Question

Among the following, the energy of 2s orbital is lowest in:
(A) K
(B) Na
(C) Li
(D) H

Answer 1

Hint:. To solve this question we should know the atomic number of the elements mentioned in the option. The main concept behind this question is that nuclear effective charge and screening effects are very important .

Complete step by step answer:
The modern periodic law states that “The properties of the elements are a periodic function of their atomic number”. According to this law the elements in the periodic table are arranged in increasing order of the atomic number.
We should know three important terms to understand the concept better.
-The atomic number of an element can be defined as the number of protons present in the nucleus of the atom. Atomic number is denoted by Z.
-The atomic radius is the nucleus of an atom and the electron cloud of the outermost shell.
-Nuclear effective charge is the magnitude of the force of attraction experienced by the valence electrons that is exerted by the nucleus of the atom. It is denoted by ${Z_{eff}}$.
Now, let’s see the relationship between atomic number, atomic radius and nuclear effective charge of an atom or element:
With the increase in the atomic number, the number of shells around the nucleus will also increase Therefore the distance between the nucleus and the valence electron increases in the same trend, that is atomic radius will also increase in the same trend.
The nuclear effective charge decreases as there is an increase in atomic number because there will increase in atomic radius.
The atomic number of K, Na, Li and H are 19,11,3 and 1 respectively.
The order of increasing atomic number: 1 < 3 < 11< 19
                                                                 H < Li < Na < K

So, the increasing order of nuclear effective charge: K < Na < Li < H
Now, let’s see the relationship between the energy of an orbital and nuclear effective charge. The electrons residing between the nucleus and the valence shell are called intervening electrons. The intervening electrons will reduce the force of attraction between the nucleus and the outermost shell electrons, this process is called screening effect. So, the energy of the orbital in the subshell will decrease with increase in atomic number.
So, the correct answer is “Option A”.

Note: Hydrogen doesn’t have 2s orbital at all. So, option D can be rejected in the first place itself. Just remembering the relationship as well as the increasing/decreasing trend of atomic number, nuclear effective charge and energy of orbital will help us to answer this question within no time.