What is a covalent bond?
3 marks · NormalEthene (CH) is an organic molecule. Explain, using electron arrangement, how you can use a dot-and-cross diagram to confirm that each atom in ethene has a full outer shell.
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In a dot-and-cross diagram for ethene, each carbon atom shares two electrons with the other carbon (double bond) and one electron each with two hydrogens. This sharing gives each carbon 8 electrons in its outer shell (4 bonds). Each hydrogen atom shares one electron with carbon, giving it 2 electrons in its outer shell.
Also accept: Alternative wording as long as each atom's outer shell is justified by bond counting.
Common mistakes: Not referring to number of electrons in shells.; Forgetting that double bonds count as two pairs.
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Marking points
M1 · 1 mark · Mentions two carbons form a double bond (share two pairs of electrons) and each carbon also bonds to two hydrogens.
M2 · 1 mark · States each carbon ends up with 8 electrons in its outer shell through sharing.
A1 · 1 mark · States each hydrogen has 2 electrons in its outer shell after bonding.
Worked solution
Each carbon atom in ethene shares two electrons with the other carbon atom (double bond) and one electron each with two hydrogen atoms (M1).
This gives each carbon atom four pairs (eight electrons) in its outer shell, filling the shell (M2).
Each hydrogen atom shares one electron with carbon, so has two electrons in its outer shell (A1).