UNIT9:TRENDS OF CHEMICAL PROPERTIES OF GROUP 14 ELEMENTS AND THEIR COMPOUNDS
Key unit Competence
Compare and contrast the chemical properties of the Group 14 elements and their
compounds in relation to their position in the Periodic Table.
Learning objectives
By the end of this unit, students should be able to:
• Compare and contrast the physical properties of Group 14 elements.
• Compare the relative stabilities of the higher and lower oxidation states in
oxides.
• Distinguish between the chemical reactions of the oxides and chlorides of
Group 14 elements.
• Explain the trends in thermal stability of the oxide, halides and hydrides of
Group 14 elements.
• Explain the variation in stability of oxidation state of +2 and +4 down the
Group 14 elements.
Introductory Activity 9
1. State any elements of group 14 that is found in Rwanda. Where are they produced from? What are they used for?
2. State 2 allotropes of carbon and give a brief description of the structure of the
two allotropes.
3. Explain the variation in electronegativity of group 13 elements as you move
down the group.
4. Discuss the way the variation in size of atoms down a group affects their:
a) Metallic character
b) First ionization energy
c) Ability to form ionic or covalent compounds.
5. Describe the variation in melting points down group 1from lithium to potassium
• Define the diagonal relationship.
• Carbon, the first element of the group has two main allotropes: graphite
and diamond.
• In graphite allotrope of carbon, each carbon is bonded to 3 other carbon
atoms to form a hexagonal structure. The structure of graphite is made of
hexagonal layers which are attracted to each other by weak Van der Waals
forces such that the layers slide over each other to make the structure
soft(Fig.9.1). In graphite structure, there are delocalised double bonds with
mobile electrons that allow graphite to conduct electricity.
• In diamond, each carbon is covalently bonded to 4 other carbon atoms
forming a giant tetrahedral structure that makes it to be very hard (Fig.9.1).
In diamond, there are no mobile electrons as in graphite, hence diamond
does not conduct electricity.
• As you move down the group in the carbon family, the atomic radius and
ionic radius increase while the electronegativity and ionization energy
decrease.
• Atomic size increases on moving down the group due to additional
electronic shells.
• Density increases as you move down the group.
• Carbon is the only element in the family that can be found in pure form in
• Lead is the only element of group 14 that does not exist in various allotropes.
• Tin occurs as white, grey and rhombic tin.
• Group 14 elements have much higher melting points and boiling points
than the group 13 elements.
• Melting and boiling points tend to decrease as you move down the group
mainly because inter atomic bonding between the larger atoms reduce in
strength as you move down the group.
Moving down the group, there is an increase in atomic size which results in less
attraction of valence electrons by the nucleus. This change results in weaker metallic
bonding down the group and therefore there is a decrease in melting point, boiling
point, enthalpy change of atomization and first ionization energy.
The decrease in first ionization energy from silicon to lead is relatively little compared
to that from carbon to silicon because there is a large increase in nuclear charge
which counterbalances the increase in atomic radius from silicon to lead.
ii) The increase in metallic character down the group causes a general increase in
conductivity.
Carbon is typically a solid, non-metal. Carbon graphite is a non-metal but conducts
electricity due to delocalized electrons in its structure.
In its compounds, carbon almost invariably completes its valence shell by forming
four covalent bonds
Silicon is solid at room temperature and pressure, it is a semi-metallic element and
semi-conductor of electricity which is the second most abundant element on earth,
after oxygen.
It should also form bonds like C-C which are similar in strength to those of C and
other elements, particularly C-O bonds.
Silicon forms -Si-O-Si-bonds predominantly.
ii) Multiple bonds
Carbon forms double bonds and triple bonds between carbon atoms and that
bonding is formed by one Sigma bond and one π bond for double bond, one Sigmabond and two π bonds in a triple bond.
Checking-up 9.1
1. Explain the reason why diamond has a higher melting point than silicon.
2. Discuss the increase in metallic character when moving down in group 14
elements from carbon to lead.
3. Diamond and graphite are allotropes of carbon,
a) Draw their three dimensional structures.
b) With reference to their structures, compare the hardness of diamond and
graphite.
c) With reference to their structures, compare their electrical conductivity and
explain.
4. Germanium has the same structure as diamond. Explain the type of bonds that
exist in the two elements.
5. The first element in a group in the periodic table exhibits anomalous properties
compared with other members. Use carbon to illustrate this statement.
9.2. Chemical properties of Group 14 elements
Activity 9.2 (a)
1. Get a piece of charcoal and burn it. Observe and write the chemical equation
that represents the change that takes place when the charcoal burns.
2. a) Put about 1 gram of carbon charcoal in a boiling tube.
b) Add 1 ml of concentrated nitric acid.
c) Heat strongly on a Bunsen burner flame using a test tube holder
d) Observe and note the changes during heating.
e) Deduce the chemical changes that have occurred.
3. Write the molecular structure of carbon dioxide, carbonate ion and carbon
monoxide.
