3b. How to draw electronic bonding diagrams of small covalent molecules (Lewis dot and cross) and how to deduce a formula from valencies

Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A level US grade 9-12 level Revision Notes

How do we draw electron diagrams of molecules?

dot and cross diagram of the chlorine molecule  (c) doc b  Cl-Cl

The diagrams above illustrate two of the many 'dot and cross' styles of representing the electronic structure of molecules.

The dots and crosses represent electrons, typically x from one atom and o from the other. (but not always!)

Two chlorine atoms combine to form the diatomic chlorine molecule (Cl2).

The atomic number of chlorine is 17, therefore the atom has 17 protons holding 17 electrons, arranged in three shells, with a 2.8.7 electron configuration.

Chlorine needs to share one electron with another atoms to give itself a stable outer shell of 8 electrons.

Therefore two chlorine atoms share a pair of electrons, one from each atom, to form the single covalent bond.

The left-hand diagram shows all the inner and outer shells of electrons and the electron pair sharing indicated in a 'Venn diagram' overlap sharing style (as used in maths to indicate a shared grouping - and I think this is the best style, but others are accepted in exams - but take care).

The  right-hand diagram only shows the electrons and the pair of bonding electrons on the intersection of the two outer shells.

There are lots of dot and cross electron diagrams including simplified Lewis diagrams of covalent bonding in molecules

Lewis diagrams are quite minimalist, all they show is a duplet of electrons associated with hydrogen and for simple molecules, the complete octet of outer shell electrons for the other atoms.

Note: Limitations of dot and cross electronic diagrams of covalent element or compound molecules

It is important to appreciate that dot and cross Lewis diagrams for covalent molecules do not show the structure of the compound in terms of ...

(i) The 3D arrangement of the atoms in covalent molecules (e.g. H2O compound)

(ii) the relative size of molecules in a covalent molecule


There are lots of dot and cross diagrams i.e. Lewis diagrams of covalent bonding in molecules

The simplest molecules are formed from two atoms and examples of their formation are shown below.

The electrons are shown as dots and crosses to indicate which atom the electrons come from, though all electrons are the same!

The diagrams may only show the outer electron arrangements for atoms that use two or more electron shells.

The electron structures are given in parentheses ().

I'll now describe lots of examples of simple covalent molecules, starting with the most 'simplest' of molecules of two atoms (diatomic) and finishing with molecules with up to 8 atoms!

I've illustrated each molecule describe lots of different styles of diagram you should be familiar with.

How to work out a covalent compound formula

Selected valencies of elements

The valency of an element is the combing power of its atoms.

Note that some elements can have more than one (valency):

Hydrogen  H (1)

Chlorine Cl and other halogens (often 1 in simple molecular compounds)

Oxygen O (2 in most compounds)

Sulphur S (2, 4 and 6)

Boron B and aluminium Al (3 in all compounds)

Nitrogen N (3, 4 and 5)

Carbon C and silicon Si (4 in nearly all compounds)

Phosphorus P (usually 3 or 5)

To work out a covalent compound formula by combining 'A' with 'B' the rule is

number of atom 'A' x valency of atom 'A' = number of atom 'B' x valency of atom 'B'

Three examples are shown below

'A' (valency) 'B' (valency) deduced formula of A + B
1 of carbon C (4) balances 4 of hydrogen H (1) 1 x 4 = 4 x 1 = CH4 
1 of nitrogen (3)  balances 3 of chlorine Cl (1) 1 x 3 = 3 x 1 = NCl3 
1 of carbon C (4) balances 2 of oxygen O (2) 1 x 4 = 2 x 2 = CO2 
(c) doc b The diagram on the left illustrates the three covalent examples above for

methane CH4

nitrogen trichloride NCl3

carbon dioxide CO2








What next?

Recommend next: Electron configuration, elements in the periodic table forming covalent bonds (GCSE level)


Sub-index for Part 3. Covalent Bonding: small molecules & properties


Index for ALL chemical bonding and structure notes


Perhaps of interest?

Key to styles of bonding diagrams, comparison of diagrams and models


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