An atom has an electronic configuration of $2, 8, 7$.
$(a)$ What is the atomic number of this element?
$(b)$ To which of the following elements would it be chemically similar? (Atomic numbers are given in parentheses.)
$N(7), F(9), P(15), Ar(18)$

(B) The atomic number is the sum of the electrons in all shells: $2 + 8 + 7 = 17$. Thus,the atomic number is $17$.
$(b)$ Elements with the same number of valence electrons exhibit similar chemical properties. The given element has $7$ valence electrons. Among the choices,$F(9)$ has an electronic configuration of $2, 7$,meaning it also has $7$ valence electrons. Therefore,it is chemically similar to $F(9)$.