4. Describe how CO2
gas dissolves in water and state the nature of the solution
formed when it is in aqueous solution.
5. Describe 2 chemical properties of amphoteric substances.
Reaction of group 14 elements with acids and bases:Carbon does not react with dilute acids but reacts with hot, concentrated acids:
9.3 Difference between the chemical reactions of the oxides
and chlorides of Group 14 elements.
Activity 9.3
1. Measure 0.5g of lead oxide or decompose the same quantity of lead nitrate
crystals by heating.
2. Divide it into 5 portions and put each portion in a test tube.
3. In the first test tube, add 2mL of dilute hydrochloric acid solution in which universal indicator has been dissolved.
4. In the second test tube, add 2ml sodium hydroxide solution in which phenolphthalein indicator has been dissolved.5. Note the observations and deduce the acid–base nature of lead oxide.
Interpretation of results of the above activityThe reactions that take place are:
in chemical bonding in Ge, Sn and Pb elements of group 14 and hence only the
outermost p-electrons are involved.
The electrons in s orbital are much more tightly bound to the nucleus than
p-electrons. As we move down the group, the difference in energy level between s
sub-shell and p sub-shell becomes wider.
So if we use weak oxidizing agents, only 2-p electrons are removed. If we use a strong
oxidizing agent 2 s-electrons and 2-p electrons are all removed from the shell.
If the elements in group 14 form +2 ions, they will lose the p electrons leaving the
s-electrons pair unused. For example, to form Pb2+ ions lead will lose the two 6p
electrons but the 6s electrons will remain in its sub-energy level.
The inert pair effect shown in Pb2+ explains why the compounds of lead are
predominantly ionic
Carbon uses:
• As a component of fuel for combustion as charcoal or coal.
• As the main component of crude oil and its derivatives used in our everyday
life such: fuel, plastics, etc…
• As good chemical reducing agent used in extraction of metals (metallurgy).
• As a lubricant in moving parts of machines, to make electrodes, in lead
pencils when mixed with clay.
• Carbon isotope, C-14 isotope is used in archaeological dating.
• Diamond is used to make glass cutters, drilling devices and as abrasive for
smoothing hard materials as precious gemstone in jewelry and ornamental
objects; it is also a precious stone appreciated in jewelry.
Silicon uses:
• Silicon is used as a semi-conductor in transistors in electrical gadgets such
as radios, computers, amplifiers etc..
• Silicon in form of silicates is used in ceramics and in glass production.
• Silicon is also used in medicine to make silicone implants.
• Many rocks that we use for building our houses and other buildings are
Silicates.
• Ferrosilicon alloy is used as a deoxidizer in steel manufacture.
• Silicon dioxide can be used to produce toothpastes and in semiconductors;
silicon dioxide is the main component of sand, a raw material in the
manufacture of glass.
Germanium uses:
• Germanium being a metalloid, is used in transistors in electrical gadgets
such radios, computers, amplifiers etc..
Tin uses:
• Tin is used in plating steel sheets to resist corrosion; it is used for example
to make tinned cans to avoid the corrosion of the materials which are incontact with an acid medium.
9.7.1.The diagonal relationship in groups 1 & 2, 13 &14 elements
Diagonal relationships are similarities between pairs of elements in different
groups which are adjacent to one another in the second and the third period of the
periodic table.
These pairs are in Groups 1 and 2(Li/Mg), Groups 2 and 13(Be/Al) and Groups 13
and 14(B/Si). They exhibit similar properties; for example, boron and silicon are both
semi-conductors, they form halides that are hydrolyzed in water and have acidicoxides.
• Beryllium and aluminium have an appreciable covalent character of compounds
(e.g. the chlorides are predominantly covalent).
9.7.4. Diagonal relationship between Boron and Silicon
Due to its small size and similar charge/mass ratio, boron differs from other group 13
members, but it closely resembles silicon, the second element of group 14 to exhibit
diagonal relationship. Some important similarities between boron and silicon are
given below:
• Both boron and silicon are typical non-metals that exist as non-metallic giant
9.8. End unit assessment
I: Fill in the following statements with a missing word:
1. The arrangement of atoms in diamond structure is called………………..
2…………………..is the only element of group 14 whose chloride does not
hydrolyse in water.
3…………………is a semi-metallic element of group 14 whose oxide reacts with
HF acid only
4…………………is the only element of group 14 that does not exist in various
allotropic forms.
5………………….is the only element of group 14 whose compounds in the
oxidation state of +2 is more stable than that of +4.
II. Answer the following questions:
6. Write the equations for the reaction of decomposition of:
a) Lead (II) hydroxide
b) Tin tetrachloride
7. Explain the amphoteric character of tinby using appropriate equations of
reaction.
8. Discuss the stability of +2 oxidation state as you move down in group 14
elements.
9. Explain the reason why the melting and boiling points of group 14 elementsdecrease down the group.
