Page 106 - Chemistry ICSE Class IX

94 ICSE Chemistry – 9
ƒ Each orbit or shell corresponds to a de nite energy. Therefore, these
circular orbits are also known as energy levels or energy shells.
ƒ The orbits or energy levels are characterised by an integer n, where
n can have values 1, 2, 3, 4... The integer n ( = 1, 2, 3,...) is called the
quantum number of the respective orbit. The orbits are numbered
as 1, 2, 3, 4,... etc. starting from the nucleus side. Thus, the orbit for
which n = 1 is the lowest energy level.

Fig. 4.11 Energy levels around the The orbits corresponding to n = 1, 2, 3, 4,... etc. are also designated as
nucleus of an atom K, L, M, N,... etc. shells (Fig. 4.11). When the electron is in the lowest energy
level, it is said to be in the ground state.
Since, electrons can be present only in these orbits, hence, these
electrons can only have energies corresponding to these energy levels, i.e.,
electrons in an atom can have only certain permissible energies.

How are the electrons arranged in an atom: the Böhr-Bury
scheme
According to the Böhr’s atomic model, the electrons revolve around the
Neils Böhr
;ϭϴϴϱʹϭϵϲϮͿ nucleus in certain orbits. These electrons are distributed in various orbits
according to a scheme proposed by Böhr and Bury. The Böhr and Bury
ƂŚƌ ǁĂƐ ďŽƌŶ ŝŶ ŽƉĞŶŚĂŐĞŶ͕
ĞŶŵĂƌŬ ĂŶĚ ƐƚƵĚŝĞĚ ƉŚLJƐŝĐƐ Ăƚ ƚŚĞ scheme gives the following rules for the distribution of electrons in various
hŶŝǀĞƌƐŝƚLJ ŽĨ ŽƉĞŶŚĂŐĞŶ͘ orbits:
ƚ ƚŚĞ ĂŐĞ ŽĨ ϮϮ͕ ŚĞ ǁŽŶ Ă ŐŽůĚ ƒ The maximum number of electrons which can be accommodated in
ŵĞĚĂů ŽĨ ƚŚĞ ĂŶŝƐŚ ĐĂĚĞŵLJ ŽĨ a given orbit is given by the formula 2n , where n is the quantum
2
^ĐŝĞŶĐĞƐ͘ ŌĞƌ ƌĞĐĞŝǀŝŶŐ ŚŝƐ WŚ͘ ͘ number of the orbit. Thus,
ĚĞŐƌĞĞ ŝŶ ϭϵϭϭ͕ ŚĞ ǁĞŶƚ ƚŽ ŶŐůĂŶĚ
ĂŶĚǁŽƌŬĞĚďƌŝĞŇLJĂƚ ĂŵďƌŝĚŐĞ Designation of the orbit Maximum number of electrons (2n )
2
hŶŝǀĞƌƐŝƚLJ ĂŶĚ ƚŚĞŶ ǁĞŶƚ ƚŽ ƚŚĞ n
2
hŶŝǀĞƌƐŝƚLJ ŽĨ DĂŶĐŚĞƐƚĞƌ ƚŽ ǁŽƌŬ 1 K 2 × 1 = 2
ǁŝƚŚ ZƵƚŚĞƌĨŽƌĚ͘ ,Ğ ĚĞǀĞůŽƉĞĚ 2
ƚŚĞ ĮƌƐƚ ĐŽŶƐŝƐƚĞŶƚ ĞdžƉůĂŶĂƟŽŶ ŽĨ 2 L 2 × 2 = 8
ĂƚŽŵŝĐ ƐƚƌƵĐƚƵƌĞ ŝŶ ϭϵϭϯ͘ ,Ğ ǁĂƐ 3 M 2 × 3 = 18
2
ĂǁĂƌĚĞĚ ƚŚĞ EŽďĞů WƌŝnjĞ ŝŶ WŚLJƐŝĐƐ
2
ŝŶ ϭϵϮϮ͘ 4 N 2 × 4 = 32
and so on.
ƒ The outermost orbit can have at the most 8 electrons and the next
inner to the outermost (penultimate) can have at the most 18 electrons
2
(if permissible in accordance with 2n rule).
ƒ It is not always necessary to ll up an orbit completely before starting
the next higher orbit.

How to write the electronic configuration of an element
The electronic con guration of an element is de ned as follows:
A step-wise distribution of electrons in different orbits (energy
NGXGNU QH CP CVQO KU ECNNGF KVU GNGEVTQPKE EQPſIWTCVKQP
To write the electronic con guration of an element, one should
proceed as follows:
ƒ Find out the total number of electrons present in an atom of that
element.
ƒ Fill these electrons in the various orbits (shells) one by one in the
2
Fig. 4.12 Electronic con¿guration of order of increasing energy according to the 2n rule, i.e., the shell
lithium