Simple Molecules and Covalent Bonds Exam Questions

• A shared pair of electrons [1]
• between two atoms / between two non-metal atoms [1]

Examiner note: “Sharing electrons” alone is worth 1 mark. You must specify it is a pair of electrons that is shared. The bond forms because both nuclei are attracted to the shared pair.

(a) O atom with 2 bonding pairs (one shared with each H) and 2 lone pairs [1]

(b) C atom with 4 bonding pairs (one shared with each H), no lone pairs on C [1]

(c) H shares one pair with Cl; Cl has 3 lone pairs [1]

Examiner note: Use dots for one atom’s electrons and crosses for the other. Each bonding pair must contain one dot and one cross in the overlap region. Do not forget lone pairs on the central atom — water has 2 lone pairs on oxygen, HCl has 3 lone pairs on chlorine.

• Methane is a simple molecular substance with weak intermolecular forces between molecules [1]
• Little energy is needed to overcome these weak forces [1]
• NaCl has strong electrostatic forces between oppositely charged ions in a giant ionic lattice, requiring much more energy to overcome [1]

Examiner note: A critical distinction: the covalent bonds within methane are strong, but the intermolecular forces between methane molecules are weak. It is the intermolecular forces that are broken during boiling, not the covalent bonds. Writing “covalent bonds are weak” is incorrect and will lose marks.

• Simple molecular substances have no free ions [1]
• and no free/delocalised electrons to carry charge [1]

Examiner note: All electrons in I2 are either in covalent bonds or lone pairs — none are free to move. There are no ions because iodine is made of covalently bonded molecules, not ions.

• Each oxygen shares two pairs of electrons with carbon [1]
• Carbon has 4 bonding pairs (2 double bonds), no lone pairs [1]
• Each oxygen has 2 lone pairs [1]
• The bonds are double covalent bonds (two shared pairs per bond) [1]

Examiner note: CO2 has two C=O double bonds, not four single bonds. Each double bond consists of two shared electron pairs (4 electrons total per bond). Carbon’s 4 outer electrons are all used in bonding.

• Both are simple molecular substances with weak intermolecular forces between molecules [1]
• Water has hydrogen bonding between its molecules, which is a stronger type of intermolecular force [1]
• H2S has only weak van der Waals forces between its molecules, so less energy is needed to overcome them [1]

Examiner note: This is a Supplement question testing knowledge of different types of intermolecular forces. Hydrogen bonding occurs in water because of the O-H bond (highly electronegative oxygen). H2S has S-H bonds which do not form hydrogen bonds because sulfur is less electronegative.

(a) Each nitrogen atom shares 3 electron pairs with the other nitrogen atom, forming a triple bond [1]. Each nitrogen has 1 lone pair [1].

(b) N2 is a simple molecule with weak intermolecular forces between molecules, so little energy is needed to separate them [1]

Examiner note: The triple bond in N2 is very strong, but the question asks about boiling point, which depends on intermolecular forces (weak), not the covalent bond (strong). Do not confuse the two.

• NaCl has a high melting point because of strong electrostatic forces between ions in a giant ionic lattice; CCl4 has a low melting point because it is a simple molecular substance with weak intermolecular forces [1]
• NaCl is soluble in water because the polar water molecules can separate the ions; CCl4 is insoluble in water because its molecules are non-polar [1]
• NaCl conducts electricity when molten or in solution because ions are free to move; CCl4 does not conduct in any state because it has no ions or free electrons [1]
• NaCl does not conduct when solid because ions are in fixed positions [1]

Examiner note: Comparison questions require you to address both substances for each property. Writing only about NaCl or only about CCl4 limits your marks. Use parallel structure: “NaCl has… because… whereas CCl4 has… because…”