Which atom in each of the following sets has smallest ionization energy and why? i: Na, Mg, Al ii: Li, Na, k iii: Cl, Br, I

Na (Sodium), Mg (Magnesium), and Al (Aluminum) are all in Period 3. Na is in Group 1, Mg in Group 2, and Al in Group 13.

Ionization energy generally increases across a period from left to right due to increasing nuclear charge and decreasing atomic radius, which results in a stronger attraction between the nucleus and the valence electrons.

Since Na is furthest to the left in Period 3 among these elements, it will have the smallest ionization energy.
The atom with the smallest ionization energy is $\boxed{\text{Na}}$.
This is because ionization energy increases across a period, and Na is the leftmost element in this set.

Li (Lithium), Na (Sodium), and K (Potassium) are all in Group 1 (Alkali Metals). Li is in Period 2, Na in Period 3, and K in Period 4.

Ionization energy generally decreases down a group due to increasing atomic radius and increased shielding effect from inner electrons, which reduces the effective nuclear charge experienced by the valence electrons, making them easier to remove.

Since K is at the bottom of this group among these elements, it will have the smallest ionization energy.
The atom with the smallest ionization energy is $\boxed{\text{K}}$.
This is because ionization energy decreases down a group, and K is the lowest element in this set.

Cl (Chlorine), Br (Bromine), and I (Iodine) are all in Group 17 (Halogens). Cl is in Period 3, Br in Period 4, and I in Period 5.

Ionization energy generally decreases down a group due to increasing atomic radius and increased shielding effect from inner electrons, which reduces the effective nuclear charge experienced by the valence electrons, making them easier to remove.

Since I is at the bottom of this group among these elements, it will have the smallest ionization energy.
The atom with the smallest ionization energy is $\boxed{\text{I}}$.
This is because ionization energy decreases down a group, and I is the lowest element in this set.