Question

what is atomic number which contains 2 electrons in 4-orbit

Answer 1

atomic number = 2+8+8+2 =20

Answer 2

The atomic number is 58

Explanation:

• If there are 20 electrons in the 4th orbit then it suggests that the 4th orbit has electrons filled as 4s2 4p6 4d10 4f2, and as per the Aufbau principle and
• hunds rule the element would belong to the inner transition series and

• would have an electronic configuration as 1s2.2s2.2p6.3s2.3p6.4s2.3d10.4p6.5s2.4d10.5p6.6s2.4f2 or else simply [Xe] 4f2 6s2.

• But in the Lanthanum series, we don’t have an element that has this configuration.
• Because the electronic configuration of cerium is [Xe] 6s2 4f1 5d1 hence has only 19 electrons in its 4th orbit
• while the next element Praseodymium has an electronic configuration is  [Xe]6s2 4f3 5d1 hence it has 21 electrons in its 4th orbit.

• But still, if you only need the atomic number of the element it would be 58 i.e Cerium but as the energy difference between 4f and 5d is very small the electron from 4f enters 5d orbital and hence we don’t have any element which has an electronic configuration of 4f2
• hence under normal conditions and in the ground state, we have no element which has 20 electrons in its 4th orbit but in an excited state of +2, cerium has this electronic configuration hence in the excited state the element would be cerium with atomic number 58.