Group 14 ELEMENTS. THE CARBON FAMILY.

Question 1

Members.

Answer 1

Carbon, silicon, germanium, tin lead and flerovium.

Question 2

Abundance of Carbon.

Answer 2

Carbon is the seventeenth most abundant element by mass in the earth’s crust. It is widely distributed in nature in free as well as in the combined state. In elemental state it is available as coal, graphite and diamond; however, in combined state it is present as metal carbonates, hydrocarbons and carbon dioxide gas (0.03%).

Question 3

Isotopes of C.

Answer 3

Naturally occurring carbon contains two stable isotopes: 12C and C. In addition to these, third isotope. “C is also present. It is a radioactive isotope with half-life 5770 years and used for radiocarbon dating.

Question 4

Abundance of Si.

Answer 4

Silicon is the second (27.7% by mass) most abundant element on the earth’s crust and is present in nature in the form of silica and silicates. Silicon is a very important component of ceramics, glass and cement.

Question 5

Germanium.

Answer 5

Germanium exists only in traces.

Question 6

Tin.

Answer 6

occurs mainly as cassiterite, SnO2 and lead as galena, PbS

Question 7

Flerovium

Answer 7

Flerovium is synthetically prepared radioactive element

Question 8

Ge, Si

Answer 8

Ultrapure form of germanium and silicon are used to make transistors and semiconductor devices

Question 9

Flavorium.

Answer 9

Symbol of Flerovium is Fl. It has atomic number 114, atomic mass 289 gmol and electronic configuration [Rn] 5f¹46d107s² 7p². It has been prepared only in small amount. Its half life is short and its chemistry has not been established yet.

Question 10

Electronic Configuration

Answer 10

The valence shell electronic configuration of these elements is ns²np². The inner core of the electronic configuration of elements in this group also differs

Question 11

Covalent Radius

Answer 11

There is a considerable increase in covalent radius from C to Si, thereafter from Si to Pb a small increase in radius is observed. This is due to the presence of completely filled d and f orbitals in heavier members.

Question 12

Ionization Enthalpy

Answer 12

The first ionization enthalpy of group 14 members is higher than the corresponding members of group 13. The influence of inner core electrons is visible here also. In general the ionisation enthalpy decreases down the group. Small decrease in iH from Si to Ge to Sn and slight increase in iH from Sn to Pb is the consequence of poor shielding effect of intervening d and f orbitals and increase in size of the atom.

Question 13

Electronegativity

Answer 13

Due to small size, the elements of this group are slightly more electronegative than group 13 elements. The electronegativity values for elements from Si to Pb are almost the same.

Question 14

Physical Properties

Answer 14

All members of group 14 are solids. Carbon and silicon are non-metals, germanium is a metalloid, whereas tin and lead are soft metals with low melting points. Melting points and boiling points of group 14 elements are much higher than those of corresponding elements of group 13.

Question 15

Oxidation states and trends in chemical reactivity

Answer 15

The group 14 elements have four electrons in outermost shell. The common oxidation states exhibited by these elements are +4 and +2. Carbon also exhibits negative oxidation states. Since the sum of the first four ionization enthalpies is very high, compounds in +4 oxidation state are generally covalent in nature. In heavier members the tendency to show +2 oxidation state increases in the sequence Ge<Sn<Pb. It is due to the inability of ns² electrons of valence shell to participate in bonding. The relative stabilities of these two oxidation states vary down the group. Carbon and silicon mostly show +4 oxidation state. Germanium forms stable compounds in +4 state and only few compounds in +2 state. Tin forms compounds in both oxidation states (Sn in +2 state is a reducing agent). Lead compounds in +2 state are stable and in +4 state are strong oxidising agents.

In tetravalent state the number of electrons around the central atom in a molecule (e.g., carbon in CCI) is eight. Being electron precise molecules, they are normally not expected to act as electron acceptor or electron donor species. Although carbon cannot exceed its covalence more than 4. other elements of the group can do so. It is because of the presence of d orbital in them. Due to this, their halides undergo hydrolysis and have tendency to form complexes by accepting electron pairs from donor species. For example, the species like,
SiF6²-, [GeCl6]²- [Sn(OH)6]²- exist where the hybridisation of the central atom is sp³d²

Question 16

Reactivity Towards oxygen.

Answer 16

All members when heated in oxygen form oxides. There are mainly two types of oxides, i.e., monoxide and dioxide of formula MO and MO₂ respectively. SiO only exists at high temperature. Oxides in higher oxidation states of elements are generally more acidic than those in lower oxidation states. The dioxides - CO₂, SiO2 and GeO₂ are acidic, whereas SnO2 and PbO2 are amphoteric in nature. Among monoxides, CO is neutral, GeO is distinctly acidic whereas SnO and PbO are amphoteric.

Question 17

Reactivity towards water

Answer 17

Carbon, silicon and germanium are not affected by water. Tin decomposes steam to form dioxide and dihydrogen gas.

Sn+2H₂O ➡️SnO2 + 2H2

Lead is unaffected by water, probably because of a protective oxide film formation.

