3.1.1 (d) Metallic Lattices

Syllabus

(d) explanation of:

(i) metallic bonding as strong electrostatic attraction between cations (positive ions) and delocalised electrons

(ii) a giant metallic lattice structure, e.g. all metals

{No details of cubic or hexagonal packing required.}

What does this mean?

This is another topic you should be familiar with from GCSE.

Essentially, the metal atoms gives up its outer (valence) electrons to become positice cations.

The electrons are not held in any place - they are often said to be "a sea of delocalised electrons".

And because they are delocalised they can move - explaining how metals conduct electricity and also heat.

If you look at this model it appears that the metal should fly apart since all the ions would repel each other.

However, each ion is closer to the electrons than its neighbouring ions.

And cations attract electrons.

So the attractions outweigh the repulsions.

They must outweigh them by a lot because most metals have high melting points.

Here we can see that the metallic elements in Period 3 have higher melting/boiling points than other elements except Silicon which has a giant covalent structure.

There is a rise from Na to Al you should be able to account for.

Na forms 1+ ions, Mg 2+ ions and Al 3+ ions

Also the ionic radius decreases from Na to Al.

So there are two good reasons why the ions become more attractive towards the electrons.

And also there are simply three times more electrons in Aluminium than Sodium because each Aluminium provides 3 electrons whereas each Sodium provides only one.

You may also recall why metals are generally malleable.

Part of the reason is because layers of regularly sized ions can roll over each other.

But also, when they do so they do not upset a rigid pattern.

The metal ions were repelled by their neighbours anyway.

After sliding they are simply repelled by different neighbours.

But there are still electrons between them attracting the ions.

So the metal ions tends to stick together unless repeatedly over-stressed.

When the syllabus says "No details of cubic or hexagonal packing required" it means that you don't need to concern yourself with exactly how metallic crystals grow.

If you're interested you can see(right) that it is a difference in the repeating pattern of layers.

Here we can see a piece of Brass under a microscope.

Like all metals it is made of individual crystals growing into each other in different orientations.

Where crystals meet is a source of weakness.

So blades of jet-engines are generally grown from single crystals so that they can withstand greater stresses and strains.

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Exam-style Questions

1. (a) (i) Complete the electronic configuration of Aluminium.

1s2 ................................................................................................................................................................

(ii) State the block in the Periodic Table to which Aluminium belongs.

.......................................................................................................................................................................

(2)

(b) Describe the bonding in metals.

.......................................................................................................................................................................

(2)

(c) Explain why the melting point of Magnesium is higher than that of Sodium.

.......................................................................................................................................................................

(3)

(d) Explain how metals conduct electricity.

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(2)

(Total 9 marks)

Answers

1. (a) (i) 1s2 2s2 2p6 3s2 3p1 (1)

Allow subscripted electron numbers

(ii) p (block) (1)

Allow upper or lower case ‘s’ and ‘p’ in (a)(i) and (a)(ii)

(b) Lattice of metal / +ve ions/ cations / atoms (1)

Not +ve nuclei/centres

Accept regular array/close packed/tightly packed/uniformly arranged

(Surrounded by) delocalised electrons (1)

Note: Description as a ‘giant ionic lattice’ = CE

(c) Greater nuclear or ionic charge or more protons (1)

Smaller atoms / ions (1)

Accept greater charge density for either M1 or M2

More delocalised electrons / e– in sea of e– / free e– (1)

Stronger attraction between ions and delocalised / free electrons etc. (1)

Max 3

Note: ‘intermolecular attraction/ forces’ or covalent molecules = CE

Accept stronger ‘electrostatic attraction’ if phrase prescribed elsewhere

Ignore references to m/z values

If Mg or Na compared to Al, rather than to each other, then: Max 2

Treat description that is effectively one for Ionisation Energy as a ‘contradiction’

(d) (Delocalised) electrons (1)

Move / flow in a given direction (idea of moving non-randomly) or under the influence applied pd QoL mark (1)

Allow ‘flow through metal’

Not: ‘Carry the charge’; ‘along the layers’; ‘move through the metal’

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