Covalent Bonding, Molecular Models and Properties

How sharing electrons holds atoms together

Many substances, including water, oxygen and the hydrocarbons in crude oil, contain molecules. A molecule is a discrete group of atoms joined by covalent bonds. In a simple molecular substance, each molecule is a separate unit: its atoms are bonded together, but it is not covalently joined to every neighbouring molecule.

Covalent bonding usually occurs between non-metal atoms. Instead of transferring electrons, the atoms share them. A single covalent bond contains one shared pair of electrons, usually with one electron contributed by each atom.

Electrons are negatively charged, while atomic nuclei are positively charged. Both nuclei attract the shared electrons. This strong electrostatic attraction holds the bonded atoms together; a bond is not a physical rod between them.

For example, each hydrogen atom has one electron. Two hydrogen atoms share their electrons to form $\mathrm{H_2}$. Each atom now has access to both electrons in the shared pair, giving it a full first shell of two electrons. The electrons are shared, not copied or transferred permanently to one atom.

Building a dot-and-cross diagram

A dot-and-cross diagram shows the outer-shell electrons involved in bonding. Dots represent electrons originally belonging to one atom and crosses represent those from another. All electrons are identical: the symbols simply help us keep track of them.

For the molecules considered here, hydrogen achieves a full shell with two electrons; carbon, nitrogen, oxygen and chlorine achieve eight. Their starting numbers of outer-shell electrons are:

To construct a diagram, first identify the atoms and their outer-shell electrons. Choose dots and crosses, then place shared pairs between bonded atoms, in the overlapping shell regions if shells are drawn. Place the remaining electrons around their original atoms. Finally, count all the electrons available to each atom, including the shared ones.

In water, $\mathrm{H_2O}$, oxygen contributes one electron to a shared pair with each hydrogen. This makes two O–H bonds. Oxygen's other four outer-shell electrons remain as two lone pairs: pairs that are not shared in bonds. Oxygen has access to eight electrons altogether, and each hydrogen has access to two.

In methane, $\mathrm{CH_4}$, carbon shares one of its four outer-shell electrons with each of four hydrogen atoms. There are four single bonds and no lone pairs on the carbon atom. Carbon has access to eight electrons; each hydrogen has two.

Single, double and triple bonds

Atoms can share more than one pair of electrons with the same neighbouring atom. A double bond contains two shared pairs, while a triple bond contains three.

In oxygen, $\mathrm{O_2}$, each atom begins with six outer-shell electrons. Each contributes two electrons to the shared region, making two shared pairs. Each oxygen also keeps two lone pairs. In nitrogen, $\mathrm{N_2}$, each atom contributes three electrons to three shared pairs and keeps one lone pair.

A displayed formula replaces each shared pair with a line:

The unshared electrons still matter even when a displayed formula hides them. For example, chlorine in HCl has three lone pairs, and nitrogen in ammonia has one.

Moving between flat diagrams and three-dimensional molecules

A flat drawing can show which atoms are connected without showing their actual arrangement in space. Methane is often drawn with hydrogen atoms above, below, left and right of carbon. The real molecule is not a flat cross. Its four C–H bonds point towards the corners of a tetrahedron, a three-dimensional shape with four triangular faces.

A ball-and-stick model makes this spatial arrangement easier to see. To represent it on paper, ordinary lines can show bonds lying in the plane of the page, a solid wedge can show a bond pointing towards you, and a dashed wedge can show one pointing away.

When translating from a 3D model into a displayed formula, preserve which atoms are bonded and whether each bond is single, double or triple. When translating back, distinguish this connectivity from the molecule's spatial arrangement: an ordinary flat displayed formula does not, by itself, tell you the 3D shape.

Diagrams also contain numerical information. Counting the atoms in a methane model gives one carbon and four hydrogens, so its molecular formula is $\mathrm{CH_4}$ and it contains five atoms altogether. Its four single bonds contain four shared pairs: eight shared electrons. Conversely, tells you which atoms to include, while the bonding information tells you how to connect them.

For ethane, counting two carbon atoms and six hydrogen atoms in its displayed structure gives $\mathrm{C_2H_6}$. The molecular formula records these numbers, but does not show the individual bonds or the molecule's shape.

Choosing a model and recognising its limits

Every molecular representation is a model: it makes selected features clear while leaving others out.

For methane, a dot-and-cross diagram is useful for explaining how carbon obtains a full outer shell. A ball-and-stick model is better for showing that the molecule is not flat. Neither shows every feature of the real molecule, and neither normally shows the forces between separate molecules.

Why strong bonds do not mean a high boiling point

A sample of a simple molecular substance contains many molecules. There are two different attractions to distinguish: strong covalent bonds within each molecule and weaker intermolecular forces between molecules.

During melting, molecules become able to move past one another. During boiling, they separate much further. These changes require energy to overcome intermolecular attractions, but the covalent bonds within the molecules remain intact. Boiling water produces water vapour containing $\mathrm{H_2O}$ molecules, not separate hydrogen and oxygen atoms.

Because intermolecular forces are relatively weak, simple molecular substances usually have low melting and boiling points. Many are gases or liquids at room temperature, although some are solids. Their state depends on their melting and boiling points, not merely on the presence of covalent bonds.

Among similar hydrocarbons, larger molecules generally have stronger intermolecular forces. More energy is needed to separate them, so their boiling points are generally higher. Octane therefore has a higher boiling point than methane. This connects molecular size with the different boiling ranges of crude-oil fractions.

Simple molecular substances also generally do not conduct electricity. Their molecules are neutral overall, and they lack mobile ions or delocalised electrons to carry charge. Electrons shared in covalent bonds are not free to move through the substance as an electric current.

Bonding and electron counting

• A covalent bond is the attraction between a shared electron pair and both bonded nuclei.
• Single, double and triple bonds contain 1, 2 and 3 shared pairs respectively.
• Hydrogen has a full shell with 2 electrons; the other atoms in these examples have 8.
• Water: two bonds and two lone pairs on oxygen. Methane: four bonds, no lone pairs on carbon.
• Oxygen: a double bond and two lone pairs per atom. Nitrogen: a triple bond and one lone pair per atom.

Reading molecular models

• Count atoms to obtain the molecular formula; each displayed bond line represents one shared pair.