Question

Why Does Period 5 Have 18 Elements?

Answer 1

Periodic table: Periodic table is the arrangement of elements in the increasing order of their atomic number in the horizontal rows known as periods and vertical columns known as groups.

Calculating the number of elements in period 5:

For period 5

• Principal quantum number, n=5, and azimuthal quantum number values, l = 0, 1, 2, 3, 4
• According to the Aufbau rule, the filling of orbitals in sub-shells occurs in the increasing order of energy.
• Since the energy of the 4f sub-shell is larger than that of the 5s sub-shell, the sub-shells that can be filled are 5s, 4d, and 5p.
• Therefore, total no. of orbitals = 1 + 5 + 3 = 9
• Hence, total no. of electrons that can be accommodated in 9 orbitals $=2\times 9=18$

Hence, the no. of electrons or elements in the 5th period is 18.