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A-Level How atoms bond and how that governs a substance's behaviour
What the A-Level syllabus expects for How atoms bond and how that governs a substance's behaviour, and how to practise it.
What the syllabus expects
- Recognise that every chemical bond is at heart electrostatic, and be able to describe the following bond types:
- ionic bond, that is the pull between ions of opposite charge
- covalent bond, the attraction between a shared electron pair and the positively charged nuclei flanking it
- metallic bond, the attraction binding a sea of delocalised electrons to a lattice of positive ions
- With dot-and-cross diagrams, describe:
- ionic bonding, taking sodium chloride and magnesium oxide as the cases
- covalent bonding as it appears in methane, ethene, carbon dioxide and hydrogen chloride, and in the elements chlorine, nitrogen, oxygen and hydrogen
- co-ordinate (dative covalent) bonding, seen as the ammonium ion forms and in the Al2Cl6 molecule
- Explain covalent bonding as the overlapping of orbitals, which yields sigma and pi bonds.
Scope: limited to s and p orbitals; see also Section 11.1 - Apply Valence Shell Electron Pair Repulsion theory to reason out the shapes and bond angles of molecules such as BF3, CO2, CH4, NH3, H2O and SF6.
Scope: these span trigonal planar, linear, tetrahedral, trigonal pyramidal, bent and octahedral - For molecules resembling those in (d), forecast their geometry and their bond angles.
- Draw on electronegativity to reason out, and deduce, whether a bond is polar.
Scope: no numerical electronegativity work is expected - Combine the individual bond polarities with the molecular shape to decide whether the whole molecule is polar.
Scope: for molecules like those in (d) - Describe these attractive forces, all of them electrostatic at root:
- forces between molecules that spring from dipoles, whether permanent or induced, shown by liquid CHCl3, liquid Br2 and the noble gases in liquid form
- hydrogen bonding, with ammonia and water standing in for molecules carrying -NH and -OH groups
- Outline why hydrogen bonding matters for the physical properties of a substance, drawing on ice and water to make the point.
- Say what bond energy and bond length mean for a covalent bond.
- Judge how reactive covalent bonds are by looking together at their energy, their length and their polarity.
- In plain terms, describe the lattice present in a crystalline solid of each kind:
Scope: the notion of a unit cell is not needed - ionic, such as sodium chloride and magnesium oxide
- simple molecular, such as iodine
- giant molecular, such as graphite and diamond
- hydrogen-bonded, such as ice
- metallic, such as copper
- Explain, interpret or forecast how a substance's structure and bonding shape its physical properties.
- From clues supplied, propose what structure and bonding a substance has.
How it's examined
Questions on this topic most often ask you to explain, show, state, compare. About 6% of the past-paper style questions in Rae's bank for this subject sit in this topic.
Worked examples
Example 1 (3 marks)
The table below gives the melting points, in °C, of the chlorides and iodides of the two elements iron and silicon. Table 1.2 (chloride) iron: 674, silicon: -68 (iodide) iron: 590, silicon: 120 Using ideas about structure and bonding, account for the differences in melting point among FeCl2, SiCl4 and SiI4.
Show the worked answer
FeCl2 has a giant ionic lattice: strong electrostatic forces of attraction between Fe2+ and Cl- ions must be overcome, so its melting point is very high (674 C). SiCl4 and SiI4 are simple molecular (discrete covalent molecules); only weak instantaneous-dipole/induced-dipole (van der Waals/dispersion) forces act between the molecules, so their melting points are low. SiI4 (120 C) melts higher than SiCl4 (-68 C) because the iodine atoms have more electrons/larger electron clouds, giving a more polarisable molecule and therefore stronger dispersion forces than in SiCl4.
Example 2 (3 marks)
PCl5, a chloride of Period 3, is often used to chlorinate organic compounds and as a catalyst in their preparation. It is made industrially by reacting Cl2 with PCl3. Reaction (1): PCl3(g) + Cl2(g) ⇌ PCl5(g) Apply VSEPR theory to predict and account for the shape and bond angle of PCl3, and include a suitable diagram showing the shape of the PCl3 molecule.
Show the worked answer
P has 5 valence electrons; in PCl3 it forms 3 bonding pairs to Cl and retains 1 lone pair, giving 4 electron domains around P. Four domains adopt a tetrahedral arrangement, but with one position occupied by a lone pair the molecular shape is trigonal pyramidal. The lone pair-bond pair repulsion is greater than bond pair-bond pair repulsion, so the Cl-P-Cl angle is compressed below 109.5 deg to about 100 deg. Diagram: P at apex with a lone pair on top and three P-Cl bonds fanning down to the three Cl atoms.
Example 3 (2 marks)
Explain how a sigma (σ) bond and a pi (π) bond are each formed.
Show the worked answer
A sigma (sigma) bond is formed by the head-on (end-on) overlap of two orbitals along the line joining the two nuclei (the internuclear axis), giving a region of electron density concentrated directly between the nuclei. A pi (pi) bond is formed by the sideways (lateral) overlap of two parallel p orbitals, giving regions of electron density above and below the internuclear axis.
More worked questions on this topic
- Using a clearly labelled diagram, explain the structure of iron and the bonding within it. (2 marks)
- Using ideas about electronegativity, account for why the bonding in NaCl differs from that in P (2 marks)
- Work out the hybridisation adopted by the nitrogen atom(s) in each of the following species: NF (2 marks)
- (h) Using ideas about structure and bonding, account for why the boiling point of N2O4 exceeds (2 marks)
- Predict how many IR absorptions nitrogen dioxide should show and state which vibrational modes (2 marks)
- Using ideas of structure and bonding, account for why CaC2 exists as a solid under room conditi (2 marks)
- Polyacetylene is recognised as an organic semiconductor. Account for how polyacetylene manages (2 marks)
- Suggest how the electrons in the second shell of an sp carbon atom are arranged so that the bon (2 marks)
More A-Level H2 Chemistry topics
The make-up of the atom and how its electrons are arranged · Ideal gases and working with gas mixtures · Competing definitions of acids and bases · Trends in the elements across a period and down a group · The mole and reacting-quantity calculations · Enthalpy, entropy and the feasibility of reactions · all of A-Level H2 Chemistry