Question

The first four ionisation energies of an element are 191, 578, 872, 5962 $Kcal/mol$ respectively. The number of valence electrons is:

Answer 1

At first we will write the data given in the question. Then we will know about the ionization energy, then we will know what valence electrons are and how the ionization energy behaves when an electron is a valence shell. Then we will compare the ionization energy and then we will find the number of valence electrons.

Complete step-by-step answer: Step1. We are given ionization energy of an element. They are $191,578,872,5962$ $Kcal/mol$.
We need to find the number of electrons in the valence shell.
Step2. Ionization energy is the energy required to remove the electron from the shell of an atom. Or in other terms it can be defined as the strength by which the electrons are held into the atom. The removal of electrons is an endothermic process.
Step3. The Valence shell of an atom is the outer shell of the atom. The electrons found in it are called valence electrons. They take part in the formation of chemical bond forming bonds with other atoms.
Step4. Now the when we apply the ionization energy to the element. The electrons in the outermost shell i:e valance shell will be first to get ionized and come out of the atom. So we can understand that the difference of ionization energy between valence shells will be less but when we move to the inner shell the difference is suddenly spiked and there is a large gap in ionization energy of the valence shell and other inner shells.
Step5. So as we can see that the difference between the first three ionization energies is very less but there is a large gap between third and fourth. So we can say that there are three electrons in the valence shell.

Hence the correct answer is $3$.

Note: The minimum amount of energy required to remove the electron from the most loosely bound outermost shell of gaseous metal atom is called ionisation energy.The photoelectric effect is the removal of electrons from the atom by hitting it with photons. The electrons which came out are called photoelectrons.