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)

Pd metals Part of the modern Periodic Table

Pd = period, Gp = group

metals => non–metals
Gp1 Gp2 Gp3 Gp4 Gp5 Gp6 Gp7 Gp0
1 1H  Note that hydrogen does not readily fit into any group but is a non-metal 2He
2 3Li 4Be atomic number Chemical Symbol eg 4Be 5B 6C 7N 8O 9F 10Ne
3 11Na 12Mg 13Al 14Si 15P 16S 17Cl 18Ar
4 19K 20Ca 21Sc 22Ti 23V 24Cr 25Mn 26Fe 27Co 28Ni 29Cu 30Zn 31Ga 32Ge 33As 34Se 35Br 36Kr
5 37Rb 38Sr 39Y 40Zr 41Nb 42Mo 43Tc 44Ru 45Rh 46Pd 47Ag 48Cd 49In 50Sn 51Sb 52Te 53I 54Xe
6 55Cs 56Ba Transition Metals 81Tl 82Pb 83Bi 84Po 85At 86Rn
The covalent molecule methane from carbon combining with hydrogen

 

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 (c) doc b and one of (c) doc b combine to form (c) doc b 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.

(c) doc bNote that the inner shell of carbon's electrons are not shown above, only the outer shell of carbon's electrons are involved in the covalent bonding.

The molecule can be shown as (c) doc b (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.

 

dot and cross diagram of the methane molecule 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


Comments

Melting point of methane -183 oC

Boiling point of methane -162 oC

Methane is a colourless flammable gas at room temperature, it has a strong 'hydrocarbon odour'.


What next?

Recommend next: The covalent bonding in the oxygen molecule

Explaining the properties of small covalently bonded molecules

 

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

 

Index for ALL chemical bonding and structure notes

 

Perhaps of interest?

The basic chemistry of alkanes like methane

 

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