As chemistry

Atoms loose or gain electrons

Atoms loose electrons

Atoms gain electrons

Electrostatic force of attraction between Oppositely charged ions . This force is strong and requires a lot energy to be overcome so ionic bonds are broken . This means that ionic compounds have high. Melting and biking points

A metal and non metal except ammonium Salts

A Lithium ion is a smaller ion than a potassium ion and has a higher charge density . This means there is a greater force of electrostatic attraction between positive and negative ions meaning more energy is required to break the force NB both have same charge so charge cant influence anything

Attraction between particles

Arrangement of particles

Lattice

Strong electrostatic force of attraction between positively charged metal ions and a sea of delocalised electrons . The delocalised electrons come from the highest energy level

They have a sea of delocalised electrons which are free to move in a given direction if a force is applied

The sea of delocalised electrons allow layers of atoms to slide over each other allowing metals to be bent and shaped 

the electrostatic forces of attraction between the positive metal ions and the delocalised electrons aren't disrupted if a force is applied

Electrostatic force of attraction between positive metal ion and delocalised electron

Lithium is a smaller ion than sodium therefor there are stronger forces of electrostatic attraction between the metal ions and delocalised electrons than there are in sodium so lithium has the greatest melting point

Aluminium has a higher charge and more delocalised electrons. Therefor there are stronger forces of electrostatic attraction between the metal ions and delocalised electrons in aluminium than there are in sodium , so aluminium has a higher melting point