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GCSE · Chemistry

C2a Chemical Bonding: Ionic, Covalent and Metallic

AQA 8464 · Calculator allowed · about 75 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Dots, Crosses and the Octet Rule: How Chemists Pictured the Bond

Original text written for Revision Library.

At the start of the twentieth century, chemists knew that atoms joined together to form compounds, but nobody could fully explain why. In 1916, the American chemist Gilbert N. Lewis proposed that atoms could bond by sharing pairs of electrons, an idea now called covalent bonding. In the same year, the German scientist Walther Kossel suggested a different explanation for compounds such as salt: atoms could instead transfer electrons completely, becoming charged ions held together by electrostatic attraction. Lewis represented these ideas using simple diagrams showing electrons as dots around each atom's symbol; students today still use a version of this method, the dot-and-cross diagram, to distinguish the electrons contributed by each combining atom. Both scientists noticed the same underlying pattern: atoms tend to react in ways that leave them with a full outer shell of electrons, usually eight, an observation later named the octet rule by the American chemist Irving Langmuir. A third type of bonding, metallic bonding, explains why metals conduct electricity and can be bent into shape without shattering: metal atoms release their outer electrons into a shared 'sea' of electrons that moves freely through a lattice of positive ions. Together, these three models of bonding, ionic, covalent and metallic, explain how almost every solid substance around us is held together at the level of its atoms.

This question checks some key words used throughout this topic.

(a)State what is meant by the term 'ion'.(1)

(b)Name the three types of strong chemical bonding studied in this topic.(3)

(Total for Question 1 is 4 marks)

Sodium reacts with chlorine to form the ionic compound sodium chloride, NaCl.

(a)Complete the electron configurations of a sodium atom and a chlorine atom.(2)

(b)Describe, in terms of electron transfer, how a sodium atom and a chlorine atom form ions when they react.(2)

(c)State the ratio of Na+ ions to Cl- ions in sodium chloride, and name the type of bonding holding them together.(1)

(Total for Question 2 is 5 marks)

Figure 1 shows the outline of a dot-and-cross diagram for sodium chloride, before the electron transfer has been shown.

(a)Complete the dot-and-cross diagram in Figure 1 to show the ionic bonding in one formula unit of sodium chloride. Show the charge on each ion, and show outer-shell electrons only.(3)

(b)State the charge on the sodium ion and the charge on the chloride ion shown in your diagram.(1)

(c)Give one limitation of using a dot-and-cross diagram, like the one you completed in part (a), to represent the ionic bonding in sodium chloride.(1)

(Total for Question 3 is 5 marks)

Ionic compounds form in fixed ratios that balance the total positive and negative charge on the ions.

(a)Magnesium ions, Mg2+, and chloride ions, Cl-, combine to form magnesium chloride. Use the charges on the ions to work out the formula of magnesium chloride. Show your reasoning.(2)

(b)Aluminium ions, Al3+, and oxide ions, O2-, combine to form aluminium oxide. Use the charges on the ions to work out the formula of aluminium oxide. Show your reasoning.(2)

(Total for Question 4 is 4 marks)

Calcium reacts with oxygen to form the ionic compound calcium oxide, CaO.

(a)Complete the electron configurations of a calcium atom and an oxygen atom.(2)

(b)Draw a dot-and-cross diagram for calcium oxide and explain, in terms of electron transfer, how the two ions are formed.(3)

(Total for Question 5 is 5 marks)

Some elements bond by sharing pairs of electrons rather than transferring them completely.

(a)State what is meant by a covalent bond.(1)

(b)Explain why two non-metal atoms share a pair of electrons instead of one atom transferring an electron completely to the other.(2)

(Total for Question 6 is 3 marks)

Hydrogen and chlorine can each form simple covalent molecules.

(a)Draw a dot-and-cross diagram to show the covalent bonding in a molecule of hydrogen, H2. Show outer-shell electrons only.(2)

(b)Draw a dot-and-cross diagram to show the covalent bonding in a molecule of hydrogen chloride, HCl. Show outer-shell electrons only.(2)

(Total for Question 7 is 4 marks)

Figure 2 shows the outline of a dot-and-cross diagram for methane, CH4, before the covalent bonds have been added.

