Chemical substances in which all of the atoms have the same number of protons are called elements.

Here's why:

* Atomic Number: The number of protons in an atom's nucleus defines its atomic number.

* Elements: Elements are the fundamental building blocks of matter and are defined by their atomic number. Since all atoms of a specific element have the same number of protons, they have the same atomic number.

For example, all carbon atoms have 6 protons, making carbon's atomic number 6. Similarly, all oxygen atoms have 8 protons, giving oxygen an atomic number of 8.