Simple Molecular Substances

From atoms to molecules

A simple molecular substance consists of separate molecules. Each molecule contains a fixed, relatively small number of atoms joined by covalent bonds. Hydrogen, $\mathrm{H_2}$, water, $\mathrm{H_2O}$, and ammonia, $\mathrm{NH_3}$, are examples.

The atoms are usually non-metals. Rather than transferring electrons and forming ions, they share outer-shell electrons. The nuclei remain unchanged during this process.

How a covalent bond forms

One covalent bond contains a shared pair of electrons. It is not a material stick holding two atoms together. Its nature is a strong electrostatic attraction between the negatively charged shared pair and the positively charged nuclei of the two bonded atoms.

Sharing allows each atom to count the bonding electrons as part of its outer shell. For example, each hydrogen atom has one outer-shell electron. In $\mathrm{H_2}$, the two atoms contribute one electron each to a shared pair, producing one single covalent bond.

Atoms can share more than one pair of electrons. One shared pair gives a single bond, two shared pairs give a double bond and three shared pairs give a triple bond.

The arrangement in simple molecules

The important structural idea is that the atoms form discrete molecules, rather than one continuous lattice extending throughout the substance. Within each molecule, particular atoms are connected by strong covalent bonds.

In water, $\mathrm{H_2O}$, one oxygen atom is bonded to two hydrogen atoms. In ammonia, $\mathrm{NH_3}$, one nitrogen atom is bonded to three hydrogen atoms. Ethane, $\mathrm{C_2H_6}$, contains two carbon atoms bonded to one another, with each carbon also bonded to three hydrogen atoms.

This arrangement contrasts with an ionic compound, which contains a repeating lattice of oppositely charged ions, and with a giant covalent structure, in which covalent bonds extend through a continuous network. A simple molecule is therefore not merely a small piece cut from a giant structure: it is an individual particle with a fixed composition.

Representing the same molecule in different ways

Chemists select models according to the information they need.

• A molecular formula, such as $\mathrm{H_2O}$, gives the number of atoms of each element but not how they are connected.
• A displayed formula shows the atoms and bonds. Each line stands for one shared pair of electrons.
• A dot-and-cross diagram shows and distinguishes which atom they came from. A bonding region contains a shared pair; any unshared outer-shell pairs are shown as lone pairs.

The representations are complementary. A dot-and-cross diagram makes electron sharing clear, while a ball-and-stick model makes spatial arrangement easier to see. Neither is the molecule itself.

Using ball-and-stick models carefully

A molecular modelling kit can be used to build a molecule by selecting the correct numbers of each atom and connecting them according to the bonds. Turning the model helps translate between a three-dimensional arrangement and a two-dimensional drawing.

Every model simplifies reality. A flat diagram may not show the true three-dimensional shape. Ball-and-stick models do not display the electrons, their atoms and bonds are not necessarily to scale, and the sticks can make atoms appear farther apart than they really are. These limitations do not make the model useless; they identify which questions it can and cannot answer.