Considering the definition of ionization energy, the highest ionization energy belongs to the element:
a. Be
b. Ar
c. Cl
Electrons are held in atoms by their attraction to the nucleus, which means that energy is needed to remove an electron from the atom.
You should keep in mind that the electrons of the last layer are always lost, because they are the weakest attracted to the nucleus.
Ionization energy, also called ionization potential, is the necessary energy that must be supplied to a neutral, gaseous, ground-state atom to remove an electron from an atom. When an electron is removed from a neutral atom, a cation with a charge equal to +1 is formed.
In a group, the ionization energy increases upwards because when passing from one element to the bottom, it contains one more layer of electrons. Therefore, the valence layer electrons, being further away from the nucleus, will be less attracted to it and it will cost less energy to pluck them.
In the same period, in general, it increases as you shift to the right. This is because the elements in this way have a tendency to gain electrons and therefore it will cost much more to tear them off than those on the left which, having few electrons in the last layer will cost them much less to lose them.
Considering all the above, from each of the pairs, the highest ionization energy belongs to the element:
a. Be
b. Ar
c. Cl
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