The defining characteristic that distinguishes one element from another is the number of protons in its nucleus. This number, called the atomic number, is unique to each element.

Here's why:

* Protons determine the element's identity: Protons are positively charged particles found in the nucleus of an atom. They are the fundamental building blocks of an element, and their number defines its chemical properties.

* Atomic number is constant: For a given element, the number of protons never changes. This is why the atomic number is used to organize the periodic table.

* Other particles can vary: While the number of protons is fixed for an element, the number of neutrons and electrons can vary. This leads to the existence of isotopes (atoms of the same element with different numbers of neutrons) and ions (atoms that have gained or lost electrons).

Example:

* Hydrogen (H) has 1 proton.

* Helium (He) has 2 protons.

* Carbon (C) has 6 protons.

Even though isotopes of carbon like Carbon-12 and Carbon-14 have different numbers of neutrons, they are still both carbon because they have the same number of protons (6).