(a)Complete the dot-and-cross diagram in Figure 2 to show the covalent bonding in a molecule of methane. Show outer-shell electrons only.(3)

(b)Explain how the carbon atom achieves a full outer shell in methane.(2)

(c)Give one limitation of the dot-and-cross diagram you completed in part (a) to represent the structure of a methane molecule.(1)

(Total for Question 8 is 6 marks)

Oxygen atoms bond together to form molecules of oxygen gas, O2. An oxygen atom has 6 electrons in its outer shell.

(a)Draw a dot-and-cross diagram to show the covalent bonding in a molecule of oxygen, O2.(2)

(b)Explain why oxygen atoms form a double covalent bond rather than a single covalent bond in O2.(2)

(Total for Question 9 is 4 marks)

Figure 3 shows a simplified representation of the structure of a metal.

(a)Describe the structure of a metal in terms of metallic bonding.(2)

(b)Using Figure 3, identify (i) what the circles marked '+' represent and (ii) what the small dots represent, and state what holds the structure together.(2)

(Total for Question 10 is 4 marks)

When a metal atom forms metallic bonds, it releases its outer-shell electrons to become part of the delocalised sea of electrons.

(a)Using group numbers from the periodic table, state the number of electrons released to become delocalised electrons by one atom of each of the following metals: (i) sodium (Group 1) (ii) magnesium (Group 2) (iii) aluminium (Group 3).(3)

(Total for Question 11 is 3 marks)

Metals are good conductors of electricity, but substances made of small covalent molecules are not.

(a)Explain, in terms of bonding, why metals conduct electricity.(2)

(b)Suggest why a simple covalent substance, such as methane, does not conduct electricity.(1)

(Total for Question 12 is 3 marks)

Required practical: a student electrolysed a solution of copper chloride using inert (graphite) electrodes connected to a d.c. power supply, as shown in Figure 4.

(a)State the type of bonding present in solid copper chloride.(1)

(b)Explain, in terms of structure and bonding, why solid copper chloride does not conduct electricity, but the copper chloride solution used in this practical does conduct electricity.(4)

(c)Predict the product formed at the negative electrode (cathode) during this electrolysis, and explain your answer.(2)

(d)State the product formed at the positive electrode (anode).(1)

(e)Write the half equation for the reaction at the cathode.(2)

(f)Write the half equation for the reaction at the anode.(2)

(g)A second student repeated the electrolysis using a more concentrated copper chloride solution, passing the same current for the same length of time. State and explain the effect this has on the mass of copper deposited at the cathode.(2)

(h)State one safety precaution that should be taken when carrying out this practical.(1)

(Total for Question 13 is 15 marks)

Complete the table below to compare ionic, covalent and metallic bonding. For each bonding type, state (i) the particles that are joined by the bond and (ii) the force that holds them together.

(a)Complete the row for ionic bonding.(2)

(b)Complete the row for covalent bonding.(2)

(c)Complete the row for metallic bonding.(2)

(Total for Question 14 is 6 marks)

Magnesium reacts with chlorine to form magnesium chloride, MgCl2, which contains ions in a 1:2 ratio.

(a)Complete the electron configurations of a magnesium atom and a chlorine atom.(2)

(b)Draw a dot-and-cross diagram to show the ionic bonding in magnesium chloride, showing the correct ratio of ions, and explain how the ions are formed.(3)

(Total for Question 15 is 5 marks)

A student is asked to describe the bonding in methane, CH4. The student writes: 'Methane contains ionic bonds between the carbon atom and each hydrogen atom, because the carbon atom transfers electrons to the hydrogen atoms.'

(a)Explain why the student's description is incorrect, and give the correct type of bonding present in methane.(3)

(Total for Question 16 is 3 marks)

Explain why sodium chloride is described as having a giant ionic lattice structure.

