11.5 Group 14 Elements: The Carbon Family

Carbon, silicon, germanium, tin lead and flerovium are the members of group 14. 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%) in air.

One can emphatically say that carbon is the most versatile element in the world. Its combination with other elements such as dihydrogen, dioxygen, chlorine and sulphur provides an astonishing array of materials ranging from living tissues to drugs and plastics.

Organic chemistry is devoted to carbon containing compounds. It is an essential constituent of all living organisms. Naturally occurring carbon contains two stable isotopes:12C and 13C.

In addition to these, third isotope, 14C is also present. It is a radioactive isotope with half-life 5770 years and used for radiocarbon dating. 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. Germanium exists only in traces. Tin occurs mainly as cassiterite, SnO2 and lead as galena, PbS.

Flerovium is synthetically prepared radioactive element Ultrapure form of germanium and silicon are used to make transistors and semiconductor devices. Symbol of Flerovium is Fl.

It has atomic number 114, atomic mass 289 gmol-1 and electronic configuration [Rn] 5f 146d10 7s2 7p2. It has been prepared only in small amount.

Its half life is short and its chemistry has not been established yet. The important atomic and physical properties along with their electronic configuration of the elements of group 14 leaving flerovium are given in Table.

 

11.5.1 Electronic Configuration

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

 

11.5.2 Covalent Radius

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.

 

11.5.3 Ionization Enthalpy

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.

 

11.5.4 Electronegativity

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.

 

11.5.5 Physical Properties

All members of group14 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.

 

11.5.6 Chemical Properties

Oxidation states and trends in chemical reactivity

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 CCl4) 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, SiF6 2–, [GeCl6]2–, [Sn(OH)6]2– exist where the hybridisation of the central atom is sp3d2.

 

Reactivity towards oxygen

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.

 

Reactivity towards water

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

Sn + H2O → SnO2 + 2H2

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

 

Reactivity towards halogen

These elements can form halides of formula MX2 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. PbI4 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. Hydrolysis can be understood by taking the example of SiCl4.

It undergoes hydrolysis by initially accepting lone pair of electrons from water molecule in d orbitals of Si, finally leading to the formation of Si(OH)4 as shown below:

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