Principles of Chemistry
9 marks · HardMagnesium and chlorine can combine in different ways to form compounds. Ammonia is another important compound formed from non-metals.
Describe how magnesium and chlorine atoms form ions and the charges of these ions.
Explain what holds the ions together in the compound magnesium chloride.
Define a covalent bond and describe how it forms specifically in an ammonia (NH₃) molecule.
Compare the type of particles present in solid magnesium chloride and in ammonia. State one reason for your answer.
State whether you would expect solid magnesium chloride or ammonia to have a higher melting point. Explain your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Magnesium atoms lose two electrons to form Mg2+ ions; chlorine atoms gain one electron to form Cl- ions.
(b) Strong electrostatic attraction between Mg2+ and Cl- ions holds the compound together (ionic bonding).
(c) A covalent bond is a shared pair of electrons between two atoms. In ammonia, the nitrogen shares its three unpaired electrons with three hydrogen atoms to form three covalent bonds.
(d) Magnesium chloride is made up of ions arranged in a lattice. Ammonia consists of molecules. This is because magnesium chloride has ionic bonding and ammonia has covalent bonding.
(e) Magnesium chloride has a higher melting point because more energy is needed to break the strong ionic bonds than the weaker forces between ammonia molecules.
Common mistakes: Confusing ions and molecules; Mixing up positive and negative charges; Describing covalent bonds as transfer of electrons
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Marking points
M1 (a) · 1 mark · States magnesium loses electrons to form a 2+ ion
M2 (a) · 1 mark · States chlorine gains electrons to form a 1− ion
B1 (b) · 1 mark · Identifies electrostatic attraction between oppositely charged ions
B2 (b) · 1 mark · Uses the term 'ionic bonding' or 'strong ionic bonds'
M3 (c) · 1 mark · Defines covalent bond as a shared pair of electrons
A1 (c) · 1 mark · Correctly describes sharing between nitrogen and three hydrogens in ammonia
B3 (d) · 1 mark · Identifies magnesium chloride as ionic lattice/ions; ammonia as molecules
B4 (d) · 1 mark · Explains the difference based on bonding type (ionic/covalent)
E1 (e) · 1 mark · States magnesium chloride has higher melting point with correct reason (stronger ionic bonds require more energy to break than weak forces between molecules)
Worked solution
a) Magnesium atoms each lose two electrons to form magnesium ions with a 2+ charge (Mg2+). Chlorine atoms each gain one electron to form chloride ions with a 1− charge (Cl−).
b) The ions in magnesium chloride are held together by strong electrostatic forces of attraction between the Mg2+ (positive) and Cl− (negative) ions. This is called ionic bonding.
c) A covalent bond is a shared pair of electrons between atoms. In ammonia (NH3), nitrogen has five electrons in its outer shell and needs three more; it shares three pairs of electrons with three hydrogen atoms, each providing one electron. This forms three covalent bonds.
d) In solid magnesium chloride, the particles present are ions arranged in a giant lattice. In ammonia, the substance consists of molecules. This is because ionic bonding in magnesium chloride forms a lattice of ions, whereas covalent bonding in ammonia forms molecules.
e) Magnesium chloride has a higher melting point than ammonia. This is because much more energy is needed to break the strong ionic bonds in the giant lattice of magnesium chloride than to overcome the weak intermolecular forces between ammonia molecules.