(a)Explain why sodium chloride is described as having a giant ionic lattice structure.(3)

(Total for Question 17 is 3 marks)

Figure 5 shows the start of a 2D grid representing a small section of the giant ionic lattice found in an ionic compound such as sodium chloride.

(a)Complete Figure 5 to show the alternating arrangement of positive and negative ions in the giant ionic lattice, and add a note to show that the pattern continues in all directions.(3)

(Total for Question 18 is 3 marks)

Both sodium and magnesium have metallic bonding, but magnesium has a much stronger metallic bond than sodium. Higher tier only.

(a)Explain why magnesium has a much stronger metallic bond than sodium, in terms of their metallic bonding.(4)

(Total for Question 19 is 4 marks)

Sodium chloride, iodine and magnesium are three solid substances at room temperature.

Compare the type of bonding present in sodium chloride, iodine and magnesium, and use this to explain: why solid sodium chloride does not conduct electricity but molten sodium chloride does; why solid iodine does not conduct electricity in either the solid or liquid state; and why solid magnesium conducts electricity.

(Total for Question 20 is 6 marks)

Mark scheme · C2a Chemical Bonding: Ionic, Covalent and Metallic

Question 1

(a) B1 an atom (or group of atoms) that has lost or gained one or more electrons and so has an overall electric charge oe
(a) Answer: An ion is an atom, or group of atoms, that has lost or gained electrons and so has an overall positive or negative charge.
(b) B1 ionic bonding
(b) B1 covalent bonding
(b) B1 metallic bonding
(b) Answer: Ionic bonding, covalent bonding and metallic bonding.

Question 2

(a) B1 sodium: 2,8,1
(a) B1 chlorine: 2,8,7
(a) Answer: Sodium: 2,8,1. Chlorine: 2,8,7.
(b) B1 the sodium atom loses (transfers) its 1 outer-shell electron to become Na+ (allow ft from (a))
(b) B1 the chlorine atom gains that electron directly to become Cl- (allow ft from (a))
(b) Answer: Na loses 1 outer-shell electron to form Na+ (2,8); Cl gains that 1 electron to form Cl- (2,8,8). The electron transfers directly from the sodium atom to the chlorine atom.
(c) B1 1:1 ratio; ionic bonding cao
(c) Answer: 1:1 ratio; ionic bonding, held together by strong electrostatic forces of attraction between the oppositely charged ions.

Question 3

(a) B1 Na shown as [Na]+ with an empty outer shell (no dots or crosses)
(a) B1 Cl shown as [Cl]- surrounded by 8 outer-shell electrons: 7 shown as one symbol (e.g. dots, its own original electrons) and 1 shown as a different symbol (a cross, transferred from sodium)
(a) B1 both ions enclosed in square brackets with the correct charge (1+ and 1-) marked outside the brackets
(a) Answer: [Na]+ with an empty outer shell, and [Cl]- surrounded by 8 outer-shell electrons (7 shown as dots from chlorine's original outer shell, plus 1 shown as a cross transferred from sodium), each ion in square brackets with its charge shown outside.
(b) B1 Na+ = 1+ (or +1); Cl- = 1- (or -1) cao
(b) Answer: Na+ = 1+; Cl- = 1-.
(c) B1 any one valid limitation, e.g. it does not show the giant lattice structure / that millions of ions are arranged in a repeating 3D pattern (it only shows one ion pair); OR it does not show the relative sizes of the ions; OR it does not show the true 3D shape or arrangement oe
(c) Answer: It does not show the giant lattice structure of the real compound; the diagram shows only one Na+ and one Cl- ion, not the millions of ions arranged in a repeating 3D pattern.

