Covalent Bonding
6 marks · NormalThis question is about using dot-and-cross diagrams to interpret structures and properties.
Draw a dot-and-cross diagram for a molecule of nitrogen, N2, showing only the outer shell electrons.
Explain why nitrogen gas contains molecules, not a giant covalent structure like diamond and graphite.
Explain why simple covalent molecules like N2 have much lower melting points than giant covalent structures such as diamond.
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) Two N atoms joined by three pairs of shared electrons (triple bond); each N atom has 2 lone electrons (outer shell only). · must show triple bond (3 shared pairs), 2 lone pairs per N
(b) A nitrogen atom forms a triple covalent bond with one other N atom, making discrete N2 molecules. It cannot form a structure where each N is bonded to more than one atom, so giant structures are not formed. · must show N atom forms 3 bonds only, discrete molecules, no extended network
(c) In simple covalent molecules, weak intermolecular forces must be overcome to melt them, needing little energy. In giant covalent structures, strong covalent bonds throughout the whole structure must be broken, requiring much more energy. · must show covalent bonds vs intermolecular forces, energy required, difference simple/giant
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Marking points
D1 (a) · 1 mark · Shows two N atoms joined by three shared pairs of electrons.
D2 (a) · 1 mark · Shows each N atom has two lone electrons.
E1 (b) · 1 mark · Explains N atom can only form three bonds and so only makes N2 molecules.
E2 (b) · 1 mark · States giant covalent structures require many bonds to other atoms.
E3 (c) · 1 mark · Identifies difference between breaking weak intermolecular forces (N2) and strong covalent bonds (diamond).
E4 (c) · 1 mark · States it takes less energy to melt N2 than diamond because of these differences.
Worked solution
a) (Student's drawing: two nitrogen atoms joined by three pairs of shared electrons, each N with two lone electrons.)
b) Each nitrogen atom in N2 forms a triple bond with just one other atom, so only N2 molecules exist. It does not bond with other N atoms to form an extended structure.
c) N2 only has weak intermolecular forces between molecules, needing little energy to break. In diamond, each carbon is joined to four others by strong covalent bonds in a giant structure, needing much more energy to break, so the melting point is much higher.