Chapter 31 - Nuclear Physics and Radioactivity - Check Your Understanding - Page 878: 2

(a) It's atomic number

Material is known to be an isotope of lead . Isotopes are the atoms of same element that have different number of neutrons but the $same$ $number$ $of$ $ protons$ and electrons. And the $atomic$ $number$ is the number of $protons$ . Hence we can specify the atomic number.^{\circ}