Magnesium is a metal.
(a) Name and describe the bonding in magnesium.
(b) Magnesium oxide, \(\mathrm{MgO}\), is formed when magnesium burns in oxygen.
(i) Complete the dot-and-cross diagram to show the electron arrangement of the ions in magnesium oxide.
The inner shells have been drawn.
Give the charges on the ions.
(ii) Write the chemical equation for the reaction that occurs when magnesium burns in oxygen.
(c) Magnesium oxide also forms when magnesium nitrate, \(\mathrm{Mg}\left(\mathrm{NO}_3\right)_2\), is heated strongly. This is an endothermic reaction.
(i) Write the chemical equation for this reaction.
(ii) What type of reaction is this?
(iii) Name two other compounds of magnesium that form magnesium oxide when heated.
▶️ Answer/Explanation
(a) The bonding in magnesium is metallic bonding.
Description: Magnesium atoms form a lattice of positive ions surrounded by a “sea” of delocalized electrons. The strong electrostatic attraction between the positive ions and electrons holds the structure together.
(b) (i) The dot-and-cross diagram shows:
– Magnesium ion (\(\mathrm{Mg}^{2+}\)) with an empty outer shell (lost 2 electrons).
– Oxide ion (\(\mathrm{O}^{2-}\)) with 8 electrons in the outer shell (gained 2 electrons).
Charges: \(\mathrm{Mg}^{2+}\) and \(\mathrm{O}^{2-}\).
(b) (ii) The chemical equation is:
\[ 2\mathrm{Mg} + \mathrm{O}_2 \rightarrow 2\mathrm{MgO} \]
(c) (i) The decomposition of magnesium nitrate:
\[ 2\mathrm{Mg(NO_3)_2} \rightarrow 2\mathrm{MgO} + 4\mathrm{NO_2} + \mathrm{O_2} \]
(c) (ii) This is a thermal decomposition reaction.
(c) (iii) Two other magnesium compounds that form \(\mathrm{MgO}\) when heated:
1. Magnesium carbonate (\(\mathrm{MgCO_3}\))
2. Magnesium hydroxide (\(\mathrm{Mg(OH)_2}\))
An isotope of sodium is written as shown.
\(_{11}^{23}\textrm{Na}\)
(a) (i) Deduce the number of protons, electrons and neutrons in this isotope of sodium.
(ii) State one medical use of radioactive isotopes.
(b) (i) Draw the electronic structure of a sodium atom.
(ii) State the name of the particle which is lost when a sodium atom forms a sodium ion.
(c) Sodium reacts with water to form:
- an alkaline solution
- a gas which ‘pops’ with a lighted splint.
(i) Complete the word equation for the reaction of sodium with water.
(ii) The reaction of sodium with water is exothermic.
What is meant by the term exothermic?
(iii) Sodium reacts with oxygen to form sodium oxide.
Is sodium oxide an acidic oxide or a basic oxide?
Give a reason for your answer.
(d) The table shows some observations for the reaction of four metals with cold water and with hot water.
Use this information to put the four metals in order of their reactivity.
Put the least reactive metal first.
▶️ Answer/Explanation
(a)(i) The isotope \(_{11}^{23}\textrm{Na}\) has:
– Protons: 11 (atomic number = proton count).
– Electrons: 11 (neutral atom has equal protons and electrons).
– Neutrons: 12 (mass number − atomic number = 23 − 11).
(a)(ii) Radioactive isotopes are used in cancer treatment (radiotherapy) or as tracers in medical imaging.
(b)(i) Sodium’s electronic structure: 2,8,1 (1st shell: 2 electrons, 2nd shell: 8 electrons, 3rd shell: 1 electron).
(b)(ii) The particle lost is an electron (Na → Na⁺ + e⁻).
(c)(i) The word equation: sodium + water → sodium hydroxide + hydrogen.
(c)(ii) Exothermic means heat is released during the reaction.
(c)(iii) Sodium oxide is basic because it reacts with acids (metal oxides are typically basic).
(d) Reactivity order (least to most): manganese < uranium < lanthanum < calcium. Calcium reacts vigorously with cold water, making it the most reactive.