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GCSE / IGCSE Chemistry – Bonding and Structure

Ionic, covalent and metallic bonding, structures and properties.

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12 questions  ·  44 marks  ·  answers included  ·  GCSE/IGCSE Chemistry (Edexcel/AQA/Cambridge style)
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This worksheet covers:

  • This worksheet covers ionic
  • Covalent and metallic bonding
  • How structure explains the physical properties of substances

A calculator may be used for relative formula mass calculations; use $A_r$ values H = 1, C = 12, N = 14, O = 16, Na = 23, Mg = 24, Al = 27, Cl = 35.5, Ca = 40. Allow about 40 minutes.

Questions

Section A – Types of bonding (skills)

1

(a) Describe, in terms of electrons, what happens when an ionic bond forms.(b) Describe, in terms of electrons, what happens when a covalent bond forms.

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(a) Ionic bonding: one or more electrons are transferred from a metal atom to a non-metal atom, forming a positive ion and a negative ion; the oppositely charged ions are then held together by strong electrostatic attraction.(b) Covalent bonding: two non-metal atoms each share one or more pairs of electrons, with the shared pair(s) attracted to the nuclei of both atoms, holding them together.
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2

Sodium (group 1) reacts with chlorine (group 7) to form sodium chloride.(a) State the charge on the sodium ion formed and the charge on the chloride ion formed.(b) Describe how the sodium atom becomes a sodium ion.

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(a) Sodium ion: $\text{Na}^{+}$; chloride ion: $\text{Cl}^{-}$(b) A sodium atom loses one electron from its outer shell (transferred to a chlorine atom), leaving it with a full outer shell and an overall charge of $+1$.
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3

Magnesium ions have charge $2+$ and chloride ions have charge $1-$. Aluminium ions have charge $3+$ and oxide ions have charge $2-$.(a) Give the formula of magnesium chloride, and calculate its relative formula mass, $M_r$.(b) Give the formula of aluminium oxide, and calculate its $M_r$.

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(a) $\text{MgCl}_2$; $M_r = 24 + 2(35.5) = 95$(b) $\text{Al}_2\text{O}_3$; $M_r = 2(27) + 3(16) = 102$
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4

State three physical properties typical of metals, and for each one give a brief reason based on metallic structure and bonding.

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Any three, e.g.: (1) Good electrical conductors – the delocalised electrons are free to move through the structure and carry charge. (2) Malleable/ductile – the layers of positive ions can slide over one another without breaking the metallic bonds, since the delocalised electrons still hold the structure together. (3) High melting/boiling points – strong electrostatic attraction between the positive metal ions and the sea of delocalised electrons requires a lot of energy to overcome.
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Section B – Structure and properties (exam-style)

5

Carbon dioxide, $\text{CO}_2$, is a simple molecular substance in which each carbon atom forms two double covalent bonds with oxygen atoms.(a) Calculate the $M_r$ of $\text{CO}_2$.(b) Explain why carbon dioxide has a low melting point and does not conduct electricity, in terms of its structure and bonding.

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(a) $M_r = 12 + 2(16) = 44$(b) $\text{CO}_2$ consists of small molecules held together by weak intermolecular forces; only a small amount of energy is needed to overcome these weak forces, giving a low melting point. There are no free ions or delocalised electrons to carry charge, so it does not conduct electricity.
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6

The melting point of sodium chloride is $801\,^{\circ}\text{C}$, while the melting point of magnesium oxide is $2852\,^{\circ}\text{C}$.Explain, in terms of ionic charge and structure, why magnesium oxide has a much higher melting point than sodium chloride.

Show answer
Magnesium oxide is made of $\text{Mg}^{2+}$ and $\text{O}^{2-}$ ions, which have greater charge than the $\text{Na}^{+}$ and $\text{Cl}^{-}$ ions in sodium chloride. The electrostatic forces of attraction between the more highly charged ions in $\text{MgO}$ are much stronger, so much more energy is needed to overcome them and separate the ions, giving a much higher melting point.
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7

Diamond and graphite are both giant covalent structures made only of carbon atoms.(a) Describe the structure of diamond and explain why it is very hard and does not conduct electricity.(b) Describe the structure of graphite and explain why it conducts electricity and is soft/slippery.

