Substances can be sorted into four types by their structure and bonding. Ionic compounds are a metal combined with a non-metal and form a giant lattice of oppositely charged ions. Simple molecular substances are made of small molecules, with strong covalent bonds inside each molecule but only weak forces between molecules. Giant covalent substances are a huge network of atoms joined by covalent bonds, such as diamond. Metallic elements are a giant lattice of positive ions in a sea of delocalised electrons.
The bonding decides the properties. Ionic, giant covalent and metallic substances all have high melting and boiling points, because many strong bonds must be broken. Simple molecular substances have low melting and boiling points, because only the weak forces between molecules are overcome.
The conduction of electricity needs charged particles that are free to move. Metals conduct as solids because of their delocalised electrons. Ionic compounds conduct only when molten or dissolved in water, because then the ions can move. Simple molecular substances do not conduct. Most ionic compounds dissolve in water, but metals and giant covalent substances do not, and most simple molecular substances are insoluble or only slightly soluble.