Simple molecular structure

A simple molecular structure is a structure made of separate small molecules held to one another by weak intermolecular forces, which gives it low melting and boiling points. Malay: struktur molekul mudah · Chinese: 简单分子结构.

Many covalent substances are built this way. Within each molecule the atoms are joined by strong covalent bonds, but the molecules themselves are only weakly attracted to their neighbours. When the substance is heated, it is these weak forces between molecules that break first, so only a little energy is needed to melt or boil it. This is why simple molecular substances tend to have low melting and boiling points and are often liquids or gases at room temperature. They also do not conduct electricity, because the molecules are neutral and carry no free-moving charges.

Example. Water, carbon dioxide, iodine and oxygen all have simple molecular structures. Solid iodine, for instance, is made of I₂ molecules held in place by weak forces, so it melts easily and even sublimes. Carbon dioxide is a gas at room temperature for the same reason.

Confusion to avoid. Do not confuse a simple molecular structure with a giant ionic lattice or a giant covalent structure. All three may contain strong bonds, but only the simple molecular structure is held together overall by weak forces, which is why its melting point is low while the giant structures melt at very high temperatures. Remember also that “simple molecular” describes the whole structure, not the individual molecule, which may itself be quite large.

The simple molecular structure is the covalent counterpart to the giant ionic lattice in this chapter, and contrasting their properties is one of the most common comparison questions in SPM Chemistry.