Question

What is the electron configuration of a chlorine ion in a compound of BeCl2?

Answer 1

The  electronic  configuration  of  a   chlorine ion  in BeCl2  compound is

[2.8.8]^-  (answer  B)

chlorine  atom  gain  on electron form Be to  form  chloride ions
chlorine  atom  has  a  electronic   configuration of  2.8.7   and  it  gains one  electron  to  form  chloride ion  with  2.8.8  electronic  configuration

Answer 2

Chlorine has 7 valence electrons in the outermost shell. when it forms an ionic bond with Be, it takes in 1 electron and achieves the noble gas configuration
since it has one more electron it gains a negative charge of -1.
anion is then - Cl⁻
electronic configuration of Cl atom is - [2,8,7]
after taking in the electron Cl⁻ - [2,8.8]⁻
answer is 
B. [2.8.8]⁻