Isotopes are special types of the same element. They have some things in common and some differences.

First, let’s look at what they have in common:

• Same Atomic Number ($Z$): This number tells us how many protons are in the center of the atom (the nucleus). This number is what makes an element what it is. For example, carbon has an atomic number of 6 because it has 6 protons.

Now, here’s what makes them different:

• Different Mass Numbers ($A$): This number is the total of protons and neutrons in the atom.

To give you a few examples:

• Carbon-12 has a mass number of 12 ($A = 12$).
• Carbon-14 has a mass number of 14 ($A = 14$).

So, isotopes have the same atomic number but different mass numbers because they have a different number of neutrons.