Question 18

Reactivity towards halogen

Answer 18

These elements can form halides of formula MX, and MX4 (where X = F, Cl, Br, I). Except carbon, all other members react directly with halogen under suitable condition to make halides. Most of the MX4, are covalent in nature The central metal atom in these halides undergoes sp³ hybridisation and the molecule is tetrahedral in shape. Exceptions are Snf4 and PbF4 , which are ionic in nature. Pbl4, does not exist because Pb-I bond initially formed during the reaction does not release enough energy to unpair 6s² electrons and excite one of them to higher orbital to have four unpaired electrons around lead atom. Heavier members Ge to Pb are able to make halides of formula MX2. Stability of dihalides increases down the group. Considering the thermal and chemical stability, GeX4, is more stable than GeX2 whereas PbX2, is more than PbX4,. Except CCl4 other tetrachlorides are easily hydrolysed by water because the central atom can accommodate the lone pair of electrons from oxygen atom of water molecule in d orbital.

Question 19

IMPORTANT TRENDS AND. BEHAVIOUR OF C.

Answer 19

Like first member of other groups, carbon also differs from rest of the members of its group. It is due to its smaller size, higher electronegativity, higher ionisation enthalpy and unavailability of d orbitals.

In carbon, only s and p orbitals are available for bonding and, therefore, it can accommodate only four pairs of electrons around it. This would limit the maximum covalence to four whereas other members can expand their covalence due to the presence d orbitals.
Carbon also has unique ability to form ρπ- ρπ multiple bonds with itself and with other atoms of small size and high electronegativity. Few examples of multiple bonding are: C=C, C=C, C=O, C = S, and C = N. Heavier elements do not form pπ-pπ bonds because their atomic orbitals are too large and diffuse to have effective overlapping.

Carbon atoms have the tendency to link with one another through covalent bonds to form chains and rings. This property is called catenation. This is because C-C bonds are very strong. Down the group the size increases and electronegativity decreases, and, thereby, tendency to show catenation decreases. This can be clearly seen from bond enthalpies values. The order of catenation is C&raquo_space; Si > Ge= Sn. Lead does not show catenation.
Due to property of catenation and pπ- pπ bond formation, carbon is able to show allotropic forms.

Question 20

Diamond

Answer 20

It has a crystalline lattice. In diamond each carbon atom undergoes sp³ hybridisation and linked to four other carbon atoms by using hybridised orbitals in tetrahedral fashion.
The C-C bond length is 154 pm. The structure extends in space and produces a rigid three- dimensional network of carbon atoms. In this structure ( directional covalent bonds are present throughout the lattice.

It is very difficult to break extended covalent bonding and, therefore, diamond is a hardest substance on the earth. It is used as an abrasive for sharpening hard tools, in making dyes and in the manufacture of tungsten filaments for electric light bulbs.

Question 21

Graphite.

Answer 21

Graphite has layered structure, Layers are held by van der Waals forces and distance between two layers is 340 pm.
Each layer is composed of planar hexagonal rings of carbon atoms, C-C bond length within the layer is 141.5 pm. Each carbon atom in hexagonal ring undergoes sp² hybridisation and makes three sigma bonds with three neighbouring carbon atoms. Fourth electron forms a π bond. The electrons are delocalised over the whole sheet. Electrons are mobile and, therefore, graphite conducts electricity along the sheet. Graphite cleaves easily between the layers and, therefore, it is very soft and slippery. For this reason graphite is used as a dry lubricant in machines running at high temperature, where oil cannot be used as a lubricant.

Question 22

Fullerenes

Answer 22

Fullerenes are made by the heating of graphite in an electric arc in the presence of inert gases such as helium or argon.
The sooty material formed by condensation of vapourised Cñ small molecules consists of mainly C 60 with smaller quantity of C70 and traces of fullerenes consisting of even number of carbon atoms up to 350 or above.
Fullerenes are the only pure form of carbon because they have smooth structure without having ‘dangling’ bonds.
Fullerenes are cage like molecules. C60 molecule has a shape like soccer ball and called Buckminsterfullerene

Question 23

Buckminsterfullerene

Answer 23

It contains twenty six-membered rings and twelve five-membered rings. A six membered ring is fused with six or five membered rings but a five membered ring can only fuse with six membered rings.
All the carbon atoms are equal and they undergo sp² hybridisation. Each carbon atom forms three sigma bonds with other three carbon atoms. The remaining electron at each carbon is delocalised in molecular orbitals, which in turn give aromatic character to molecule. This ball shaped molecule has 60 vertices and each one is occupied by one carbon atom and it also contains both single and double bonds with C-C distances of 143.5 pm and 138.3 pm respectively. Spherical fullerenes are also called bucky balls.

Question 24

Allotropes of C.

Answer 24

graphite is thermodynamically most stable allotrope of carbon and, therefore, ^H of graphite is taken as zero. ^H values of diamond and fullerene, C60 are 1.90 and 38.1 kJ mol, respectively.

Other forms of elemental carbon like carbon black, coke, and charcoal are all impure forms of graphite or fullerenes. Carbon black is obtained by burning hydrocarbons in a limited supply of air. Charcoal and coke are obtained by heating wood or coal respectively at high temperatures in the absence of air.