Each electron has a definite energy which fixes the orbital in which it moves

# Each electron has a definite energy which fixes the

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Each electron has a definite energy, which fixes the orbital in which it moves. Each electron orbital is called a quantum state with a set of quantum numbers as follows: N = 1, 2, 3, 4, 5, 6 Shell = K, L, M, N, O, P NUMBER OF ELECTRONS IN AN ATOM Electrons moving around the nucleus are distributed in different shells as per the following rules: (i) The maximum number of electrons which can be present in a shell is 2 n 2 , when: n = 1, the 1 st (inner-most) shell can have at the most 2 x 1 2 = 2 electrons n = 2, the 2 nd shell can have at the most 2 x 2 2 = 8 electrons n = 3, the 3 rd shell can have at the most 2 x 3 2 = 18 electrons n = 4, the 4 th shell can have at the most 2 x 4 2 = 32 electrons (ii) The last/outer-most shell cannot have more than 8 electrons and the shell immediately preceding it cannot have more than 18 electrons. The shells numbering 1, 2, 3, 4, 5, and 6 are designated as K, L, M, N, O and P. With the present concept of atomic model, the maximum number of electrons does not hold good beyond N-shell. Outer-most main shell never has more than 8 electrons. The shell becomes stable as soon as the number becomes 8 even though the sum of its sub- shell is more than 8. The next electron to be added starts to fill the next main shell, taking position in its S sub- shell. The inner-incomplete main shell goes on filling up until all spaces in this sub-shell have been filled. ELECTRONS IN SUB-SHELLS The sub-shells in each main shell are designated by small letters s , p , d, f, g, and h with quantum number l = 0, 1, 2, 3, 4, and 5 respectively. The number of sub-shells in a given main shell is the same as its principal quantum number, n . The number of electrons in a sub-shell is given by the formula: Number of electrons in a sub-shell, where l = number of sub-shell = 2 (2 l + 1).
~ Page 3 of 79 ~ Therefore, number of electrons when l = 0 for the 1 st sub-shell s = 2 (2 x 0 + 1) = 2 when l = 1 for the 2 nd sub-shell p = 2 (2 x 1 + 1) = 6 when l = 2 for the 3 rd sub-shell d = 2 (2 x 2 + 1) = 10 when l = 3 for the 4 th sub-shell f = 2 (2 x 3 + 1) = 14 Hence the total number of electrons in a main shell can be obtained by adding the electron capacities of sub-shells as below: Main shell Sub-shell Total number of electrons s p d f K L M N 2 2 2 2 - 6 6 6 - - 10 10 - - - 14 2 8 18 32 The sub-shells are further designated as per the energy levels: Sub-shells 1 s , 2 s , 3 s , 4 s of the first, second, third and fourth energy levels respectively. Sub-shells 2 p , 3 p , 4 p of the second, third and fourth energy levels respectively. Sub-shells 3 d , 4 d , 5 d of the third, fourth and fifth energy levels respectively. Sub-shells 4 f , 5 f of the fourth and fifth energy levels respectively. The sequence of filling up the electrons in the sub-shells (orbital) is governed in the following order. Principal Energy Level Max. No. of Electrons (-2 n 2 ) Sub-shell/Electrons First orbit ( n = 1) Second orbit ( n = 2) Third orbit ( n = 3) Fourth orbit ( n = 4) 2 8 18 32 1 s : containing maximum 2 electrons 2 s : containing maximum 2 electrons 2 p : containing maximum 6 electrons 3 s : containing maximum 2 electrons 3 p : containing maximum 6 electrons 3 d

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