Question

which of the following elements has the lowest ionization energy?
a) argon
b) beryllium
c) helium
d) hydrogen

Explanation:

Ionization energy is the energy required to remove an electron from an atom. Noble gases (Argon, Helium) have high ionization energy due to stable electron configurations. Hydrogen has 1 electron in its outer shell, but Beryllium (Be) has a larger atomic radius and more electron shielding compared to H, He, and Ar. Beryllium is in group 2, period 2. As we move down a group or across a period (left to right, ionization energy generally increases), Beryllium has lower ionization energy than the noble gases and lower than Hydrogen? Wait, no—wait, Hydrogen has 1 electron, ionization energy of H is about 1312 kJ/mol, Be is about 899 kJ/mol. Wait, let's recall: Ionization energy trend: increases across a period (left to right) and decreases down a group. Helium (group 18, period 1) has very high IE. Argon (group 18, period 3) has high IE but lower than He. Hydrogen (group 1, period 1) has IE ~1312. Beryllium (group 2, period 2) has IE ~899. So Beryllium has lower IE than H, He, Ar. Wait, but let's check the options. The options are Ar, Be, He, H. So among these, Beryllium has the lowest ionization energy. Wait, no—wait, maybe I messed up. Wait, Hydrogen: 1s¹, removing that electron gives H⁺, IE is 1312. Beryllium: 1s²2s², removing an electron from 2s² (stable? No, 2s² is a filled subshell? Wait, 2s² is a filled subshell, but Be is in group 2. Wait, the first ionization energy of Be is lower than that of H? Wait, no, actually, the first ionization energy of H is 1312 kJ/mol, Be is 899 kJ/mol. So Be has lower IE. Helium is ~2372 kJ/mol, Argon ~1521 kJ/mol. So among the options, Beryllium (B) has the lowest ionization energy.

Answer:

B) Beryllium