The elements: from non-metal to metal down the group
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predict the characteristic properties of an element in a given group by using knowledge of chemical periodicity … deduce the nature, possible position in the Periodic Table and identity of unknown elements from given information about physical and chemical properties.
Four elements, one steady change of character
Exam questions on this group use four elements: carbon (usually as graphite), silicon, tin and lead. Each has four electrons in its outer shell. In Group 2 or Group 17 every element is the same kind of element and only the size of a property changes. Group 14 changes kind as you go down it: carbon is a non-metal, silicon is a semiconductor on the boundary, and tin and lead are metals.
Why? Down the group the atoms get bigger and the outer electrons are further from the nucleus and better shielded, so they are held less tightly. At the top, carbon and silicon share their outer electrons in covalent bonds. At the bottom, tin and lead let their outer electrons go into a "sea" of delocalised electrons, which is metallic bonding.
Element | State/appearance | Structure | Bonding | Electrical conductivity |
|---|---|---|---|---|
C (graphite) | grey, shiny solid | giant covalent | covalent | good — delocalised electrons within each layer |
Si | grey, shiny solid | giant covalent | covalent | semiconductor — much less than a metal, more than an insulator |
Sn | silvery solid | giant metallic | metallic | good — delocalised electrons throughout the lattice |
Pb | grey/silvery solid | giant metallic | metallic | good — delocalised electrons throughout the lattice |
The structure stays giant all the way down the group, but the bonding changes from covalent to metallic between silicon and tin, where the group crosses from non-metal to metal.
Graphite conducts, but it is not metallic
Graphite conducts well, but not because carbon is a metal. In graphite the carbon atoms are in flat hexagonal layers. Each carbon forms covalent bonds to only three neighbours, so one outer electron per atom is left over. These electrons are delocalised across the layer, and they carry the current.
Diamond is also pure carbon, but each atom bonds to four neighbours in a 3-D network. All four outer electrons are used in bonds, none are delocalised, so diamond does not conduct. Questions usually name graphite; check which form of carbon a question gives.
Silicon: a semiconductor
Silicon has a giant covalent structure like diamond: each atom bonds to four others and there are no delocalised electrons. So you might expect it to be an insulator. In fact it conducts a little: much less than a metal, but more than an insulator such as diamond. An element like this is called a semiconductor. This in-between behaviour fits silicon's place on the boundary between non-metals and metals. (It is also why silicon is used to make computer chips.)
Predicting the properties of lead from its place in the group
Lead is at the bottom of Group 14. Predict (a) the type of bonding in lead, (b) its type of structure, (c) its electrical conductivity, and (d) whether it is a metal or a non-metal.
Show full working
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Locate lead. Lead is the largest atom in the group, so its four outer electrons are the furthest from the nucleus and the most shielded.
Every prediction in a group question starts from the trend in atom size and how tightly the outer electrons are held.
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(a) Bonding: metallic. Its outer electrons are held weakly enough to become delocalised, so lead has positive ions in a sea of delocalised electrons.
This is the far end of the covalent-to-metallic change: tin is already metallic, so lead is too.
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(b) Structure: giant. A metallic lattice is a giant structure, with bonding throughout the solid.
Every Group 14 element is giant. Only the bonding changes down the group, not the structure type.
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(c) Conductivity: good. The delocalised electrons can move through the lattice and carry a current.
Link conductivity to mobile charged particles every time; here they are delocalised electrons.
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(d) Lead is a metal.
Metallic bonding and good conductivity are the signs of a metal.
(a) metallic (b) giant (c) good conductor (delocalised electrons) (d) metal.
Completing a state, structure, bonding and conductivity table
The Group 14 elements show a change from non-metallic to metallic character down the group.
Table 3.1 shows some properties of two Group 14 elements, C and Sn, in their standard states. The table is incomplete.
Table 3.1
| C (graphite) | Sn | |
|---|---|---|
| state and appearance in standard state | grey shiny solid | silvery solid |
| electrical conductivity | good | |
| type of bonding | metallic | |
| type of structure | giant |
Complete Table 3.1.
Show full working
- 1
Find the three blanks. They are graphite's electrical conductivity, graphite's type of bonding, and tin's type of structure. There is one mark for each.
