Question

Which element has the lowest first ionization energy?

Answer 1

The element that has the lowest ionization energy will belong to period 6 and the group number will be 1. The atomic number of the element will be 55 so, the energy to extract the last electron will be the least.

Complete answer:
When the last electron of any element then there is a requirement of energy because the electrons are held by nuclear forces to the nucleus of the atom, so this energy must be greater than this nuclear force, this energy is known as ionization energy. If the electron can be easily extracted from the element then the element has low ionization energy and if the electron cannot be easily extracted from the element then the element has high ionization energy.
So, the trend seen in the periodic table is when we move from left to right in the periodic table then the ionization energy increases because the nuclear attraction increases but when we move down the group then the ionization energy decreases because the number of shells increases.
The element that has the lowest ionization energy will belong to the period 6 and group number will be 1 because there is only one electron in the valence shell of group 1 which can be easily removed. So the element is Cesium and its atomic number is 55. Its electronic configuration is:
$1{{s}^{2}}2{{s}^{2}}2{{p}^{6}}3{{s}^{2}}3{{p}^{6}}3{{d}^{10}}4{{s}^{2}}4{{p}^{6}}4{{d}^{10}}5{{s}^{2}}5{{p}^{6}}6{{s}^{1}}$

Note:
The elements that will have very high ionization energy are the noble gas group element because their shells are fully filled and they are inert, and the fluorine element of halogens also has very high electronegativity.