Answers · Covalent molecules

Covalent molecules — final check key

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

1. VK1 · Final check

Model answer. (a) Carbon has 4 outer electrons and needs 4; each chlorine has 7 and needs 1. Carbon shares one pair with each of four chlorine atoms: four covalent bonds, each a dot and a cross in the overlap. Carbon uses all 4 of its electrons in the bonds, so it has no lone pairs; each chlorine keeps 6 crosses as three lone pairs. Carbon counts 4 × 2 = 8 in its second shell, 2,8 like neon; each chlorine counts 2 + 6 = 8 in its third shell, 2,8,8 like argon. (b) Carbon needs 4 electrons and gets 2 from its two C–H bonds, so it shares 2 pairs with oxygen: a double bond, C=O, which also gives oxygen the 2 it needs. Oxygen keeps 6 − 2 = 4 electrons as two lone pairs; carbon uses all 4 of its electrons in bonds, so it has no lone pairs. Carbon counts 2 + 2 + 4 = 8 and oxygen 4 + 4 = 8.

A full answer includes

(a) I wrote that carbon needs 4 electrons, so it shares 4 pairs.
(a) I described one shared pair with each of the four chlorine atoms.
(a) I put one dot and one cross in each overlap.
(a) I put no lone pairs on carbon.
(a) I put three lone pairs of crosses on each chlorine atom.
(a) I wrote neon’s configuration, 2,8, for carbon.
(a) I wrote argon’s configuration, 2,8,8, for each chlorine atom.
(a) I did not give the chlorine atoms neon’s configuration.
(b) I wrote a double bond between carbon and oxygen.
(b) I wrote 2 lone pairs on oxygen.
(b) I wrote 0 lone pairs on carbon.

Also accept

(a) Four C–Cl shared pairs, no lone pair on C, three on each Cl; neon for C, argon for Cl. (b) A double bond; 2 lone pairs on O, 0 on C.

Watch for

“Every atom in CCl₄ reaches neon’s configuration, 2,8.” — Carbon’s outer shell is its second shell, so it reaches 2,8, like neon. Chlorine’s outer shell is its third, so with 8 there it is 2,8,8, like argon.
“Carbon and oxygen share one pair in methanal, as in methanol.” — In methanal carbon has only two hydrogen atoms. One pair with oxygen would leave carbon at 6, so they share two pairs.

2. VK2 · Final check

Model answer. (a) Carbon dioxide is a simple molecular substance: separate CO₂ molecules, each held together by strong covalent bonds. Turning to a gas only separates the molecules, which attract each other weakly, so little energy is needed and it happens at −78 °C. The molecules have no overall charge and their electrons are not free to move, so there are no charged particles to carry a current. (b) Sulfur dioxide has the stronger intermolecular forces. It needs a higher temperature, −10 °C rather than −78 °C, so more energy is needed to overcome the forces between its molecules. Neither change breaks covalent bonds.

A full answer includes

(a) I wrote that carbon dioxide is made of separate CO₂ molecules.
(a) I wrote that turning to a gas only separates the molecules.
(a) I wrote that this needs little energy.
(a) I wrote that so it happens at a low temperature, −78 °C.
(a) I wrote that there are no charged particles free to move.
(a) I did not write that the covalent bonds break.
(b) I chose sulfur dioxide.
(b) I wrote that its higher boiling point means more energy is needed to separate its molecules.
(b) I did not write that sulfur dioxide’s covalent bonds are stronger.

Also accept

(a) Separate molecules, little energy to separate; no mobile charged particles. (b) Sulfur dioxide: a higher boiling point, so more energy is needed.

Watch for

“The covalent bonds break when the substance melts or boils.” — Melting and boiling only separate the molecules. Every covalent bond inside them stays.
“Sulfur dioxide boils at a higher temperature because its covalent bonds are stronger.” — Boiling does not break covalent bonds. The higher temperature shows stronger forces between molecules.