The highest ionization energy is exhibited by –
(A) Halogens
(B) Alkaline earth metals
(C) Transition metals
(D) Noble gases

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Hint: Ionization energy is the measure of difficulty in removing an electron from an atom or an ion. Elements which have a greater number of protons holding all the electrons together are said to be more stable. Hence, they would avoid losing electrons.

Complete step by step answer: The amount of energy that an isolated gaseous atom requires to lose an electron in its ground state.
In the periodic table, as we move across a period, more electrons are added to shells with the same quantum level. Thus, the shielding of additional electrons from electrons already present does not increase much. Repulsion from existing electrons and attraction from the nucleus are the two forces acting on the new electron simultaneously. These two compensate for each other.
So, across a period, nuclear charge overpowers the shielding effect. As a result of this electrons experience strong attraction from the nucleus which makes it extremely difficult to remove electrons. Hence, the value of ionization energy increases from left to right in a period.
As per these conditions, noble gases have the highest ionisation energy. However, since they are inert in nature and do not react much, we consider halogens to have the highest ionization energy.

Hence, Halogens exhibit highest ionization energy i.e. option A.

Additional information: Halogens are located to the extreme right side of the periodic table i.e. in group 17. Their general electronic configuration is \[{\text{n}}{{\text{s}}^2}{\text{ n}}{{\text{p}}^5}\]. These elements have seven valance electrons and are highly reactive.

Note: As per general trends seen in periodic properties, we consider noble gases to have highest ionization enthalpy. But remember, as the name suggests, these elements are noble and hence do not react much.