Atomic Structure, Bonding & the Periodic Table

10 methods · 29 questions

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Decide which of the four structure types a substance has, from the elements it contains or from its physical fingerprint, then the property being asked follows at once. Ionic (metal with non-metal): high melting point, conducts molten or dissolved but not solid. Simple molecular (non-metal with non-metal): low melting and boiling points, no conduction unless it ionises in water. Giant covalent (carbon, silicon and their oxides): very high melting point, no conduction (graphite excepted). Metallic: conducts as a solid.

Trigger: a table of melting or boiling points and conductivities, 'which structure / which compound', or a bonding-versus-property claim.

Instances:
(i) low melting point with poor solid and liquid conduction but good aqueous conduction points to a molecular substance that ionises in water ( $HCl$ , not $CC l_{4}$ );
(ii) boiling a simple molecular substance ( $PC l_{3}$ , $PC l_{5}$ ) overcomes intermolecular forces, never the covalent bonds inside the molecules;
(iii) a giant covalent lattice ( $Si O_{2}$ , silicon) melts only by breaking covalent bonds, giving a very high melting point.

Linked questions (3)

NSAA 2016 Section 1, Part C (Chemistry questions) · Q39

In which, if any, of the following reactions are covalent bonds both broken and formed?

1 burning sodium in oxygen

2 electrolysis of aqueous sodium chloride

3 displacement of iron from iron oxide by heating with aluminium powder

Anone of them
B1 only
C2 only
D3 only
E1 and 2
F1 and 3
G2 and 3
H1, 2 and 3

Worked solution

Answer:C

Covalent bonds are broken when a molecular substance reacts and formed when a molecular product is made.

Reaction 1: sodium is a metal and sodium oxide is ionic. The

O = O

bond in oxygen is broken, but no covalent bond is formed. Not both.

Reaction 2: during the electrolysis of aqueous sodium chloride, water molecules are reduced at the cathode (

2 H_{2} O + 2 e^{-} \to H_{2} + 2 O H^{-}

), so

O - H

bonds are broken and an

H - H

bond is formed, while chloride ions are oxidised at the anode to form the

Cl - Cl

bond of chlorine. Covalent bonds are both broken and formed.

Reaction 3: iron oxide is ionic, aluminium and iron are metals and aluminium oxide is ionic. No covalent bond is involved at all.

Only reaction 2 qualifies.

The answer is C.

NSAA 2018 Section 1, Part C (Chemistry questions) · Q46

The non-metallic element phosphorus forms two stable chlorides:

PC l_{3}

(boiling point

76^{\circ} C

) and

PC l_{5}

(boiling point

161^{\circ} C

).

Which of the following statements explain(s) the difference in boiling points?

1 There are more covalent bonds in

PC l_{5}

so more energy is required to break them.

2 The forces between the molecules in liquid

PC l_{5}

are stronger.

3 The covalent bonds in

PC l_{3}

are weaker so less energy is required to break them.

Anone of them
B1 only
C2 only
D3 only
E1 and 2 only
F1 and 3 only
G2 and 3 only
H1, 2 and 3

Worked solution

Answer:C

Both chlorides are simple molecular substances. When a molecular liquid boils, the molecules separate from one another; the covalent bonds inside each molecule are not broken. So neither the number of covalent bonds (statement 1) nor their strength (statement 3) has anything to do with the boiling point, and both statements give the wrong reason.

What must be overcome is the intermolecular attraction.

PC l_{5}

is a larger molecule with more electrons than

PC l_{3}

, so the forces between its molecules are stronger and more energy is needed to separate them, giving the higher boiling point. Statement 2 is the correct explanation.

Only statement 2 is correct.

The answer is C.

NSAA 2019 Section 1, Part C (Chemistry questions) · Q38

In which of the following solids does the bonding consist of single covalent bonds only?

1 graphite

2

Si O_{2}

A l_{2} O_{3}

Anone of them
B1 only
C2 only
D3 only
E1 and 2 only
F1 and 3 only
G2 and 3 only
H1, 2 and 3

Worked solution

Answer:C

A solid whose bonding consists of single covalent bonds only must be a covalent structure with no ionic bonding and no delocalised electrons.

Graphite is a giant covalent structure, but each carbon atom is bonded to only three others and the fourth outer electron of every atom is delocalised between the layers. This delocalised bonding, which is what lets graphite conduct electricity, is not single covalent bonding, so statement 1 fails.

Silicon dioxide,

Si O_{2}

, is a giant covalent lattice in which every silicon atom is joined by single covalent bonds to four oxygen atoms and every oxygen atom to two silicon atoms. There is no other type of bonding, so statement 2 holds.

Aluminium oxide,

A l_{2} O_{3}

, is a compound of a metal with a non-metal: it is an ionic solid made of

A l^{3 +}

and

O^{2 -}

ions, not a covalent solid, so statement 3 fails.

The answer is C.