensional structure of atoms, bonded together by strong covalent bonds
- They have high melting and boiling points because the strong covalent bonds between the atoms require a high amount of energy to overcome
- They don’t conduct electricity (apart from graphite) because there are no charged particles free to move
- They’re insoluble in both polar and non-polar solvents because the covalent bonds are too strong to be overcome.
Diamond and graphite are two allotropes of carbon.
Allotrope: two or more different forms in which an element can exist
- has a tetrahedral structure held together by covalent bonds
- it’s not an electrical conductor because there are no delocalised electrons to carry charge; all the outer-shell electrons are used to form bonds
- it’s hard because the tetrahedral structure allows external forces to be spread throughout the lattice.
- has strong hexagonal layers, covalently bonded, with weak van der Waals’ forces between them
- it conducts electricity because there are delocalised electrons between the layers, that can move parallel to them and carry a charge
- it’s soft because the bonding within each layer is strong, but the forces between each layer are weak and allow them to slide easily.
Metallic bonding: the electrostatic attraction between a lattice of positive ions and a sea of delocalised electrons.
Metallic substances don’t bond covalently or ionically, and this gives them different properties to ionic or covalent substances:
- High melting/boiling points: there’s strong electrostatic attraction between the positive lattice and negative sea, that requires a lot of energy to overcome
- They conduct electricity well: thanks to the sea of delocalised electrons, which can move and carry a charge. When molten, the ions can move too.
- They’re insoluble in water or non-polar substances because the electrostatic attraction is too strong for the interactions with the solvents to overcome