Here's why:

* Atomic Number: The number of protons in an atom's nucleus determines its atomic number. This number defines the element the atom belongs to. For example, all carbon atoms have 6 protons.

* Atomic Mass: The mass of an atom is primarily determined by the number of protons and neutrons in its nucleus.

* Protons and neutrons have roughly the same mass (slightly more for neutrons).

* Electrons have a very small mass compared to protons and neutrons, so they contribute very little to the overall atomic mass.

Therefore, atoms of different elements have different masses because they have different numbers of protons and neutrons.

Here's a simple example:

* Hydrogen (H) has 1 proton and 0 neutrons, so its atomic mass is roughly 1 atomic mass unit (amu).

* Carbon (C) has 6 protons and 6 neutrons, so its atomic mass is roughly 12 amu.

* Oxygen (O) has 8 protons and 8 neutrons, so its atomic mass is roughly 16 amu.

Isotopes: Even atoms of the same element can have slightly different masses. This is due to the existence of isotopes. Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. For example, Carbon-12 (6 protons and 6 neutrons) and Carbon-14 (6 protons and 8 neutrons) are isotopes of carbon.