GROUP 4A ELEMENTS

GROUP IVA ELEMENTS

Group IVA of the periodic table comprises elements, carbon, silicon, germanium, tin and There is a marked change of properties from carbon to lead. This gradation is of such Nature, however, that there are very few similarities between the extreme members. Thus carbon And lead differ very considerably; far more, for example, than do lithium and caesium in group IA. The electronic configurations of IVA group elements along with their physical properties areGiven in Table 3.4.

1. Carbon and silicon are the only Non-metals in Group IVA. Carbon has The peculiar property of forming longCarbon chains, silicon forms long chains Of alternating silicon and oxygen atoms.

2. Carbon and silicon both form Acidic oxides, whereas the oxides of Germanium, tin and lead are amphoteric In nature. Both carbon and silicon form Covalent bonds. Their oxides are acidic And both form hydrides and chlorides.

The elements of group IVA are characterized by a set of four valence electrons, which Form two pairs.

In the first three elements of IVA group, carbon, silicon and germanium, all the four Outermost electrons are used as valency electrons, while in tin and lead either all four (stannic and Plumbic compounds) or only one of the pairs of electrons (stannous and plumbous compound) is Used for bonding.

The pair of valence electrons that do not readily take part in chemical combination is Termed as inert pair. As in other groups, the inert pair effect is most marked in the element of Highest atomic mass, namely lead. The increase in electropositive character from carbon through Silicon, tin, and lead is pronounced. This trend is shown also by the increase in the metallic Character of the elements with increased atomic mass.

Common Properties of Group IVA Elements:

1. All the elements of this group show a valency of four.

2. All of them form hydrides, MH,.

3. They form tetrachlorides, MCI,.

4. They also form the dioxides, MO,.

3.4.1 Occurrence of Carbon

Carbon occurs naturally in two states. One is crystallline (graphite, diamond) form and The other is amourphous (coal, charcoal) form.

3.4.2 Occurrence of Silicon

Silicon is very abundant, about 25% of The mass of the Earth’s crust being due to this element. Silicon, unlike carbon, is not found in Free state. Silicon is found as a major constituent

Of rocks either in the form of silica or silicates. Most minerals other than sulphides, sulphates, Phosphates and carbonates contain a high proportion of silicon.

As oxide, it is found as quartz in the following forms:

• Rock crystal, amethyst quartz, smoky quartz, rose quartz and milky quartz.

• Sand is largely silicon dioxide (silica). Opal is a hydrated variety of quartz.

3.4.3 Peculiar Behaviour of Carbon

Carbon differs from the remaining members of Group IV-A in the following respects:

1. Carbon and silicon are nonmetals while the other members of the family are metalloids Or metals.

2. Catenation or self linkage. Carbon has a tendency to form long chains of identical atoms The type of linkage of identical atoms with each other is called catenation or self- Linkage. The property of catenation decreases on moving down the group from carbon to Lead. The maximum tendency of catenation associate with carbon forms the basis of the Carbon compounds which constitute organic chemistry.

COMPOUNDS OF CARBON AND SILICON

Structure of Oxides of Carbon

Three oxides of carbon are known:

1. Carbon monoxide, CO

2. Carbon dioxide, CO,

3. Carbon suboxide, C,O,

Out of these, the last one is of little importance.

Structure of Carbon Monoxide (CO)

Carbon monoxide is diatomic molecule having triple bond between the two atoms. It is Very slightly polar. The electronic structure of carbon monoxide can be represented as: It is usually written as: It might appear from the above structure that the molecule should have a large dipole Moment, but in fact the molecule has a small dipole moment (0.112D).

Structure of the Carbon Dioxide

Carbon dioxide exists in the gaseous state as Linear molecules. The observed C-O bond distance is 115 pm and is in agreement with the structure shown. Solid CO, has a face-centered cubic structure. Being Linear its dipole moment is zero.

3.5.2 Oxides of Silicon

Silicon Dioxide

Silicon dioxide, SiO, called silica, is probably the most common and the most important Compound of silicon. In silica every silicon atom is attached tetrahedrally to four oxygen atoms And each oxygen atom has two close silicon neighbours.

Vitreous silica possesses the following interesting and useful properties.

1. High transparency to light.

2. Very refractory, does not soften below 1500 to 1600°C.

3. Very low thermal expansion.

4. Excellent insulator.

5. Hard, brittle and elastic.

6. Insoluble in water and inerts towards many reagents.

7. It is resistant towards all acids except HF.

Quartz,

the common crystalline form of silicon dioxide, is a hard, brittle, refractor, colourless Solid which differs very markedly from carbon dioxide.Both carbon and silicon have four electrons in their valence shells and both also form Four covalent bonds. So, why should there be a big difference between CO, and SiO₂?

The answer lies in the fact that silicon atoms are much larger than carbon atoms and thus Tend to surround themselves with more oxygen neighbours; silicon forms only single bonds with Oxygen atoms whereas carbon may form double bonds. Carbon, in fact, forms double bonds with Each of the two oxygen atoms to produce a small, symmetrical, linear molecule CO,,which is Volatile and reasonably reactive.

The silicon atom can be approached closely by four oxygen atoms and forms a single Bond with each at tetrahedral angles. This structure can be continued in three dimensions to Produce a continuous giant silicon oxygen network extending out to give the massive silicon Dioxide crystal.

In the interior of the silica networkE silicon atom is bonded tetrahedrally With four oxygen atoms and every oxygen Atom is bonded to two silicon atoms. The Overall ratio of silicon to oxygen atoms is 1:2 And the simplest formula for silica therefore Is SiO,. The silicon oxygen bonds are strong And keep the atoms firmly in place.

Note, however, that it is not the Molecular formula for silica but the whole Chunk of silica must be considered to be Essentially one molecule.