In terms of electron configuration, why arsenic and antimony are chemically similar.

Arsenic, As: atomic number is 33. So its electron configuration is:

As:[Ar]4s2 3d10 4p3

Antimony, Sb: atomic number is 51, so its electron configuration is:

Sb: [Kr] 4d10 5s2 5p3.

As  you see, the last shell of both element has two s and 3 p electrons.

The electrons in the last shell are responsible for most of chemical behaviors, because those are the electrons that interact in the chemical changes.