3.1.6 Periodic trends in melting and boiling points

Bonding across the periodic table

Across the periodic table the bonding changes from metallic to giant covalent to simple molecular lattice

Melting and boiling points

From period 2 to 3 mp and bp increase from first to the fourth elements in the period but then decrease from the fourth to eighth elements

Metals

Mp and bp increase across the period because metal- metal bonds get stronger because metal positive ions have a greater charge, an increasing number of delocalised electrons and decreasing ionic radius → higher charge density = attracts ions together more strongly

Giant covalent structures

Giant covalent structures have strong covalent bonds linking the atoms together so a lot of energy is needed to break these bonds

Simple molecular structures

Simple molecular structures have strong covalent bonds between atoms but mp and bp depend on London dispersion forces between molecules which are weak and are easily overcome with little energy

More atoms = more London dispersion forces = collectively stronger

Nobel gases are monatomic and have lowest mp and BP and exist as gases = weak London dispersion fora's