1. Concept used

Ionization potential (or ionization energy) is the minimum energy required to remove the outermost electron from an isolated gaseous atom.

In the periodic table:

• Ionization potential decreases down a group because atomic size increases.
• Ionization potential increases across a period from left to right.
• Therefore, atoms near the bottom-left of the periodic table have the lowest ionization potential.

2. Examine the given atoms

• A: ${}^{14}_{7}N$ (Nitrogen)

  • Period 2, non-metal
  • Relatively high ionization energy

• B: ${}^{133}_{55}Cs$ (Cesium)

  • Group 1 alkali metal
  • Very large atomic size
  • Outer electron is weakly held
  • Hence, very low ionization potential

• C: ${}^{40}_{18}Ar$ (Argon)

  • Noble gas
  • Completely filled shell
  • Very high ionization energy

• D: ${}^{16}_{8}O$ (Oxygen)

  • Period 2, non-metal
  • High ionization energy compared to metals

3. Comparison

Among the given atoms, cesium ($Cs$) is the heaviest alkali metal listed and belongs to Group 1, whose elements have the lowest ionization potentials.

Thus,

4. Correct option

Therefore, the atom with the lowest ionization potential is:

5. Comparison with stored correct answer

Stored correct answer: B

My derived answer: B

They match.