$(a)$ An atom is considered electrically neutral. Explain.
$(b)$ State the valency of Helium.
$(c)$ If an atom has an atomic mass of $4$ and $2$ protons in its nucleus,how many neutrons does it have?

(N/A) An atom is considered electrically neutral because it contains an equal number of positively charged protons and negatively charged electrons,which cancel out each other's charge.
$(b)$ The valency of Helium is $0$ because its outermost shell is completely filled (it has a stable duplet configuration).
$(c)$ The formula for atomic mass is: $\text{Atomic mass} = \text{Number of protons} + \text{Number of neutrons}$.
Given: $\text{Atomic mass} = 4$,$\text{Number of protons} = 2$.
Substituting the values: $4 = 2 + \text{Number of neutrons}$.
Therefore,$\text{Number of neutrons} = 4 - 2 = 2$.