Which of the following elements has the highest first ionization energy?

The element with the highest first ionization energy among the given choices is chlorine (Cl). Ionization energy refers to the amount of energy required to remove an electron from a neutral atom in its gaseous state.

As you move across a period in the periodic table from left to right, the ionization energy generally increases. This happens because the number of protons in the nucleus increases, which results in a greater positive charge that attracts the electrons more strongly, making it more difficult to remove an electron. Chlorine is situated further to the right in the periodic table compared to calcium, potassium, and sodium, which are all located in Group 1 or Group 2 and have lower ionization energies due to having more electron shells and being less effective at holding onto their outermost electrons.

While both sodium and potassium are group 1 elements known for having low ionization energies due to their single electron in the outer shell, calcium, being in Group 2, also has a lower ionization energy than chlorine. Thus, because chlorine is a non-metal with a greater effective nuclear charge and fewer electron shells (compared to the alkaline and alkaline earth metals listed), it has the highest first ionization energy among the options provided.