Which element requires the least amount of energy to remove the most loosely held electron from a gaseous atom in the ground state?

(1) bromine (3) sodium

(2) calcium (4) silver

The answer is (3) sodium. The sodium has only one electron at the outer layer. While calcium has 2 and bromine has 7. And silver is usually not easy to react. So the sodium is the easiest one to lose one electron,