Question

Why do the knowledge of atomic number enable us to deduce the number of electrons present in an atom?

Answer 1

Atomic number is the number of protons in the nucleus and also the number of electrons in the electron cloud, but only when the atom has a neutral charge.

\(\sf\huge\begin{matrix} 4\\\sf 2\end{matrix} He\)
2 is the atomic number (the number of protons and the number of electrons (when the charge is neutral))
4 is the mass number (the number of protons+neutrons)

Answer 2

It a neutral atom, the number of protons (i.e. the atomic number) equals the number of electrons.