Covalent Bonding and Structures

A molecule is a separate group of atoms joined by covalent bonds. Many familiar substances consist of small molecules: hydrogen, H₂; chlorine, Cl₂; oxygen, O₂; nitrogen, N₂; hydrogen chloride, HCl; water, H₂O; ammonia, NH₃; methane, CH₄; and carbon dioxide, CO₂.

The formula tells you the atoms in one molecule. H₂O means two hydrogen atoms and one oxygen atom. NH₃ means one nitrogen atom and three hydrogen atoms. Where there is no subscript, there is one atom of that element.

Recognising familiar formulae helps you identify small molecular substances, but a formula alone does not always reveal the structure. Carbon dioxide, CO₂, consists of small molecules; silicon dioxide, SiO₂, forms a giant covalent network.

Drawing dot and cross diagrams

A dot and cross diagram shows outer-shell electrons. Dots and crosses distinguish electrons contributed by different atoms; they do not represent different kinds of electron. A shared pair normally contains one electron from each bonded atom. Electrons not used in bonding remain as lone pairs.

To draw a molecule, write the atom symbols and arrange their outer shells so that bonded atoms overlap. Put the shared pairs in the overlaps, then add the remaining outer electrons around their own atoms. Finally, check that each hydrogen has two electrons and each other atom has eight, counting shared electrons for both atoms.

The following patterns cover all eight required molecules. A single bond contains one shared pair, a double bond contains two, and a triple bond contains three.

For water, oxygen supplies one electron to each of its two bonds. Four of its original six outer electrons remain unshared, forming two lone pairs. For oxygen gas, each oxygen supplies two electrons to the double bond, leaving four unshared electrons on each atom.

Different models show different information

In a structural formula, each line represents a single covalent bond: one shared pair of electrons. Hydrogen can be shown as H–H and water as H–O–H. Ammonia has three N–H lines around a central nitrogen; methane has four C–H lines around a central carbon. Two lines represent the double bond in O₂, and three represent the triple bond in N₂.

A ball and stick model uses balls for atoms and sticks for bonds. Use the labels or colour key to identify the elements. It helps show the three-dimensional arrangement: ammonia has three hydrogen atoms arranged around a nitrogen atom in a pyramid, rather than all four atoms lying in one plane.

These representations describe the same bonding but emphasise different features. Dot and cross diagrams show shared and unshared electrons; structural formulae show connections clearly; ball and stick models show spatial arrangement.

Polymers are very large molecules

Not all molecules are small. A polymer contains very large molecules made from a repeating group of atoms joined together many times by covalent bonds.

Poly(ethene) has the repeating unit –CH₂–CH₂–. Its representation places this unit inside brackets, with a subscript $n$ outside. Here, $n$ means a large number of repetitions; it is not an atom or an element symbol.

Each carbon in this repeating unit has single bonds to two hydrogen atoms and two neighbouring carbon atoms. The carbon–carbon bonds extend through the brackets to show that the chain continues into the next repeating unit. When drawing the unit, include those continuing bonds rather than leaving the carbon atoms at the ends unconnected.

Giant covalent structures are continuous networks

Some substances form a giant covalent structure rather than separate molecules. Strong covalent bonds join atoms into a continuous network extending throughout the structure.

Diamond is a giant covalent structure made only of carbon. Each carbon atom is bonded to four other carbon atoms in a three-dimensional network.

Silicon dioxide is also giant covalent. Each silicon atom is bonded to four oxygen atoms, and each oxygen atom links two silicon atoms. SiO₂ gives the overall ratio of silicon to oxygen atoms; it does not describe a separate three-atom molecule.

To represent part of either structure, draw labelled atoms joined by single lines. The drawing shows only a small fragment: bonds at its boundary continue into the rest of the network. A polymer chain is a very large molecule, whereas diamond and silicon dioxide are continuous networks rather than collections of separate chains.

Deducing a molecular formula from a model

First identify each element using the atom symbols or the model's key. Then count every atom of each element in one complete molecule and write those counts as subscripts. Leave out the subscript 1.

For example, ethane has two carbon atoms joined by a single bond, with three hydrogen atoms attached to each carbon. Counting gives two carbon atoms and six hydrogen atoms, so its molecular formula is C₂H₆. The three-dimensional position of an atom does not change the count: a hydrogen behind another atom still belongs to the molecule.

This method needs a complete molecule. A fragment of a giant network or a polymer repeating unit does not show all the atoms of a complete molecule.

What the models leave out

All models simplify the real structure. Dot and cross diagrams show electron sharing but not the true three-dimensional arrangement or relative atom sizes. Dots and crosses also do not show electrons moving.

Two-dimensional structural formulae show which atoms are connected, but their flat layout does not usually show the real molecular shape. Lines are symbols for bonds, not physical rods.

Three-dimensional ball and stick models make shape clearer, but atoms are not hard balls separated by the large empty gaps suggested by the sticks. These models usually omit electrons and are not accurate scale drawings.

For polymers and giant structures, a further limitation is size: a practical drawing or model shows only a short chain or a small section of a network. Brackets, continuing bonds and explanations are needed to show how that section relates to the much larger structure.

Structure and representation

• Small molecules: separate groups with fixed atom counts.
• Polymers: very large molecules; brackets enclose a repeating unit, $n$ indicates many repeats, and bonds pass through the brackets.
• Giant covalent: continuous networks, including diamond and SiO₂.
• One line represents one single bond.
• Molecular formula: identify elements and count atoms in one complete molecule.

Model limitations

Dot and cross: no true 3D shape or relative sizes. Flat diagrams: connections, not true spatial arrangement. Ball and stick: unrealistic gaps, no electrons. Giant-structure and polymer models: only a small section is shown.