The molecular world
The molecular world

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The molecular world

4.6 Summary of Section 4

  1. The chemical formulae of many substances can be understood by arguing that their atoms attain noble gas structures by chemical combination.

  2. In ionic compounds, this is achieved by the transfer of electrons from one atom to another; in molecular substances, it happens through the sharing of electron pairs in covalent bonds. But in both cases, bonds between atoms consist of shared pairs of electrons. In covalent compounds the sharing is fairly equitable; in ionic compounds it is much less so.

  3. In metals, the sharing takes a different form. An 'electron gas' is created by removing electrons from the atoms of the metallic elements. The result is an array of ions steeped in a pool of free electrons. The negatively charged electron gas occupies the space between the ions and pulls them together.

  4. Atoms with high but similar electronegativities from the right of Figure 30 combine to form covalent substances; those with low but similar electronegativities from the left of Figure 30 yield metallic substances. The combination of atoms of low and high electronegativity from the left and right of Figure 30 produces ionic compounds.

  5. Chemical substances can now be classified, first structurally as either molecular or non-molecular, and second by bond type as ionic, covalent or metallic.

  6. In Lewis structures, each covalent bond is represented by a shared electron pair. Double bonds, as in CO2, require two shared pairs; a triple bond, as in HCN, requires three. These allocations often leave some atoms with non-bonded electron pairs.

  7. In many cases, this operation provides each atom with a noble gas shell structure, especially if we introduce dative bonds in which both electrons are contributed by one atom. But in some cases, such as PF5, it does not.

  8. Sometimes the bond lengths in a chemical substance are such that the substance cannot be represented by a single Lewis structure or structural formula. It is better described as a resonance hybrid-an average or superposition of two or more structural formulae called 'resonance structures'.

Question 11

Consider the compounds IBr, CaCl2 and CaMg2. One is ionic, one is covalent, and one is metallic. Identify which is which, and match each compound to one of the descriptions below. In each case, suggest whether the compound is molecular or non-molecular.

  • (i) White solid that melts at 782 °C. It is a poor conductor of electricity in the solid state, but a good one when melted or dissolved in water.

  • (ii) Brown-black solid that melts at 41 °C to give a liquid with low electrical conductivity.

  • (iii) Silvery-looking solid that melts at 720 °C. Whether solid or molten, it is an excellent conductor of electricity.

Answer

(i) CaCl2; (ii) IBr; (iii) CaMg2.

The properties listed are characteristic of (i) an ionic substance, (ii) a molecular covalent substance, and (iii) a metallic substance. CaCl2 is a combination of elements from the extreme left and extreme right of Figure 30, so the electronegativity difference will be large and CaCl2 will be the ionic compound; such compounds are non-molecular. IBr will be covalent because it is a combination of elements of high electronegativity from the extreme right of Figure 30. CaMg2 will be a metallic alloy because it is a combination of metallic elements with low electronegativity from the left of Figure 30. Such alloys are non-molecular.

Question 12

Write single Lewis structures, and the corresponding structural formulae, for the following molecules or ions: (a) hypochlorous acid, HOCl; (b) sulfur hexafluoride, SF6; (c) nitrosyl chloride, ONCl; (d) the amide ion, NH2. In each case, state the number of bonding electron pairs and non-bonded pairs on the atom of highest valency. In which of the four Lewis structures do some atoms not have a noble gas shell structure?

Answer

  • (a) For hypochlorous acid, see Structures Q.10 and Q.11. The oxygen has two non-bonded pairs and two bonding pairs.

  • (b) For SF6, see Structures Q.12 and Q.13. The sulfur atom has six bonding pairs and no non-bonded pairs.

  • (c) For ONCl, see Structures Q.14 and Q.15. The nitrogen has one non-bonded pair and three bonding pairs.

  • (d) For NH2, the amide ion, see Structures Q.16 and Q.17 . The nitrogen has two bonding pairs and two non-bonded pairs.

All the atoms in Lewis structures Q.10, Q.12, Q.14 and Q16 have noble gas shell structures, except for sulfur in SF6, which is assigned twelve outer electrons.

Question 13

In the nitrate ion, NO3, the nitrogen atom is central and surrounded by three oxygens. Draw a single Lewis structure for this ion which gives each atom a noble gas shell structure. Also draw two structural formulae for this Lewis structure, each containing a different representation of any dative bonds.

Answer

The Lewis structure of NO3 is shown as Structure Q.18. The atom of highest valency is nitrogen, so the single negative charge on the NO3 ion is assigned to nitrogen, giving the shell structure (2,6). All atoms gain the shell structure of neon if nitrogen forms one double bond and two single dative bonds to oxygen. Structure Q.19 shows the two dative bonds as arrows. In the alternative representation, one of the two positive charges at the nitrogen end of the two dative bonds is cancelled by the single negative charge of the central nitrogen. This gives Structure Q.20.

Question 14

For the Al2Br6 molecule (Figure 8), write a single Lewis structure that contains dative bonds and gives each atom a noble gas structure (the bromine atom, like chlorine, has seven electrons in its outer shell). Use the two different representations of the dative bond to draw two structural formulae for the Lewis structure. Experiments on this molecule show that all bond lengths in the Al—Br—Al bridges are identical. Which of your two structural formulae best fits this observation?

Answer

See Structures Q.21-Q.23.

In the Lewis structure Q.21, each bridging bromine atom forms one shared electron pair bond with one aluminium and one dative bond to the other aluminium. All atoms gain a noble gas shell structure with eight outer electrons. Structural formula Q.22 shows the dative bonds as arrows, and suggests that the two bonds formed by each bridging bromine are different. The alternative Q.23 makes these two bonds identical. It is therefore a better representation of the experimental data. Nevertheless, although the single formula Q.22 is not compatible with the equal Al—Br bond lengths, this way of representing the dative bonds can be made consistent with them by using the two resonance hybrids shown in Structure Q.24.

Question 15

In the nitrate ion, NO3, all three nitrogen-oxygen bonds are of equal length. Is either of the structural formulae in your answer to Question 12 consistent with this observation? If not, how do you explain the discrepancy?

Answer

In the answer to Question 12, both Structure Q.19 and Structure Q.20 contain one double bond and two single dative bonds. Neither is therefore consistent with three equal bond lengths. To explain the discrepancy we represent the nitrate ion as the resonance hybrid shown as Structure Q.25.

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