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Which of the following sequences shows the elements Ca, Ba, S, Se and Ar arranged in the correct order of increasing first ionization enthalpy?

1. Ca < S < Ba < Se < Ar
2. S < Se < Ca < Ba < Ar
3. Ba < Ca < Se < S < Ar
4. Ca < Ba < Se < S < Ar

Correct answer: Ba < Ca < Se < S < Ar

Solution

The correct order of increasing first ionization enthalpy reflects the general trend in the periodic table, where ionization energy increases across a period and decreases down a group. Barium (Ba) has the lowest ionization energy due to its larger atomic size and lower effective nuclear charge, followed by calcium (Ca), while sulfur (S) and selenium (Se) have higher ionization energies due to their position in the p-block, with argon (Ar) having the highest ionization energy as a noble gas.

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