Question

Which of the following elements is very reactive and needs to gain one electron to have a full outermost shell?

Answer 1

Answer:   Iodine

Explanation:  Cesium is the alkali metal which belongs to the group 1 of the periodic table.

Iodine is the Group 17 element which are known as the Halogens as they are the group of the most electronegative atoms.

Zinc belongs to the Transition metal d block .

Xenon belongs to the Group 18 of noble gases.

The electronic configuration will help us to determine which can gain electron to have a full outermost shell.

Cesium (Cs) = [Xe] 6s1

Zinc (Zn) = [Ar] 3d10 4s2

Iodine(I) =  [Kr] 4d10 5s2 5p5

Xenon (Xe) = Kr 4d10 5s2 5p6

Thus Iodine can easily gain one electron to have complete outermost shell.

Answer 2

Iodine has 7 valence electrons. It needs therefore only 1 electron to be stable this is why it is so reactive.