On Relating Bonding to Atomic Structure

element X element Y element Z
number of protons 16 8 30
number of neutrons 13 7 34
number of electrons 16 8 30
number of electrons in outer shell 6 6 2
Use the information to identify elements X, Y, and Z.

They are sulfur, oxygen, and zinc respectively.

Suggest and explain the trend in the melting points of elements X, Y, and Z.

Y has the lowest melting point. X has a slightly higher melting point. Z has the highest, and significantly more so.

X and Y have simple molecular structures while Z has a giant metallic structure. Less energy is required to overcome the weak intermolecular forces of attraction in X and Y than the strong electrostatic forces of attraction between the positive zinc ions and sea of delocalised electrons in Z.

As X is polyatomic (S8) and has a higher relative molecular mass than diatomic Y (O2), X has a stronger intermolecular forces of attraction than Y that requires more energy to overcome.

On Metallic Bonding

For which does 1 mole of the metal have the most electrons in its ‘sea of electrons’?

A aluminium
B barium
C caesium
D strontium

ANSWER: A

For those who haven’t learned mole, you can think of 1 mole of a fixed amount. The question is essentially asking, for the same amount of metal, which has the most electrons?

The biggest loser is… Aluminium! It is in Group III and has three outer electrons, which are all delocalised in metallic bonding such that it can gain the noble gas electronic configuration.

Meanwhile, caesium is in Group I while barium and strontium are in Group II. Thus, only one and two outer electrons are delocalised respectively.

On Physical Properties

Which property is common to both diamond and graphite?

A Each atom in the structure forms four covalent bonds
B They act as lubricant when finely divided.
C They are electrical insulators.
D They react with oxygen to produce carbon dioxide.

ANSWER: D

Diamond and graphite are allotropes of carbon: different physical forms of the same element. Therefore, many of their physical properties are different.

However, they have the same chemical composition: pure carbon. At 763 °C, they react with oxygen in the air to form carbon dioxide. Poof, into thin air. Oof, diamond doesn’t last forever.

Hold up! How can diamond then melt at a high temperature of 4027 °C when it reacts away at a lower temperature? Indeed, you can only melt a diamond under non-oxidising condition. Otherwise, it will react (a chemical change) instead of melt (a physical change).

Both calcium chloride and potassium chloride are ionic compounds. Which statements about these compounds are correct?

1. Potassium chloride has a higher melting point than calcium chloride because the potassium
2. In the calcium chloride lattice, the ratio of calcium ions to chloride ions is half that of the ratio of potassium ions to chloride ions in the potassium chloride lattice
3. Molten calcium chloride has a greater electrical conductivity than molten potassium chloride because calcium contributes more electrons to the delocalised ‘sea of electrons’ than does potassium

A 1 and 2
B 1 and 3
C 2 only
D 3 only

ANSWER: C

Statement 1 is false. As Ca2+ has a greater charge than K+, calcium chloride has a greater ionic bond strength and hence a higher melting point.

Statement 2 is true. From the chemical formulae of CaCl2 and KCl, the ratio of calcium ions to chloride ions is 1:2, which is half of the ratio of potassium ions to chloride ions, which is 1:1.

Statement 3 is tricky but wrong. Indeed, molten ionic compounds can conduct electricity. But they do because they have free-moving ions, not electrons. Only metals have delocalised sea of electrons.

On Dot and Cross Diagrams

Draw a dot and cross diagram to show the arrangement of outer shell electrons in the ions in sodium oxide, Na2O.
All hypochlorite bleaches contain the chlorate(I) oxyanion. The oxyanion contains a covalent bond between the oxygen and chlorine atoms. Both atoms in the ion have a stable octet electron arrangement because the ion also has an overall negative charge.

[Cl—O]–
chlorate(I) oxyanion

Draw a ‘dot-and-cross’ diagram to show the arrangement of outer shell electrons a chlorate(I) oxyanion.

Firstly, the structural formula given in the question, we see that there is only a single bond between chlorine and oxygen atoms. This means that there is only one pair of shared electrons in the overlapping region.

Secondly, upon drawing the shared electrons, we fill in the remaining 6 and 5 non-bonding valence electrons for chlorine and oxygen atoms.

Lastly and most crucially, we realise that there is an extra electron gained from the cation, as reflected in the negative charge. Given that “both atoms in the ion have a stable octet electron arrangement”, we assign the extra electron to oxygen.