An atom with more or fewer neutrons than atoms of the same element is called an isotope.

Here's a breakdown:

* Element: Defined by the number of protons in its nucleus.

* Isotope: Atoms of the same element (same number of protons) but with a different number of neutrons.

For example, carbon (C) has an atomic number of 6, meaning it has 6 protons. There are several isotopes of carbon, including:

* Carbon-12 (¹²C): 6 protons and 6 neutrons (most common form)

* Carbon-14 (¹⁴C): 6 protons and 8 neutrons (radioactive and used in carbon dating)

The number after the element's name (like 12 or 14) represents the mass number, which is the total number of protons and neutrons in the atom's nucleus.