Which element has the lowest first ionization energy?

• A. Fr (Francium)
• B. He (Helium)
• C. Na (Sodium)
• D. F (Fluorine)

Answer: (A)

Verified step by step guidance

1. Understand that first ionization energy is the energy required to remove one mole of electrons from one mole of gaseous atoms, forming cations.
2. Recall the periodic trends: ionization energy generally increases across a period (left to right) due to increasing nuclear charge and decreases down a group due to increasing atomic radius and electron shielding.
3. Identify the positions of the given elements on the periodic table: He is in period 1, group 18; F is in period 2, group 17; Na is in period 3, group 1; Fr is in period 7, group 1.
4. Compare the elements based on their group and period: elements in group 1 have lower ionization energies than those in groups 17 or 18, and ionization energy decreases down the group, so Fr (period 7) will have a lower ionization energy than Na (period 3).
5. Conclude that Francium (Fr), being in group 1 and the lowest period among the options, has the lowest first ionization energy.