Covalent Bonding
5 marks · EasyDraw a dot-and-cross diagram to show the bonding in a molecule of hydrogen chloride (HCl).
State how many covalent bonds are present in a molecule of hydrogen chloride.
Chlorine is a gas at room temperature, but water (HO) is a liquid. Using ideas about relative molecular mass, explain this difference.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Diagram should show H with 1 electron (dot or cross), Cl with 7 electrons (six dots/crosses as lone pairs, one as part of the bond), and the shared pair between H and Cl. · must show Two electrons between H and Cl atoms (one dot, one cross) representing a single covalent bond., Six other electrons (as dots and crosses) arranged as three lone pairs on Cl atom.
(b) There is 1 covalent bond in a molecule of hydrogen chloride.
(c) Chlorine has a lower relative molecular mass than water, so the strength of its intermolecular forces is less than that of water. Therefore, chlorine is a gas at room temperature while water, with a higher molecular mass and stronger forces, is a liquid. · must show Reference to molecular mass, Reference to intermolecular forces
Must show: Correct electron configurations on H and Cl in diagram (2 and 8), Single shared pair between H and Cl
Also accept: Correct diagram with dots and crosses in any reasonable configuration
Common mistakes: Showing more than one shared pair between H and Cl; Not filling out lone pairs on Cl
Show mark scheme and worked solution
Marking points
D1 (a) · 1 mark · Correct dot-and-cross layout: H has 1 electron involved in a single bond to Cl; Cl has one shared electron and 3 lone pairs.
A1 (a) · 1 mark · Both atoms shown with correct numbers of electrons in their outer shells after bonding.
B1 (b) · 1 mark · States there is 1 covalent bond.
M1 (c) · 1 mark · Relates physical state to relative molecular mass / strength of intermolecular forces.
A2 (c) · 1 mark · Draws correct conclusion about chlorine being a gas and water a liquid due to the above.
Worked solution
a) The dot-and-cross diagram for HCl shows:
b) There is 1 covalent bond in a molecule of hydrogen chloride.
c) Chlorine has a lower relative molecular mass and weaker intermolecular forces than water, so chlorine is a gas at room temperature. Water's higher relative molecular mass and hydrogen bonding makes it a liquid.