Answer : The correct option is, (1) Gains an electron and its radius increases.
Explanation :
The given element is chlorine.
Atomic number of chlorine = '17'
The electronic configuration of chlorine is,
Number of electrons = 17
Number of protons = 17
The electronic configuration of chloride ion is,
Number of electrons = 18
Number of protons = 17
When the chlorine atom gains an electron to acquire a stable electronic configuration. As the electrons add in the valence shell of the chlorine, the number of electrons increases but the number of protons remains the same. So, the protons will not be able to bind the extra electron and it will remain far from the nucleus.