Representations of covalent structures

A covalent structure can be represented in different ways. Each representation emphasises some information and leaves other details out. For example, a dot-and-cross diagram shows electrons involved in bonding, while a ball-and-stick model makes the arrangement of atoms easier to see.

Dot-and-cross diagrams

A covalent bond is a shared pair of electrons. A dot-and-cross diagram shows the outer-shell electrons of the atoms: dots and crosses distinguish the electrons contributed by different atoms. A shared pair is placed between the two bonded atoms. Outer electrons that are not shared are shown as lone pairs around their atom.

For example, this diagram represents water, $$\ce{H2O}$$. Oxygen contributes the dots and each hydrogen contributes a cross:

Each hydrogen shares one pair with oxygen, so there are two covalent bonds. Oxygen also has two lone pairs. In this representation, each hydrogen has two electrons in its outer shell, while oxygen has eight.

To draw a dot-and-cross diagram, place the atom symbols in their bonding arrangement, then add the outer electrons. Put each shared pair between its two atoms and place any remaining outer electrons as lone pairs. Hydrogen has one outer electron; chlorine has seven; oxygen has six; nitrogen has five; and carbon has four.

The following arrangements give each hydrogen two outer-shell electrons and each of the other atoms eight. A double bond contains two shared pairs; a triple bond contains three shared pairs.

• Hydrogen, $$\ce{H2}$$: two hydrogen atoms share one pair. Neither atom has a lone pair.
• Chlorine, $$\ce{Cl2}$$: two chlorine atoms share one pair. Each chlorine has three lone pairs.
• Oxygen, $$\ce{O2}$$: two oxygen atoms share two pairs, forming a double bond. Each oxygen has two lone pairs.
• Nitrogen, $$\ce{N2}$$: two nitrogen atoms share three pairs, forming a triple bond. Each nitrogen has one lone pair.
• Hydrogen chloride, $$\ce{HCl}$$: hydrogen and chlorine share one pair. Chlorine has three lone pairs.
• Water, $$\ce{H2O}$$: oxygen shares one pair with each of two hydrogen atoms. Oxygen has two lone pairs.
• Ammonia, $$\ce{NH3}$$: nitrogen shares one pair with each of three hydrogen atoms. Nitrogen has one lone pair.
• Methane, $$\ce{CH4}$$: carbon shares one pair with each of four hydrogen atoms. Carbon has no lone pairs.

For instance, nitrogen has a lone pair at each end and three shared pairs between the atoms. Using dots for one nitrogen’s electrons and crosses for the other’s gives:

Lines, models and spatial diagrams

A single covalent bond can be represented by a line instead of drawing its shared pair of electrons. Two lines can represent a double bond and three lines a triple bond. Line representations show which atoms are connected, but they do not normally show all the electrons.

Lines are used in structural diagrams of small molecules, in diagrams of repeating units of polymers and in sections of giant covalent structures. A section of a giant structure shows the bonding pattern locally; it does not show the whole structure.

Ball-and-stick models use balls for atoms and sticks for bonds. They, along with two-dimensional and three-dimensional diagrams, help show how atoms connect and are arranged. A two-dimensional drawing on a page may not convey the full three-dimensional shape. Models are simplified: they may not show actual relative atomic sizes or distances, all electrons, or the full extent of a giant structure.

Deducing a molecular formula

To find a molecular formula from a model or diagram, identify the element represented by each atom, then count the atoms of each element in one molecule. Use a colour key or labels if the model provides them. Write the element symbols with the counts as subscripts; a count of one is not written.

For example, a model with one nitrogen atom joined to three hydrogen atoms has the molecular formula $$\ce{NH3}$$. Count atoms, not bonds or sticks: each stick shows a connection between atoms, not an extra atom.

Types of covalent substance

Covalent bonds join atoms together, but the resulting structures are not all the same. A covalent substance may consist of small separate molecules, very large molecules or a giant covalent network.

Small molecular substances

A small molecular substance consists of separate, discrete molecules. Covalent bonds join the atoms within each molecule. Its molecular formula shows the number of atoms of each element in one molecule.

Common small molecular substances include water, $$\ce{H2O}$$; oxygen, $$\ce{O2}$$; carbon dioxide, $$\ce{CO2}$$; and methane, $$\ce{CH4}$$. For example, one molecule of carbon dioxide contains one carbon atom and two oxygen atoms.

Very large molecules: polymers

Some covalent molecules contain many atoms joined into long chains. Polymers are made of long covalent chains containing repeating units: a particular arrangement of atoms occurs again and again along each chain. Each chain is a very large molecule, distinct from other chains.

Giant covalent structures

In a giant covalent structure, covalent bonds extend throughout a continuous network of atoms rather than forming separate molecules. Diamond and silicon dioxide are examples.

A formula for a giant covalent substance does not represent one discrete molecule. For example, $$\ce{SiO2}$$ gives the ratio of silicon atoms to oxygen atoms in the network: one silicon atom for every two oxygen atoms.

Covalent bonding in simple molecules

Atoms can bond by sharing outer-shell electrons. A covalent bond is a shared pair of electrons between two atoms. The shared pair counts towards the outer electronic configuration of both bonded atoms.

Sharing gives each atom a stable outer electronic configuration like that of a noble gas. Hydrogen attains two electrons in its outer shell; carbon, nitrogen, oxygen and chlorine each attain eight.

Reading dot-and-cross diagrams

A dot-and-cross diagram shows outer-shell electrons. Dots and crosses distinguish electrons supplied by different atoms; they do not represent different kinds of electron. A dot and a cross between two atoms form a shared pair. Electrons shown around an atom but not between bonded atoms are non-bonding electrons.

