Answers · Covalent molecules

Covalent molecules — return questions key

Check meaning, not exact wording. These are teaching points, not official Cambridge marks.

1. VR1 · Return

Model answer. (a) A covalent bond is a pair of electrons shared between two atoms. Hydrogen and bromine each need only 1 electron. With one shared pair hydrogen counts 2, like helium, and bromine 6 + 2 = 8, so both shells are full, and hydrogen has no second electron to share. (b) In each shared pair one electron comes from each atom, so an atom of X puts 2 of its own electrons into the two pairs. It keeps 4 more in two lone pairs. So X has 2 + 4 = 6 outer electrons: X is oxygen, 2,6, and two shared pairs make a double bond, O=O.

A full answer includes

(a) I wrote: a pair of electrons shared between two atoms.
(a) I wrote that one pair gives hydrogen 2, a full shell like helium’s.
(a) I wrote that one pair gives bromine 8, a full outer shell.
(a) I did not write that hydrogen gives its electron to bromine.
(b) Method: I added 2 of X’s own electrons in the shared pairs and 4 in the lone pairs.
(b) Answer: 6 outer electrons.
(b) I identified X as oxygen.
(b) I named a double bond.
(b) I did not count all 4 shared electrons as X’s own.

Also accept

(a) A pair of electrons shared between two atoms; one pair fills both shells (H 2, Br 8). (b) 6; oxygen; a double bond.

Watch for

“One atom gives its electron to the other, as in ionic bonding.” — Two non-metal atoms both need electrons, so neither gives one away. They share a pair, and no ions form.
“Each atom of X brings all 4 shared electrons, so X has 8 outer electrons.” — Each pair has one electron from each atom. Of the 4 shared electrons, 2 are X’s own: 2 + 4 in lone pairs = 6, oxygen.

2. VR2 · Return

Model answer. (a) Hydrogen chloride is a simple molecular substance: separate HCl molecules, each with one strong covalent bond. Separating the molecules needs little energy, so it boils at −85 °C, below room temperature. With no ions and no free electrons, liquid hydrogen chloride cannot conduct. (b) Above −85 °C. Boiling overcomes the intermolecular forces. Stronger forces between H₂S molecules need more energy to overcome, so a higher temperature is needed: hydrogen sulfide boils at −60 °C.

A full answer includes

(a) I wrote that hydrogen chloride is made of separate HCl molecules.
(a) I wrote that separating the molecules needs little energy.
(a) I wrote that it boils below room temperature, so it is a gas at 20 °C.
(a) I wrote that the molecules have no overall charge.
(a) I wrote that no electrons are free to move.
(a) I did not write that liquid hydrogen chloride conducts because it is a liquid.
(b) I predicted that hydrogen sulfide boils above −85 °C.
(b) I wrote that boiling overcomes the intermolecular forces.
(b) I wrote that stronger forces need more energy, so a higher temperature.
(b) I did not explain it by stronger covalent bonds in H₂S.

Also accept

(a) Separate molecules, little energy to separate them; no mobile charges. (b) Above −85 °C: stronger intermolecular forces need more energy to overcome.

Watch for

“Liquid hydrogen chloride conducts because it is a liquid.” — A liquid conducts only if it has charged particles that can move. Liquid hydrogen chloride is uncharged HCl molecules.
“Stronger forces between the molecules give a lower boiling point.” — Stronger attractions hold the molecules together more, so more energy is needed to separate them: the boiling point is higher.
“Hydrogen sulfide boils higher because its covalent bonds are stronger.” — Boiling does not break covalent bonds. The higher boiling point comes from the stronger forces between the molecules.