Question 4

(a) M1 recognises that 2 Cl- ions (2 x 1- = 2-) are needed to balance the 2+ charge on 1 Mg2+ ion oe
(a) A1 correct formula MgCl2 cao
(a) Answer: MgCl2
(b) M1 finds the lowest common multiple of the charges (6), or shows an equivalent correct method, e.g. 2 Al3+ ions balance 3 O2- ions oe
(b) A1 correct formula Al2O3 cao
(b) Answer: Al2O3

Question 5

(a) B1 calcium: 2,8,8,2
(a) B1 oxygen: 2,6
(a) Answer: Calcium: 2,8,8,2. Oxygen: 2,6.
(b) B1 diagram shows [Ca]2+ with an empty outer shell
(b) B1 diagram shows [O]2- surrounded by 8 outer-shell electrons (6 original electrons plus 2 transferred, shown as a different symbol so they are distinguishable)
(b) B1 explanation that the calcium atom transfers (loses) both of its outer electrons to the oxygen atom, so the oxygen atom gains 2 electrons (allow ft from (a))
(b) Answer: Ca loses its 2 outer electrons to form Ca2+ (2,8,8); O gains those 2 electrons to form O2- (2,8). Diagram: [Ca]2+ with no outer electrons shown, and [O]2- with 8 outer electrons shown (6 dots plus 2 crosses), each ion in square brackets with its charge marked.

Question 6

(a) B1 a shared pair of electrons between two (non-metal) atoms oe
(a) Answer: A covalent bond is a shared pair of electrons between two atoms.
(b) B1 both atoms need to gain electrons to achieve a full outer shell (both are non-metals wanting to gain, not lose, electrons) oe
(b) B1 the shared pair of electrons is attracted to (the nuclei of) both atoms, holding the atoms together oe
(b) Answer: Both atoms need to gain electrons to complete their outer shells, so neither can fully transfer electrons to the other; instead they share a pair of electrons, which is attracted to both positive nuclei, holding the atoms together as a covalent bond.

Question 7

(a) B1 one shared pair of electrons shown correctly between the two H atoms (one dot from one H, one cross from the other)
(a) B1 each H atom shown with a full outer shell of 2 electrons (a duplet) as a result of sharing
(a) Answer: Two H atoms each contribute 1 electron to a single shared pair, shown as one dot and one cross between the two H symbols; each H then has a full outer shell of 2 electrons (a duplet).
(b) B1 one shared pair of electrons shown correctly between H and Cl (one dot, one cross)
(b) B1 chlorine's remaining 6 outer electrons shown correctly as 3 lone pairs, giving Cl a full outer shell of 8
(b) Answer: H contributes 1 electron and Cl contributes 1 of its 7 outer electrons to form one shared pair; Cl also shows 3 lone pairs (6 non-bonding electrons); H ends with a duplet (2 electrons) and Cl ends with an octet (8 electrons).

Question 8

(a) B1 4 hydrogen atoms shown, each bonded to the central carbon atom
(a) B1 each C-H bond shown as one correctly placed shared pair of electrons (one dot from carbon, one cross from hydrogen)
(a) B1 carbon shown with a full outer shell of 8 electrons (an octet) and each hydrogen shown with a full outer shell of 2 (a duplet)
(a) Answer: Carbon (4 outer electrons) shares one electron with each of 4 separate hydrogen atoms, forming 4 single covalent bonds (4 shared pairs); each shared pair is shown as one dot (from C) and one cross (from H); carbon ends with 8 outer electrons and each H ends with 2.
(b) B1 carbon starts with 4 electrons in its outer shell and needs 4 more electrons to reach a full outer shell of 8
(b) B1 by forming 4 single covalent bonds, one with each hydrogen atom, carbon shares 4 more electrons, giving it a full outer shell of 8 electrons oe
(b) Answer: Carbon has 4 outer electrons and needs 4 more for a full shell of 8. By forming 4 single covalent bonds, one with each hydrogen atom, carbon shares 4 more electrons, giving it a full outer shell (octet).
(c) B1 any one valid limitation, e.g. it does not show the true 3D (tetrahedral) shape of the molecule / it shows the molecule as flat and does not show the bond angles oe
(c) Answer: It does not show the true 3D shape of the molecule (methane is actually tetrahedral); the 2D diagram makes it look flat and does not show the bond angles.

