Question

Number of valence electrons in Chlorine ion are:
(a)16
(b)8
(c)17
(d)18
Answer given in three guide books is 8. how answer is 8?

Answer 1

b) 8.

     The number of electrons in the outermost orbit (Shell or energy level) of an atom are called the valence electrons.  These are the ones participating in chemical bonds between atoms during reactions.

   For Chlorine: with an atomic number 17:  1s2, 2s2 2p6 3s2 3p5.
   There are 7 valence electrons.

   For a Chlorine Ion, Cl⁻ there are 18 electrons. It acquires one e⁻ from a donor atom.  Then  it has  3s2 3p6 in the outermost valence shell.
  
   So there are 8 valence electrons.

Answer 2

the no of valence electrons of chlorine ion is 8