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A neutral atom of an element has 2K, 8M, and 5K electrons. Find out the following? The atomic number of neutral atom:
A- 20
B- 18
C- 15
D- 25


Answer
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Hint: Try to figure out whether any electrons are in other shells or not. Calculate whether the element is having filled inner shells or not by calculating the maximum number of electrons in each shell.

Complete step by step answer:
We know that atoms consist of shells where electrons are accommodated. These shells have subshells in them. The number of subshells in a given shell is equal to the principal quantum number of the shell. Shells are named as K, L, M, N, and so on. Where K shell has n = 1, L shell has n = 2, M shell has n = 3 and N shell has n = 4.
Maximum number of electrons in each shell will be 2n2.
But the filling of electrons not only depends on the n value but also depends on the l value of the subshell. L values of subshells s, p, d, f are 0, 1, 2, 3 respectively. Electrons first enter the subshells having least n + l values. If n + l values of 2 subshells are the same then electrons first enter the subshell with the lowest n value. The configuration will be:
 1s, 2s, 2p, 3s, 3p, 4s, 3d, and so on.
Here electrons enter 4s subshell before entering 3d shell because 4s have lower n + l value. We need to check how many electrons the M shell can accommodate without an electron in the N-shell.
Number of electrons in 3s subshell = 2
Number of electrons in 3p subshell = 6.
Number of electrons M shell can accommodate without an electron in N-shell = 8
Given M-shell electrons are 5 which is less than 8. So there are no other shells filled by electrons. Total number of electrons = 2 + 8 + 5 = 15. Given the atom is neutral.
Hence, the correct answer is option C.

Note:
We cannot add electrons in the shells simply because sometimes there may be other shell electrons which are not mentioned. The number of electrons = number of protons for a neutral atom atomic number is equal to the number of protons.