Before bonding, hydrogen has one outer-shell electron, carbon has four, nitrogen has five, oxygen has six and chlorine has seven. In the diagrams below, all hydrogen electrons are shown as crosses when several hydrogen atoms bond to the same central atom.

One shared pair: $$\ce{H2}$$, $$\ce{Cl2}$$ and $$\ce{HCl}$$

In hydrogen, $$\ce{H2}$$, each hydrogen atom supplies one electron. The shared pair gives each hydrogen two outer-shell electrons:

In chlorine, $$\ce{Cl2}$$, each chlorine atom supplies one electron to the shared pair. Each also has three non-bonding pairs. Counting the shared pair gives each chlorine eight outer-shell electrons:

In hydrogen chloride, $$\ce{HCl}$$, hydrogen and chlorine share one pair. Hydrogen counts two electrons in the pair; chlorine counts the pair along with its six non-bonding electrons, giving it eight:

Several shared pairs: $$\ce{H2O}$$, $$\ce{NH3}$$ and $$\ce{CH4}$$

In water, $$\ce{H2O}$$, oxygen shares one pair with each of two hydrogen atoms. Oxygen also has two non-bonding pairs. It counts eight outer-shell electrons in total, while each hydrogen counts two:

In ammonia, $$\ce{NH3}$$, nitrogen shares one pair with each of three hydrogen atoms. Nitrogen also has one non-bonding pair. The three shared pairs and the non-bonding pair give nitrogen eight outer-shell electrons; each hydrogen counts two:

In methane, $$\ce{CH4}$$, carbon shares one pair with each of four hydrogen atoms. Carbon counts the four shared pairs as eight outer-shell electrons, and each hydrogen counts two:

Chemical formulae

A chemical formula uses element symbols to show which elements are present in a substance. Small numbers called subscripts show the relative numbers of their atoms.

For example, the formula for water is $$\ce{H2O}$$. It contains hydrogen, represented by $$\ce{H}$$, and oxygen, represented by $$\ce{O}$$. The subscript 2 applies to hydrogen. There is no subscript after oxygen, which means one oxygen atom. The ratio of hydrogen atoms to oxygen atoms is therefore 2:1.

Element symbols must be written with the correct capital and lower-case letters. For example, $$\ce{Na}$$ is the symbol for sodium, whereas $$\ce{N}$$ is the symbol for nitrogen.

Molecular formulae

Some substances consist of molecules. A molecular formula shows the type and number of atoms in one molecule. One water molecule, $$\ce{H2O}$$, contains two hydrogen atoms and one oxygen atom. One carbon dioxide molecule, $$\ce{CO2}$$, contains one carbon atom and two oxygen atoms.

Not every compound consists of separate molecules. For example, the formula $$\ce{NaCl}$$ shows that sodium chloride contains sodium and chlorine atoms in a 1:1 ratio; it does not describe one sodium chloride molecule.

Deducing a formula from a model

To find the formula of a simple compound from a model or diagram, identify each type of atom using the labels or key. Count the atoms of each type, then write their element symbols with subscripts for the numbers present. Do not write a subscript when the number is one.

For example, if a model shows one carbon atom joined to two oxygen atoms, write the symbols $$\ce{C}$$ and $$\ce{O}$$. Carbon needs no subscript, while oxygen needs a subscript 2. The formula is $$\ce{CO2}$$.

Familiar formulae

Some elements also exist as molecules. Hydrogen, oxygen and nitrogen have the formulae $$\ce{H2}$$, $$\ce{O2}$$ and $$\ce{N2}$$ respectively. Each of these molecules contains two atoms of the same element.

Familiar compounds include water, $$\ce{H2O}$$; carbon dioxide, $$\ce{CO2}$$; methane, $$\ce{CH4}$$; and sodium chloride, $$\ce{NaCl}$$.

Recognising giant covalent structures

A giant covalent structure consists of an extended network of atoms joined by covalent bonds. In a diagram, look for a repeating arrangement of bonded atoms that continues across the structure, rather than separate groups of atoms forming individual molecules. A diagram usually shows only a small part of the extended structure.

Giant covalent structures in diagrams

• Diamond: Carbon atoms are joined by covalent bonds in a three-dimensional network extending throughout the structure.
• Graphite: Carbon atoms are joined by covalent bonds in extended sheets. The sheets form layers; the covalent bonds shown in the network are within each sheet.
• Silicon dioxide: Silicon and oxygen atoms are linked by covalent bonds throughout a three-dimensional network. Its formula, $$\ce{SiO2}$$, gives the ratio of the atoms; it does not represent a separate small molecule in the structure.

The arrangements look different, but all three contain extended covalent frameworks, not collections of small, discrete molecules.

Polymer repeat-unit notation

A polymer is a long covalent chain built from repeating sections. Instead of drawing the whole chain, a repeat-unit diagram shows one section inside brackets. Bond lines at the brackets show where the section connects to neighbouring units in the chain.

The symbol $$n$$ outside the brackets is the number of times the unit repeats. It is a large number. Each single bond line represents a strong covalent bond, formed by a shared pair of electrons.

The repeat unit of poly(ethene)

The repeat unit of poly(ethene) is written as:

The unit contains two carbon atoms joined by a single bond. Each carbon atom is also bonded to two hydrogen atoms, shown by $$\ce{CH2}$$. The bond lines at the ends show where the two carbon atoms connect to further units.

The bracketed unit is therefore not one small, separate molecule: it represents a section repeated many times in a continuous polymer chain.