Count the blanks against the marks first, so you don't waste time on cells that are already filled.
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Graphite's electrical conductivity: good. Each carbon in a layer bonds to only three others, so one electron per atom is delocalised across the layer and carries the current.
The mark scheme accepts "good" or "conductor".
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Graphite's type of bonding: covalent. The carbon atoms are held together by covalent bonds, even though graphite conducts.
The trap is to write "metallic" because graphite conducts. Graphite is a good conductor with covalent bonding.
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Tin's type of structure: giant. Tin is a metal: a giant lattice of positive ions in a sea of delocalised electrons.
"Giant" is enough here, because the bonding row already says metallic.
C (graphite): electrical conductivity good; type of bonding covalent. Sn: type of structure giant.
"Good conductor" does not tell you the bonding type on its own. Graphite conducts but is covalent.
Your turn
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Explain why graphite conducts electricity well, but diamond, which is also carbon, does not.
Stuck? Show hint
Compare how many neighbours each carbon atom bonds to in each structure, and what is left over.
Show solution
- 1
In graphite, each carbon atom bonds to only three neighbours in flat layers, leaving one outer electron per atom delocalised across the layer.
Start from how many bonds each atom makes, because that decides whether any electrons are left over.
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These delocalised electrons can move and carry a current, so graphite conducts.
Conduction needs mobile charged particles.
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In diamond, each carbon atom bonds to four neighbours, using all four outer electrons in bonds. No electrons are delocalised, so diamond does not conduct.
Same element, different structure: it is the structure that decides conductivity.
AnswerGraphite: each C bonds to 3 others, leaving one delocalised electron per atom, which can carry a current. Diamond: each C bonds to 4 others, all outer electrons are in bonds, none are delocalised, so no current flows.
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Tin and silicon are both in Group 14. State the type of bonding in each element and explain why tin is a better electrical conductor than silicon.
Stuck? Show hint
Which one is a metal, and which one is a semiconductor?
Show solution
- 1
Silicon has covalent bonding in a giant covalent structure. It is a semiconductor, so it conducts only a little.
Silicon is at the top of the change in the group, where outer electrons are shared in covalent bonds.
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Tin has metallic bonding: positive ions in a sea of delocalised electrons.
Tin's atoms are larger, so its outer electrons are held less tightly and become delocalised.
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Tin's delocalised electrons move freely through the whole lattice, so tin conducts well, much better than silicon.
Link the conductivity to the mobile electrons, not just to the word "metal".
AnswerSi: covalent (giant covalent, a semiconductor). Sn: metallic. Tin has delocalised electrons that move through the whole lattice, so it conducts much better than silicon.
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P and Q are two Group 14 elements from carbon, silicon, tin and lead. P is a semiconductor with a giant covalent structure. Q has metallic bonding and melts at , much lower than P. Which element is P? Is Q nearer the top or the bottom of the group?
Stuck? Show hint
Use the sharpest clue first.
Show solution
- 1
"Semiconductor" points straight to silicon, so P is silicon.
Silicon is the one semiconductor among these four elements.
- 2
Metallic bonding means Q is tin or lead, so Q is near the bottom of the group. (Tin melts at .)
Only the elements at the bottom of the group are metals.
AnswerP is silicon; Q (tin) is near the bottom of the group.
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The rest of this note
Can you do all of these?
Describe the structure, bonding and conductivity of C (graphite), Si, Sn and Pb, and explain why graphite conducts but diamond does not
Identify silicon as a semiconductor and explain why the group changes from non-metal to metal
Describe the tetrachlorides as simple molecular, covalent and tetrahedral, and justify structure (low melting point) and bonding (hydrolysed) with separate evidence
Write the equation for forming a tetrachloride from its elements
Write the hydrolysis equation for SiCl₄ (and SnCl₄), state the observations, and explain why the mixture conducts although pure SiCl₄ does not
Write the hydrolysis equation for an unfamiliar covalent halide that behaves like SiCl₄
Work a back-titration calculation to identify X in XCl₄
Classify CO₂, SiO₂, SnO₂ and PbO₂ as acidic or amphoteric, write equations for SiO₂ and SnO₂ with acids and bases, and give the formula of a salt formed
Work out oxidation numbers of Group 14 elements, including compounds that contain both +2 and +4