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?
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
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.
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!
How to work out a covalent compound formula
Selected valencies of elements
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
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