John Dalton reasoned that every atom of an element was identical because they had the same chemical properties. But atoms of an element can differ from one another in a way that has minimal effect on their chemical properties — they can vary in the number of neutrons in the nucleus forming closely related species called isotopes.
Isotopes are atoms with the same atomic number (atoms of the same element) that have different mass numbers. They have the same number of protons in the nucleus, but the number of neutrons will not be the same. Adding neutrons to an atom will make it heavier, but that won't change the chemistry of the atom.
Most elements exist in nature in two or more isotopic forms. Chlorine, Cl, is a typical example. All samples of chlorine obtained from nature are composed of the same nonchanging mixture of two isotopes, chlorine-35 (Cl-35) and chlorine-37 (Cl-37). The 35 and 37 are the mass numbers of the two isotopes. Of course, both isotopes have the same atomic number, 17.
The two isotopes of chlorine differ only in the number of neutrons in the nucleus.
In nature, 75.771% of all chlorine atoms, by weight, are Cl-35, and 24.229% are the heavier isotope, Cl-37. The percentage distribution of the isotopes of an element is called the percent natural abundance or just natural abundance of the isotopes. All samples of chlorine, no matter the source, have the same percent natural abundance of the two isotopes. It is a nonchanging mixture of isotopes, but remember that even though they are isotopes of one another, they are still atoms of the same element and have the same chemical properties.