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CBSE Notes 2011-2012

CARBON AND ITS COMPOUNDS-CLASS 10 CHEMISTRY

INTRODUCTION :
I know that this chapter will be kind of difficult for some of you because we are learning something very different.It is not like the first chapters we have learned.But if you give a mind that "i can try it" .....this chapter is very interesting

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Here are some important notes for you

CARBON

* "C" - symbol
* no:of electrons-6
* no:of neutron-6
* no:of proton-6
* all mass =neutron+proton
=12u
*atomic no: =no:of proton
=6
*atomic configuration
=k,l
2,4
*valence shell=l
*valence electron=4
*carbon belongs to: group-14
period-2
*covalent bonding
*tetravalent

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Carbon has four electrons in its outermost shell and needs to gain or lose four electrons to attain noble gas configuration. If it were to gain or lose electrons –

(i) It could gain four electrons forming C4– anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons, that is, four extra electrons.

(ii) It could lose four electrons forming C4+ cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons.

Carbon overcomes this problem by sharing its valence electrons with other atoms of carbon or with atoms of other elements. Not just carbon, but many other elements form molecules by sharing electrons in this manner. The shared electrons ‘belong’ to the outer shells of both the atoms and lead to both atoms attaining the noble gas configuration.

This type of bond formed by sharing of electrons is called covalent bond.

1. Covalent bond: A covalent bond is a bond formed by sharing of
electrons between atoms. In a covalent bond, the shared pair of
electrons belongs to the valence shell of both the atoms.

2. Conditions for formation of covalent bond:
a. The combining atoms should have 4 to 7 electrons in their valence
shell.
b. The combining atoms should not lose electrons easily.
c. The combining atoms should gain electrons readily.
d. The difference in electronegativities of two bonded atoms should be
low.

3. Properties of covalent compounds:
a. Physical states: They are generally liquids or gases. Some covalent
compounds may exist as solids.
b. Solubility: They are generally insoluble in water and other polar
solvents but soluble in organic solvents like benzene, toluene etc.
c. Melting and boiling point: They generally have low melting and boiling
points.
d. Electrical conductivity: They do not conduct electrical current.

Covalently bonded molecules are seen to have strong bonds within the molecule, but intermolecular forces are small. This gives rise to the low melting and boiling points of these compounds. Since the electrons are shared between atoms and no charged particles are formed, such covalent compounds are generally poor conductors of electricity.

Carbon forms a large number of compounds because of two unique
properties:
a. Tetravalency
b. Catenation

Tetravalency of carbon:
Atomic number = 6
Electronic configuration: 2, 4
Valence electrons = 4
Valency = 4

So, carbon needs four electrons to attain noble gas configuration.
Or in other words, carbon has the ability to form four bonds with
carbon or atoms of other mono-valent elements.

Catenation: Carbon has the unique ability to form bonds with other
atoms of carbon, giving rise to large molecules. This
property is called catenation.

ALLOTROPES: The phenomenon of existence of an element in 2 or more
forms which have different physical properties but
identical chemical properties are called allotropes.

CARBON OCCUR IN 3 ALLOTROPIC FORMS

a) diamond
In diamond, each carbon atom is bonded to four other carbon atoms by a single covalent bond. This is why a diamond is a rigid, compact, three dimensional structure and is very hard to break. It is a bad conductor of heat and electricity, as there are no free electrons present in it.

b) GRAPHITE
Graphite contains carbon atoms in hexagonal rings and the rings in turn are joined together to form a layer. A series of such layers are piled one over the other.The layers of carbon can slide over each other, because there are no b covalent bonds between the atoms of two adjacent layers. Since graphite contains free electrons, it is a good conductor of heat and electricity. Lead pencils contain graphite encased in a wooden covering.

C)FULLERENES
Fullerenes were discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley for which they were awarded the Nobel Prize for chemistry for the year 1996. A fullerene molecule consists of 60 carbon atoms arranged in pentagons and hexagons, like in a standard football. Fullerenes are also called Buckminsterfullerenes as they are shaped like the geodesic dome designed and built by the US architect Buckminster Fuller.
fullerenes, buckminster fullerene..

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