Na⁺ is smaller than Na atom because:

The total number of electrons is 10 in the case of Na⁺ and 11 in the case of Na, while the number of protons is the same, i.e. 11 in both cases. Sodium is a Group 1 element, so its only ionic state is Na⁺. Cations of a given element have a smaller radius than the neutral atom so that Na⁺ will decrease in size compared with the Na atom.

• Sodium atoms and sodium ions have the same number of protons.
• Each one has 11 protons. This is because the atomic number of sodium is 11.
• The sodium atom has 11 electrons.
• Sodium-ion has 10 electrons as sodium atom loses one electron to form sodium ion.
• Only sodium atom react vigorously with water whereas sodium ion does not.