which of the following elements has the highest ionization energy?

Analyze the question: We need the element with the highest first ionization energy, which is the energy needed to remove one electron from a gaseous atom.

Concept: Ionization energy trends in the Periodic Table:

• Across a period (left to right): it increases. Nuclear charge increases while the electrons are added to the same shell, so the nucleus pulls them more strongly and the atomic radius shrinks.
• Down a group: it decreases. The atoms get bigger, there is more shielding, and the outer electron is farther from the nucleus and easier to remove.

Solution:

1. Locate each element: • Calcium (Ca): Group 2, Period 4 • Chlorine (Cl): Group 17, Period 3 • Fluorine (F): Group 17, Period 2 • Magnesium (Mg): Group 2, Period 3

2. Compare metals with non-metals. Chlorine and fluorine are on the right of the table (Group 17), so they have much higher ionization energies than the Group 2 metals Ca and Mg.

3. Compare F and Cl. They are in the same group, and fluorine is higher up. Fluorine has the smaller atomic radius and less shielding, so its outer electrons are held most tightly.

4. Approximate values: • F ≈ 1681 kJ/mol • Cl ≈ 1251 kJ/mol • Mg ≈ 738 kJ/mol • Ca ≈ 590 kJ/mol

Answer: Fluorine has the highest ionization energy, so the correct option is C.

Tip: Ionization energy increases toward the top right of the Periodic Table (ignoring noble gases). Fluorine is the most extreme case among the reactive elements. Remember "up and to the right = harder to remove an electron."