Question 9

(a) B1 two shared pairs of electrons shown correctly between the two O atoms (a double covalent bond)
(a) B1 the remaining 4 non-bonding electrons on each O atom shown correctly as 2 lone pairs each, giving each oxygen atom a full outer shell of 8
(a) Answer: Each oxygen atom shares 2 of its outer electrons with the other oxygen atom, forming 2 shared pairs (a double covalent bond); each O atom also shows 2 lone pairs (4 non-bonding electrons); each O ends with a full outer shell of 8 electrons.
(b) B1 each oxygen atom has only 6 outer electrons, so needs 2 more electrons (not 1) to complete its outer shell of 8
(b) B1 sharing 2 pairs of electrons (2 electrons from each atom) gives both atoms the extra 2 electrons they each need, completing both outer shells oe
(b) Answer: Each O atom has 6 outer electrons and needs 2 more to reach a full outer shell of 8. Sharing just one pair (a single bond) would only give each atom 1 extra electron, so the atoms share 2 pairs (a double bond), giving each oxygen atom the 2 extra electrons it needs.

Question 10

(a) B1 a metal consists of a giant, regular lattice of positive metal ions oe
(a) B1 surrounded by a 'sea' of delocalised electrons, free to move throughout the whole structure oe
(a) Answer: A metal consists of a giant regular lattice of positive metal ions, surrounded by a 'sea' of delocalised electrons that are free to move throughout the whole structure, not fixed to one particular ion.
(b) B1 circles marked + = positive metal ions (cations); small dots = delocalised (free) electrons
(b) B1 held together by strong electrostatic forces of attraction between the positive ions and the delocalised electrons (the metallic bond)
(b) Answer: (i) The circles marked + represent the positive metal ions. (ii) The small dots represent the delocalised (free) electrons. The structure is held together by strong electrostatic forces of attraction between the positive ions and the delocalised electrons.

Question 11

(a) B1 sodium: 1 electron per atom
(a) B1 magnesium: 2 electrons per atom
(a) B1 aluminium: 3 electrons per atom
(a) Answer: Sodium releases 1 delocalised electron per atom (forming Na+); magnesium releases 2 per atom (forming Mg2+); aluminium releases 3 per atom (forming Al3+).

Question 12

(a) B1 metals contain delocalised electrons that are free to move throughout the whole structure
(a) B1 when a voltage is applied, these delocalised electrons can flow through the metal, carrying charge and producing an electric current oe
(a) Answer: Metals contain delocalised electrons that are free to move through the whole structure. When a voltage is applied, these electrons can flow through the metal, carrying charge and producing an electric current.
(b) B1 there are no free-moving charged particles (no delocalised electrons and no free ions) able to carry charge through the substance oe
(b) Answer: Simple covalent substances like methane have no free electrons or ions able to move and carry charge, so no current can flow.

