Aids to chemistry: Structure and bonding - Practice questions
Francine Taylor Campbell, Contributor

Jamaica's Centre, Nadine Bryan (left), asked to do duties at goal attack, grabs a ball ahead of Australia's wing defender, Selina Gilsenan, during the first game of a three-Test series at the National Indoor Sports Centre on Friday, October 4. Jamaica lost the game 47-38. - Carlington Wilmot Photo

AGAIN WE will examine some questions on the topic.

Question 1

A. (i) Explain the difference in boiling points between hydrogen chloride
(-85.1 0C) and magnesium chloride (1420 ºC).

(ii) What other physical properties will each exhibit?

B. Substance P has high melting and boiling points, is a good conductor of electricity when molten, but is a poor conductor when solid. Suggest a possible structure of P.

C. Element Y forms a fluoride that has the formula YF3. Neither Y nor YF3 conducts electricity.

(i)Suggest the number of valence electrons that Y has. What type of structure would solid YF3 have?

(ii) What type of bonding would be seen in YF3. Suggest two other physical properties of YF3.

Answers

1A.i. Hydrogen chloride is a gas which has weak forces between its molecules resulting in a low boiling point (-85.1ºC), while magnesium chloride exhibits ionic bonding, which results in strong forces of attraction between the ions and hence a very high boiling point (1420 ºC).

ii. Hydrogen chloride has a low melting point and will not conduct electricity, while magnesium chloride has a high melting point and is able to conduct electricity, but only when molten or in solution.

NOTE: The ions in the crystal lattice of ionic must break free from the attractive forces between the ions and this can only happen when the ions are dissolved in solution or when molten.

B. Substance P is an ionic compound.

C.i. Element Y has 5 valence (outer shell) electrons and YF3 would have a simple molecular structure.

COMMENT: Metals are known to conduct electricity so Y could not be a metal (such as aluminium).

ii. YF3 would show covalent bonding since both Y and F are non-metals and would have low melting and boiling points.

Question 2

The table below gives information about some substances.

A. Explain in terms of its structure, how copper conducts electricity.

B. Which one of these substances could be methane, CH4?

C. Which one of the substances could be sodium chloride, NaCl?

D. Magnesium oxide has a very high melting point. Give one use of magnesium oxide which depends on this property.

Answers

A. Copper is a metal (metallic structure) and conducts electricity as its sea of delocalised electrons is able to move and carry an electric charge.

B. Methane could be B which shows the properties of a covalently bonded molecule with a simple molecular structure, hence low melting and boiling points, poor electrical conductivity and insolubility in water. E is soluble in water, hence it could not be methane.

C. Sodium chloride could be A as it has high melting and boiling points and is very soluble in water. Note that sodium chloride would not conduct electricity in its solid state.

D. The high melting point of magnesium oxide makes it useful as refractory material in furnaces (hard to melt) .

* Francine Taylor-Campbell is an independent contributor.