What are bonds?
1) When different elements join, you get a compound.
2) There are two main types of bonding in compounds - ionic and covalent. You need to make sure you've got them both totally sussed.
1) Ions are formed when electron are transferred from one atom to another.
2. The simplest ions are single atoms which have either lost or gained 1, 2 or 3 electrons so that they've got a full outer shell. Here are some examples of ions:
A sodium atom (Na) loses 1 electron to form sodium ion (Na+)
A chlorine atom (Cl) gains 1 electron to form a chloride ion (Cl-)
3. You don't have to remember what each element forms - nope, for many of them you just look at the Periodic Table. Elements in the same group all have the same number of outer electrons. So they have to lose or gain the same number to get the full outer shell that they're aiming for. And this means that they form ions with the same charges.
4. Electrostatic attraction holds positive and negative ions together - it's very strong. When atoms are held together like this, it's called ionic bonding.
1. The formula of sodium chloride is NaCl. It just tells you tha sodium chloride is made up of Na+ and Cl- ions (in a 1:1 ratio).
2. You can use 'dot-and-cross' diagrams to show how ionic bonding works in sodium chloride.
3. Magnesium oxide, Mgo is another good example.
The positive harges in the compound balance the negative charges exactly - so the total overall charge is zero. This is a dead handy way of checking the formula.
- In NaCl, the single positive charge on the Na+ ion balances the single negative charge on the Cl- ion.
- In magnesium chloride, MgCl2, the 2+ charge on the Mg2+ ion balances the two individual - charges on the two Cl- ions.
Giant Ionic Lattice Structure
1. Ionic crystals are giant lattices of ions. A lattice is just a regular structure.
2. The structure's called 'giant' because it's made up of the same basic unit repeated over and over again.
3. In sodium chloride, the Na+ and Cl- ions are packed together. The sodium chloride lattice is cube shaped - different ionic compounds have different shaped structres, but they're all still giant lattices.
Ionic Bonding Properties
1. Ionic compounds conduct electricity when they're molten or dissolved - but not when they're solid.
The ions in a liquid are free to move (and they carry a charge). In a solid, they're fixed in position by the strong ionic bonds.
2. Ionic compounds have high melting points
The giant lattices are held by strong electrostatic forces. It takes loads of energy to overcome these forces, so melting points are very high (801 degrees celsius for sodium chloride).
3. Ionic compounds tend to dissolve in water
Water molecules are polar - part of the molecules has a small negative charge and the other bits have small positive charges. The water molecules pulls the ions away from the lattice and cause it to dissolve.