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3l. The covalent bonding in the ethene molecule
Doc Brown's Chemistry: Chemical Bonding and structure GCSE level, IGCSE, O, IB, AS, A level US grade 9-12 level Revision Notes
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Covalent bonding diagram for ETHENE covalent molecule, molecular formula C2H4
* metals \ non-metals (zig-zag line)
Two atoms of carbon (2.4) combine with four atoms of hydrogen (1) to form ethene C2H4 (only the outer shell of carbon's electrons are shown).
On the right, the Lewis diagram of ethene - a simplified 'dot and cross' electronic diagram for the covalently bonded ethene molecule.
Electronically, hydrogen (1) becomes like helium (2) and carbon (2.4) becomes like neon (2.8), so ALL the hydrogen and carbon atoms effectively have full outer shells in forming the covalent bonds when the atoms share their outer electrons.
The molecule can be shown as (displayed formula) with one carbon = carbon double bond and four carbon – hydrogen single covalent bonds (it has a planar shape, its completely flat!, the H–C=C and H–C–H bond angles are 120o). The valency of carbon is still 4.
The ethane molecule is held together by the four strong C–H carbon–hydrogen single covalent bonds and one C=C carbon–carbon covalent double bond.
The above diagrams are other styles of 'dot and x' electronic diagrams for ethene.
ball and stick model of ethene
space filling model of ethene
Extra advanced level notes on the structure and covalent bonding of ethene
From the quantum level rules, carbon's electron configuration is 1s22s22p2
In terms of the separate orbitals you can express it as 1s2, 2s2, 2px1, 2py1, 2pz0
This can be further expressed as an electron box diagram: 1s2s2p with two unpaired electrons.
However, as you should know by now, carbon usually forms four bonds (valency of 4) rather than two.
This is because it is energetically favourable to promote one electron from the 2s orbital into the third empty 2pz orbital, the energy required for this 'promotion' is far less than that released when the carbon atom forms four bonds rather than two..
This gives a theoretical electron configuration of 1s2, 2s1, 2px1, 2py1, 2pz1 or 1s2s2p
This gives four unpaired electrons, all of which can pair up with an electron from another atom to form four covalent bonds, but they may be of two varieties if a double or triple bond is involved in the carbon based molecule - read on ...
Reminders (if needed): The diagrams below show the bonding situation in alkenes, that conveniently involve two of the most important types of covalent bond between atoms, including carbon.
Sigma and pi covalent bonds in alkenes
In carbon based compounds a single bond (sigma bond, σ bond) is formed by the overlap of two orbitals which can be either an s orbital and p orbital, (illustrated above).
In the case of alkenes, the pi bond (π bond) is formed by the overlap of two 2p orbitals of the carbon atoms, but, due to repulsion with the bonded pairs of the sigma bond electrons, they cannot form another sigma bond molecular orbital along the same central axis.
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