Answer:
Because of the existence of isotopes.
Explanation:
Let's look at the simplest atom, hydrogen. The majority of hydrogen atoms are the
1
H
isotope; their nuclei contain the 1 proton, the one positively charged nuclear particle. A few hydrogen nuclei contain the 1 proton (this is of course what defines them as hydrogen atoms), but also 1 neutron in addition, to give the deuterium,
2
H
, isotope; a smaller fraction of hydrogen nuclei contain 2 neutrons, to give the tritium isotope,
3
H
.
