Question

What is the electron configuration of an element with atomic number 20?

Answer 1

1s2,2s2.2p6,3s2,3p6,3d4,4s2

Answer 2

Answer: $1s^22s^22p^63s^23p^64s^2$

Explanation: The element with atomic number 20 consists 20 electrons and  as atomic number is specific to an element, atomic number is of element calcium.

The nearest noble gas to calcium is argon with 18 electrons with configuartion $1s^22s^22p^63s^23p^6$ according to afbau's rule, in which orbitals are arranged in order of increasing energies.

Thus electronic configuartion of calcium in terms of noble gas configuration is:

$Ca:20:[Ar]4s^2$