AS-Level The Periodic Table
The melting points and boiling points of a substance are linked to the type of structure the substance has and how much energy is required to break apart that structure.
Simple molecular structures are held together with weak intermolecular forces that require low amounts of energy to break – they have low melting and boiling points.
Giant structures (ionic and covalent) are held together by strong atomic bonding that requires high amounts of energy to break – they have high melting and boiling points.
Across a period in the periodic table, melting points of elements increase from groups 1 to 4 (giant structures), then decrease from groups 5 to 8 (simple molecular substances).
Melting and Boiling Points (periodic trends)
The melting and boiling points of an element are linked to the type of structure that an element exists in.
For example, hydrogen (H ) has a very low melting point because it’s a simple molecule. In a solid state, hydrogen molecules are only held together with very weak temporary induced dipole-dipole interactions that require low amounts of energy to break.
Carbon (in the form of diamond) has a very high melting point because it’s a giant covalent structure. Each carbon atom is covalently bonded to another four carbon atoms. In order to melt such a structure, these strong covalent bonds need to be broken. Covalent bonds require a lot of energy to break, so carbon has a high melting point.
As you go across a period, melting points increase from group 1 to group 4, then
decrease from group 5 and stay relatively low to group 8.
This tells us that in groups 1 to 4 the elements exist in giant structures, and from group 5 to 8 they exist as simple molecules.
In giant structures, there are strong interactions between atoms that require high amounts of energy to break (high melting and boiling points).
In simple molecules, there are only weak temporary induced dipole-dipole forces between molecules that require little energy to break (low melting and boiling points).