Question

The valence shell of the element X contains two electrons in a $5s$ subshell. Below that shell, element X has a partially filled $4d$ subshell. What type of element is X?

Answer 1

The filling of electrons is done on the basis of rules namely Aufbau rule, Hund's rule, Paulies exclusion principle, Bohor bury rule. Iff the filling of electrons violates any one of the above rules then the electronic configuration is said to be wrong however there may be some exceptions like chromium, copper, etc.

Complete answer:
The electrons are filled in the respective orbitals with the help of certain laws or rules which are as follows, Aufbau rule, Hund's rule, Paulies exclusion principle, Bohor bury rule. Let us discuss them one by one: -
The Afbau rule: - the electrons of an atom are filled on the basis of the increases in the subshells.
The Hund's rule: - the pairing of the electrons will only take place iff and only iff each orbital has single electrons present in it.
The Paulie rule: - the spin of electrons in an orbit is opposite to one another.
The Bohr bury rule: - the energy of a subshell is calculated by the formula $n + 1$ iff principle quantum number is different and $n + l$ iff both principal and azimuthal quantum numbers are different.
In the above case, $5s$ is filled prior to $4d$ because of the Bohor bury rule. But on arranging them $4d$ is written first. The element X is a transition element belonging to ${5^{th}}$ period. The conformation of period is done by the principal quantum number of any atom of the subshell filled in the last.

Note:
It is to remember that in an orbital only two electrons can be accommodated however a group of orbital is known as a sub-shell which accommodates different electrons. The s subshell can accommodate only two electrons whereas p subshell can accommodate six electrons, d subshell can accommodate ten electrons and f subshell can accommodate fourteen electrons.