Question

How would you justify the presence of \(18\) elements in the \(5^{th}\) period of the periodic Table?

Answer 1

\(5^{th}\) period of the periodic table corresponds to \(n = 5.\) It involves the filling of one \(5s,\) five \(4d\) and three \(5p\) orbitals. But in the fifth period, \(5f\) and \(5d\) orbitals are not filled.
For \(5^{th}\) period, principal quantum number, \(n=5\) and values of azimuthal quantum number, \(l = 0,1,2,3\)

According to Aufbau principle, filling of electrons in subshells take place in increasing order of energy of subshell. The sub shells available for filling are \(5s, 4d, 5p.\)

The total number of orbitals available \(= 5s (1) + 4d (5) + 5p (3) =1 + 5 + 3 = 9\)

We know, \(\text{ ''One orbital can accommodate maximum of 2 electrons"}\).

Therefore, total number of electrons that can be accommodated in \(9\) orbitals

\(= 2\times 9 = 18 𝑒^{-}\)

Final answer : Hence, \(18\) elements are there in the \(5^{th}\) period.