Question 13

(a) B1 ionic bonding cao
(a) Answer: Ionic bonding.
(b) B1 copper chloride is ionic, held together by strong electrostatic forces of attraction in a giant lattice
(b) B1 in the solid, the ions are held in fixed positions in the lattice and cannot move, so cannot carry charge
(b) B1 when dissolved in water, the lattice breaks apart and the ions become free to move throughout the solution
(b) B1 the free-moving ions can carry electrical charge between the electrodes, so the solution conducts oe
(b) Answer: In solid copper chloride, the Cu2+ and Cl- ions are held in fixed positions in a giant ionic lattice by strong electrostatic forces, so they cannot move and cannot carry charge, and the solid does not conduct. When dissolved in water, the ions separate from the lattice and are free to move throughout the solution, so they can carry electrical charge between the electrodes, and the solution conducts.
(c) B1 copper metal is deposited at the cathode
(c) B1 because the positive Cu2+ ions are attracted to the negative electrode, where each ion gains electrons oe
(c) Answer: Copper metal is deposited at the cathode. The positively charged Cu2+ ions are attracted to the negative electrode, where each ion gains 2 electrons and is deposited as solid copper.
(d) B1 chlorine gas, Cl2 cao
(d) Answer: Chlorine gas, Cl2.
(e) M1 correct species with electrons shown on the correct side, Cu2+ + 2e-
(e) A1 fully balanced equation with correct state symbols cao
(e) Answer: Cu2+(aq) + 2e- -> Cu(s)
(f) M1 correct species with electrons lost, 2Cl- -> Cl2 + 2e- (or equivalent using 1 Cl-)
(f) A1 fully balanced equation with correct state symbols cao
(f) Answer: 2Cl-(aq) -> Cl2(g) + 2e-
(g) B1 no significant change to the mass of copper deposited (provided enough Cu2+ ions are present in both solutions)
(g) B1 because the same current for the same time means the same quantity of charge (and the same number of moles of electrons) is passed, which determines the mass of copper deposited, not the concentration oe
(g) Answer: There is no significant effect on the mass of copper deposited (assuming there are enough Cu2+ ions available in both solutions). The mass deposited depends on the total charge passed (current x time), which is the same in both cases, not on the concentration of the solution.
(h) B1 any valid precaution matched to the hazard, e.g. work in a well-ventilated area/fume cupboard because chlorine gas is toxic/irritant; OR wear eye protection in case of splashes
(h) Answer: Work in a well-ventilated area (or use a fume cupboard), because the chlorine gas produced is toxic and irritant; also wear eye protection in case of splashes.

Question 14

(a) B1 particles: oppositely charged ions (a positive ion and a negative ion) oe
(a) B1 force: electrostatic force of attraction between the oppositely charged ions oe
(a) Answer: Particles: oppositely charged ions. Force: electrostatic attraction between the ions.
(b) B1 particles: two (non-metal) atoms sharing a pair of electrons oe
(b) B1 force: electrostatic attraction between the shared pair of electrons and both positive nuclei oe
(b) Answer: Particles: two atoms sharing a pair of electrons. Force: electrostatic attraction between the shared electrons and both nuclei.
(c) B1 particles: positive metal ions and delocalised electrons oe
(c) B1 force: electrostatic attraction between the positive ions and the delocalised (free) electrons oe
(c) Answer: Particles: positive metal ions and delocalised electrons. Force: electrostatic attraction between the ions and the delocalised electrons.

Question 15

(a) B1 magnesium: 2,8,2
(a) B1 chlorine: 2,8,7
(a) Answer: Magnesium: 2,8,2. Chlorine: 2,8,7.
(b) B1 diagram shows one [Mg]2+ ion with an empty outer shell
(b) B1 diagram shows two separate [Cl]- ions, each surrounded by 8 outer electrons (7 original plus 1 transferred, shown distinguishably), giving the correct 1:2 ratio
(b) B1 explanation that the magnesium atom transfers one electron to each of two separate chlorine atoms (losing 2 electrons in total), so all three atoms achieve full outer shells (allow ft from (a))
(b) Answer: Mg loses its 2 outer electrons (one to each of two Cl atoms) to form Mg2+ (2,8); each Cl atom gains 1 electron to form Cl- (2,8,8). Diagram: one [Mg]2+ ion (empty outer shell) and two separate [Cl]- ions, each with 8 outer electrons shown, reflecting the 1:2 ratio in MgCl2.

Question 16

(a) B1 methane contains covalent bonds, not ionic bonds
(a) B1 carbon and hydrogen are both non-metals, and non-metal atoms share electrons (they do not transfer them completely) to gain full outer shells oe
(a) B1 in methane, carbon shares one electron with each of the 4 hydrogen atoms (4 shared pairs), rather than any electrons being fully transferred oe
(a) Answer: The student is incorrect. Methane contains covalent bonds, not ionic bonds. Carbon and hydrogen are both non-metals, and when non-metal atoms bond together they share pairs of electrons rather than transferring them completely. In methane, the carbon atom shares one electron with each of the 4 hydrogen atoms, forming 4 covalent bonds, so no ions are formed and no electrons are fully transferred.

