3i. The covalent bonding in the methane 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 METHANE covalent molecule, molecular formula CH4
* metals \ non-metals (zig-zag line)
Four atoms of hydrogen (1) combine with one atom of carbon (2.4) to form the molecule of the compound methane CH4
Each hydrogen atom is one electron short of a helium structure (full shell) and carbon is four electrons short of a full outer shell (of 8), so four hydrogen atoms share their electrons with the four outer electrons of carbon, so all five atoms effectively have full outer shells.
four of and one of combine to form so that the hydrogen atoms are electronically like helium and the carbon atom becomes like neon (only the outer shell of carbon's bonding electrons are shown).
Electronically, hydrogen (1) becomes like helium (2) and carbon (2.4) becomes like neon (2.8), so the hydrogen and carbon atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons.
(Lewis diagram of methane) simplified 'dot and cross' electronic diagram for the covalently bonded methane molecule
The methane molecule is held together by the four strong C–H carbon–hydrogen covalent bonds by sharing electrons.
The molecule can be shown as (displayed formula) with four carbon – hydrogen single covalent bonds (A level note: its called a tetrahedral shape, the H–C–H bond angle is 109o).
SiH4 will be similar because silicon (2.8.4) is in the same group as carbon. This displayed formula does indicate the shape of the methane molecule as well as how the four single C-H covalent bonds are arranged, but no relative size of atoms or electronic detail of bond formation by electron sharing.
All the bonds in the above examples are single covalent bonds. Below are three examples 7–9, where there is a double bond in the molecule, in order that the atoms have stable Noble Gas outer electron arrangements around each atom. Carbon and silicon have a valency of 4.
More complex examples can be worked out e.g. involving C, H and O. In each case link in the atoms so that there are 2 around a H (electronically like He), or 8 around the C or O (electronically like Ne).
On the left are full 'dot and cross' electronic Lewis diagram for the covalent bonding in the methane molecule.
The electronic dot & cross Lewis diagram give a 2D view of the molecule in terms of electrons, but little idea on the shape of the molecule which might be anything but flat!
Below, the stick and ball diagram of methane gives a much better impression of the shape of the molecule and the spatial arrangement of the four single bonds and five atoms.
ball and stick model of methane
space-filling model of methane
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