What element forms an ion with an electronic configuration of 1s22s22p63s23p63d104s24p6 (or [kr]) and a −2 charge? give the symbol for the element?

Answer :  The symbol of the element is Selenium (Se).

Explanation :  

Electronic configuration is distribution of electrons in an atom in                    atomic orbitals.

The electronic configuration of [Kr] =  

Total number of electrons in [Kr] = 36

Due to -2 charge, 2 electrons are withdrawn from the electronic configuration of [Kr].

Now the total number of electrons left = 36 - 2 = 34

And the Selenium[Se] is the element which has 34 number of electrons.

Electronic configuration of [Se] =

the correct option is c.

isotopes refers to different forms of the same element with the same atomic number but different mass number as a result of difference in their neutron numbers. isotopes have different relative atomic mass but share the same chemical properties.