Question 17

(a) B1 the structure consists of a very large (giant) number of Na+ and Cl- ions arranged in a regular, repeating 3D pattern oe
(a) B1 each ion is surrounded by, and strongly attracted to, oppositely charged ions in all directions
(a) B1 the strong electrostatic forces of attraction between the ions act throughout the whole structure, not just between one pair of ions oe
(a) Answer: Sodium chloride consists of a very large number of Na+ and Cl- ions packed together in a regular, repeating 3D arrangement (lattice). Each ion is surrounded by oppositely charged ions and is held by strong electrostatic forces of attraction that act in all directions throughout the whole structure, which is why it is called a giant ionic lattice.

Question 18

(a) B1 at least a 3x3 grid of ions shown in a regular, repeating pattern
(a) B1 ions correctly labelled with alternating + and - charges throughout the grid
(a) B1 diagram or accompanying note indicates that the pattern continues/repeats in all directions (e.g. dashed edges or a written note), showing it is part of a much larger giant structure
(a) Answer: A completed 3x3 (or larger) grid of alternating positive and negative ions, each ion touching four oppositely charged neighbours, with the edges shown as continuing (for example with dashed lines or a note) to indicate the pattern repeats in all directions as part of a much larger giant lattice.

Question 19

(a) B1 magnesium atoms each release 2 delocalised electrons per atom (forming Mg2+ ions), compared with only 1 delocalised electron per atom for sodium (forming Na+ ions)
(a) B1 this means magnesium's structure has both a greater number of delocalised electrons and more highly charged positive ions than sodium's structure
(a) B1 the electrostatic forces of attraction between the positive ions and the delocalised electrons are therefore stronger in magnesium than in sodium
(a) B1 stronger metallic bonding needs more energy to overcome/break, which explains why magnesium's bulk properties (e.g. melting point) differ from sodium's oe
(a) Answer: Magnesium atoms each lose 2 outer electrons (forming Mg2+ ions with 2 delocalised electrons per atom), whereas sodium atoms lose only 1 electron each (forming Na+ ions with 1 delocalised electron per atom). This means magnesium's structure has both more highly charged positive ions and more delocalised electrons than sodium's. The electrostatic forces of attraction between the positive ions and the delocalised electrons are therefore stronger in magnesium, giving it stronger metallic bonding than sodium.

Question 20

Level 3 (5-6): A clear, accurate and logically structured explanation for all three substances, correctly linking bonding and structure (ions, delocalised electrons or molecules) to the presence or absence of free-moving charged particles able to carry electrical charge.
Level 2 (3-4): A mostly correct explanation for at least two of the three substances, linking bonding/structure to electrical conductivity, but the explanation for the third substance is missing, incomplete or contains an inaccuracy. Some scientific terms used correctly.
Level 1 (1-2): The type of bonding is identified for one or more of the substances, with limited or no linked explanation of electrical conductivity. The answer may be a list of isolated, unlinked statements.
Level 0 (0): No relevant content.
Indicative content:
- Sodium chloride has ionic bonding; in the solid, the Na+ and Cl- ions are held in fixed positions in the giant lattice by strong electrostatic forces, so they cannot move and cannot carry charge, so solid sodium chloride does not conduct.
- When molten, the ionic lattice breaks down and the ions become free to move, so they can carry charge between the electrodes, so molten sodium chloride does conduct.
- Iodine is a simple molecular (covalent) substance made of individual I2 molecules held together by weak intermolecular forces.
- Iodine has no ions and no delocalised electrons available in either the solid or liquid state, so there are no free-moving charged particles to carry charge; iodine does not conduct electricity as a solid or a liquid.
- Magnesium has metallic bonding: a lattice of positive Mg2+ ions surrounded by a sea of delocalised electrons.
- The delocalised electrons in magnesium are free to move throughout the structure even while it is solid, so they can carry charge, so solid magnesium conducts electricity.
- Correct identification of all three bonding types (ionic, covalent/molecular, metallic) supports access to the higher levels.

Chemical Bonding: Ionic, Covalent and Metallic - Worksheets, Questions and Revision