Here's why:
* Atomic Number: This represents the number of protons in the nucleus of an atom. It defines the element. For example, all carbon atoms have an atomic number of 6 because they have 6 protons.
* Mass Number: This represents the total number of protons and neutrons in the nucleus. It's the approximate atomic mass of the atom.
Why they can't be equal:
* Neutrons: Most atoms have neutrons in their nucleus, contributing to the mass number. Since neutrons have a mass almost identical to protons, the mass number will always be greater than the atomic number unless there are no neutrons.
* Isotopes: Even within the same element, atoms can have different numbers of neutrons. These variations are called isotopes. For example, carbon-12 has 6 protons and 6 neutrons (mass number 12), while carbon-14 has 6 protons and 8 neutrons (mass number 14).
The exception:
* Hydrogen-1: This is the most common isotope of hydrogen. It has only one proton and no neutrons. Therefore, its atomic number (1) is equal to its mass number (1).
