Module 1.4 Bonding

The diagram shows a model of a chemical structure.

Which substance could this structure represent?

• Diamond

• Potassium

• Chlorine

• Argon

Answer explanation

The structure described, with cations lined up and electrons freely around them, represents Potassium. This model suggests a metallic structure where positive metal ions (cations) are surrounded by a sea of delocalized electrons, which is characteristic of metallic bonding.

Diamond is a covalent network solid with a different structure, Chlorine is a molecular substance with covalent bonds between atoms, and Argon is a noble gas with a monatomic and non-metallic structure.

The melting point of magnesium oxide is 2852^oC. When strongly heated this compound melts before it decomposes.

What is the main reason magnesium oxide has such a high melting point?

• Strong covalent bonds between atoms

• Strong ionic bonds between ions

• Strong metallic bonds between ions

• Very strong intramolecular bonds

Answer explanation

The main reason magnesium oxide has such a high melting point is strong ionic bonds between ions. Magnesium oxide is an ionic compound composed of magnesium cations (Mg²⁺) and oxide anions (O²⁻). The strong electrostatic attraction between these oppositely charged ions requires a significant amount of energy to overcome, resulting in a high melting point. The other options are not applicable because magnesium oxide does not involve covalent or metallic bonding in its structure, and the term "intramolecular bonds" typically refers to bonds within molecules, not ionic or metallic structures.

The Lewis electron dot diagram of boron trifluoride (BF₃) is shown.

Which of the following is correct about this molecule?

• Non-polar, planar shape

• Non-polar, tetrahedral shape

• Polar, planar shape

• Polar, pyramidal shape

Answer explanation

The correct description of boron trifluoride (BF₃) is non-polar, planar shape. BF₃ has a trigonal planar shape due to the three bonds around the central boron atom, which results in a flat, planar geometry. Because the molecule has symmetrical distribution of charge with all three fluorine atoms equally spaced around the boron atom, the dipole moments cancel out, making the molecule non-polar. The other options are incorrect because BF₃ is not tetrahedral or pyramidal, and it is non-polar rather than polar.

Which of the following is the correct subshell notation and Lewis dot diagram for a halogen atom?

• 
• 
• 
• 

Answer explanation

For chlorine (Cl), which is a halogen, the electron configuration is 1s² 2s² 2p⁶ 3s² 3p⁵. This configuration shows that chlorine has 7 valence electrons, which should be represented in the Lewis dot diagram as 7 dots around the element symbol Cl.

The other options are incorrect because:

• F (drawn with 7 electrons) 1s² 2s² 2p⁵: The electron configuration is correct for fluorine, but it should have 7 electrons, not 8.

• Br (drawn with 8 electrons) 1s² 2s² 2p⁶ 3s² 3p⁶: The configuration is correct for bromine, but it should have 7 valence electrons, not 8.

• S (drawn with 6 electrons) 1s² 2s² 2p⁶ 3s² 3p⁴: The configuration is correct for sulfur, but sulfur is not a halogen; it should not be in this list.

The properties of four solids (P, Q, R and S) are listed.

Which is most likely to have a covalent molecular shape?

• P

• Q

• R

• S

Answer explanation

The solid most likely to have a covalent molecular shape is R. Covalent molecular substances generally have low melting points, do not conduct electricity in solid or liquid forms, and often break into powders when crushed. Solid R, with a melting point of 90°C and no conductivity in either state, fits these characteristics. In contrast, solid P has a high melting point and shows some conductivity, suggesting it is more likely to be metallic or ionic. Solid Q has an extremely high melting point and zero conductivity, indicating it might be a covalent network solid rather than a molecular solid. Solid S, with a moderate melting point and low conductivity, is less characteristic of purely covalent molecular substances.

The structural formula for the compound hydrogen chloride may be represented by:

H–Cl

Which of the following shows the correct molecular shape, polarity and intermolecular

force associated with a molecule of this compound?

• Linear shape, Polar, Dipole-dipole forces

• Planar shape, Polar, Hydrogen bonding

• Linear shape, Polar, Dispersion forces

• Bent shape, Non-polar, Hydrogen bonding

Answer explanation

The correct description for the compound hydrogen chloride (HCl) is linear shape, polar, dipole-dipole forces. The HCl molecule is linear because it consists of only two atoms, hydrogen and chlorine, which are connected by a single bond. The molecule is polar due to the significant difference in electronegativity between hydrogen and chlorine, which creates a dipole moment. The primary intermolecular force in HCl is dipole-dipole interactions, as it does not form hydrogen bonds or rely on dispersion forces as its main type of intermolecular attraction.

A student reads the following paragraph in a chemistry journal:

"The compound exists naturally as white crystals which are very soluble in water. Its melting point is 95^oC and it does not conduct an electric current when in either a solid or a liquid state."
What structure would the student expect the compound to have?

• Metallic

• Ionic network

• Covalent network

• Covalent molecular

Answer explanation

Based on the description, the student would expect the compound to have a covalent molecular structure. The compound is described as having a low melting point (95°C), being very soluble in water, and not conducting electricity in either a solid or liquid state. These characteristics are typical of covalent molecular compounds, which often have low melting points, are soluble in water (depending on polarity), and do not conduct electricity because they lack free-moving charged particles. In contrast, ionic network and covalent network structures usually have high melting points and can conduct electricity when dissolved or melted, while metallic structures are conductive in both solid and liquid forms.

A chemical substance is described as follows:

It is a liquid at room temperature and has a relatively low melting and boiling point.

It does not conduct electricity as either a solid or a liquid.

What structure would you expect this substance to have?

• Ionic network

• Covalent network

• Covalent molecular

• Metallic