Page 53 - Understanding NCERT Science 09

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formula 2n , where ‘n’ is the orbit • The composition of atoms of the first
number or energy level index, 1,2,3,…. eighteen elements is given in Table 4.1.
Hence the maximum number of
electrons in different shells are as uestions
follows: 1. Write the distribution of electrons
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first orbit or K-shell will be = 2 × 1 = 2, in carbon and sodium atoms.
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second orbit or L-shell will be = 2 × 2 = 8, Q 2. If K and L shells of an atom are
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third orbit or M-shell will be = 2 × 3 = 18, full, then what would be the total
fourth orbit or N-shell will be = 2 × 4 2 number of electrons in the atom?
= 32, and so on.
(ii) The maximum number of electrons that
can be accommodated in the outermost
orbit is 8. 4.4 Valency
(iii) Electrons are not accommodated in a We have learnt how the electrons in an atom
given shell, unless the inner shells are are arranged in different shells/orbits. The
filled. That is, the shells are filled in a electrons present in the outermost shell of
step-wise manner. an atom are known as the valence electrons.
Atomic structure of the first eighteen From the Bohr-Bury scheme, we also know
elements is shown schematically in Fig. 4.4. that the outermost shell of an atom can

Fig.4.4: Schematic atomic structure of the first eighteen elements

Activity ______________ 4.2 accommodate a maximum of 8 electrons. It
was observed that the atoms of elements,
completely filled with 8 electrons in the
• Make a static atomic model displaying outermost shell show little chemical activity.
electronic configuration of the first In other words, their combining capacity or
eighteen elements. valency is zero. Of these inert elements, the

42 SCIENCE

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