3. The covalent bonding in the ammonia molecule
Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A level US grade 9-12 level Revision Notes
Covalent bonding diagram for AMMONIA covalent molecule, molecular formula NH3
* metals \ non-metals (zig-zag line)
Three atoms of hydrogen (1) combine with one atom of nitrogen (2.5) to form the molecule of the compound ammonia NH3
Each hydrogen atom is one electron short of a helium structure (full shell) and nitrogen is three electrons short of a full outer shell (of 8), so three hydrogen atoms share their electrons with the five outer electrons of nitrogen, so all four atoms effectively have full outer shells.
three of and one combine to form so that the hydrogen atoms are electronically like helium and the nitrogen atom becomes like neon (only the outer shell of nitrogen's electrons are shown).
Electronically, hydrogen (1) becomes like helium (2) and nitrogen (2.5) becomes like neon (2.8), so the hydrogen and nitrogen atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons.
(Lewis diagram of ammonia) simplified 'dot and cross' electronic diagram for the covalently bonded ammonia molecule
The ammonia molecule is held together by the strong N–H nitrogen–hydrogen single covalent bonds by sharing electrons.
The molecule can be shown as (displayed formula) with three nitrogen – hydrogen single covalent bonds (AS note: called a trigonal pyramid shape, the H–N–H bond angle is 107o. This displayed formula does indicate the shape of the ammonia molecule as well as how the three N-H single covalent bonds are arranged, but no relative size of atoms or electronic detail of covalent bond formation by electron sharing. It does indicate the pyramidal shape of the molecule (A level comment).
The double dots represent a pair of electrons not involved in the covalent bonding in ammonia. PH3 will be similar since phosphorus (2.8.5) is in the same Group 5 as nitrogen. Valency of nitrogen or phosphorus is 3 here.
Above on the right two of the full 'dot and cross' electronic Lewis diagram for the covalent bonding in the ammonia molecule.
Covalent bond molecule diagrams come in a variety of forms e.g. for ammonia, as well as those above ...
dot and cross electronic diagram, 3D ball and stick model, 3D space-filling model and 2D displayed formula
Its worth making the following comments on the different representations of simple covalent molecules of >2 atoms
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