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(a) In diamond, each carbon atom forms four strong covalent bonds to four other carbon atoms in a rigid 3-D lattice. This requires a large amount of energy to break many strong covalent bonds, making diamond very hard; all outer electrons are used in bonding, so there are no free electrons or ions to conduct electricity.(b) In graphite, each carbon atom forms three covalent bonds to three other carbons, forming layers of hexagonal rings; the fourth outer electron from each atom is delocalised and free to move between the layers, allowing graphite to conduct electricity. The layers are held together only by weak forces between layers, so they can slide over each other, making graphite soft and slippery.
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8

Ammonia, $\text{NH}_3$, and glucose, $\text{C}_6\text{H}_{12}\text{O}_6$, are both covalent compounds.Calculate the relative formula mass of each.

Show answer
$\text{NH}_3$: $M_r = 14 + 3(1) = 17$; $\text{C}_6\text{H}_{12}\text{O}_6$: $M_r = 6(12) + 12(1) + 6(16) = 180$
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Section C – Extended and comparison (challenge)

9

The table below gives information about four substances, P, Q, R and S.P: high melting point, conducts electricity only when molten or dissolved in water.Q: low melting point, does not conduct electricity in any state.R: very high melting point, conducts electricity as a solid.S: very high melting point, does not conduct electricity in any state, extremely hard.For each substance, name the type of structure and bonding present (ionic, simple molecular covalent, giant covalent, or metallic), giving a reason for each choice.

Show answer
P: ionic – high melting point from strong electrostatic forces between ions, but ions are only free to move (and so conduct) when molten or in solution, not in the fixed solid lattice.Q: simple molecular (covalent) – weak intermolecular forces between molecules give a low melting point, and there are no ions or free electrons to conduct.R: metallic – high melting point from strong metallic bonding, and delocalised electrons are free to move and conduct electricity even as a solid.S: giant covalent – very high melting point from many strong covalent bonds throughout the structure (e.g. diamond or silicon dioxide); all outer electrons are held in fixed covalent bonds so none are free to conduct.
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10

Explain why ionic compounds such as sodium chloride conduct electricity when molten or dissolved in water, but not when solid.

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In the solid, the ions are held in fixed positions in the lattice by strong electrostatic forces and cannot move to carry charge. When molten or dissolved in water, the ions are free to move throughout the liquid, so they can carry electric charge and the substance conducts electricity.
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11

Poly(ethene) is a long-chain molecule made of thousands of carbon and hydrogen atoms joined by strong covalent bonds.Explain why poly(ethene) has a much lower melting point than a giant covalent structure such as diamond, even though both contain strong covalent bonds.

Show answer
In poly(ethene), the strong covalent bonds only join atoms along each individual chain; the separate chains themselves are held to each other only by weak intermolecular forces. Melting a polymer only requires overcoming these weak forces between chains, not the strong covalent bonds within them, so the melting point is much lower than diamond, where every atom is joined to its neighbours by strong covalent bonds throughout the entire giant structure.
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12

Sodium chloride is a hard but brittle solid, while sodium metal is soft and malleable. Using ideas about structure and bonding, explain this difference in behaviour when each solid is struck with a hammer.

Show answer
In sodium chloride, the ions are arranged in an alternating regular lattice of oppositely charged ions. A blow shifts one layer of ions relative to the next, bringing ions of the same charge next to each other; the strong repulsion between like-charged ions forces the lattice to shatter, so the crystal is brittle. In sodium metal, the structure is a lattice of positive ions surrounded by a ‘sea’ of delocalised electrons; when a layer of ions is shifted by a blow, the delocalised electrons simply move with it and continue to hold the structure together, so the metal can change shape without breaking – it is malleable.
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