Valency : Structural Formulae

We may represent the combination of atoms by drawing from the symbol of an element as many straight lines as it has valencies:

chemical elements with bonds

and then making up the formulae of compounds so that no free valencies are left over, each pair of associated lines being written as one:

atomic bonds

These lines are called bonds, since they represent, as it were, the manner in which the atoms in a compound are linked together.

Multivalent atoms can link with each other, by utilising one or more bonds on each atom, the remaining bonds being free to attach other atoms:

C2H6, ethane

Such formulae are called structural formulae; they represent the way in which the atoms are united, but not necessarily their actual positions, in the molecules. Thus, the formula for hydrazine shows that two nitrogen atoms are united by a bond, and each nitrogen atom is directly united with two hydrogen atoms, in each case by a bond. Various lines of evidence show that the atoms are held at approximately constant distances and in definite positions (apart from possible rotation of parts of a molecule about a bond) in a molecule. The molecule H-H is linear; the molecule H-O-H is bent so that the two bonds make an angle somewhat greater than 90°; in the ammonia molecule (NH₃) the nitrogen occupies the apex of a flat tetrahedron with the hydrogen at the other three corners; in methane (CH₄) the carbon is at the centre of gravity of a regular tetrahedron, with the hydrogens at the four corners.

Protein crystallography
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Valency
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