Page 65 - NCERT Science Class 10 English Medium

electronic configuration of the nearest noble gas,
helium, which has two electrons in its K shell. We can
depict this using dots or crosses to represent valence
electrons (Fig. 4.1).
The shared pair of electrons is said to constitute a
single covalent bond between the two hydrogen atoms.
A single covalent bond is also represented by a line
between the two atoms, as shown in Fig. 4.2.
Figure 4.1
Figure 4.1 The atomic number of chlorine is 17. What would be its electronic
Figure 4.1
Figure 4.1
Figure 4.1
A molecule of hydrogen configuration and its valency? Chlorine forms a diatomic molecule, Cl .
2
Can you draw the electron dot structure for this molecule? Note that
only the valence shell electrons need to be depicted.
In the case of oxygen, we see the formation of a double bond between
two oxygen atoms. This is because an atom of oxygen has six electrons
in its L shell (the atomic number of oxygen is eight) and it requires two
more electrons to complete its octet. So each atom of oxygen shares two
Figure 4.2
Figure 4.2
Figure 4.2
Figure 4.2
Figure 4.2
Single bond between electrons with another atom of oxygen to give us the structure shown in
two hydrogen atoms Fig. 4.3. The two electrons contributed by each oxygen atom give rise to
two shared pairs of electrons. This is said to constitute a double bond
between the two atoms.
Can you now depict a molecule of water showing the nature
of bonding between one oxygen atom and two hydrogen
atoms? Does the molecule have single bonds or double bonds?
What would happen in the case of a diatomic molecule of
nitrogen? Nitrogen has the atomic number 7. What would be
its electronic configuration and its combining capacity? In
order to attain an octet, each nitrogen atom in a molecule of
nitrogen contributes three electrons giving rise to three shared
pairs of electrons. This is said to constitute a triple bond
Figure 4.3
Figure 4.3
Figure 4.3 between the two atoms. The electron dot structure of N and
Figure 4.3
Figure 4.3
2
Double bond between its triple bond can be depicted as in Fig. 4.4.
two oxygen atoms A molecule of ammonia has the formula NH . Can you draw
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the electron dot structure for this molecule showing how all
four atoms achieve noble gas configuration? Will the molecule
have single, double or triple bonds?
Let us now take a look at methane, which is a compound
of carbon. Methane is widely used as a fuel and is a major
component of bio-gas and Compressed Natural Gas (CNG). It
is also one of the simplest compounds formed by carbon.
Methane has a formula CH . Hydrogen, as you know, has a
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valency of 1. Carbon is tetravalent because it has four valence
electrons. In order to achieve noble gas configuration, carbon
shares these electrons with four atoms of hydrogen as shown
Figure 4.4 in Fig. 4.5.
Figure 4.4
Figure 4.4
Figure 4.4
Figure 4.4
Triple bond between
two nitrogen atoms Such bonds which are formed by the sharing of an electron pair
between two atoms are known as covalent bonds. Covalently bonded
molecules are seen to have strong bonds within the molecule, but inter-
molecular forces are weak. This gives rise to the low melting and boiling

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