Atomic Number
Definition: The Atomic Number (1$\text{Z}$) is the number of protons found in the nucleus of an atom.2
Protons: Tiny, positively charged particles found in the center (nucleus) of an atom.3
Nucleus: The dense, central part of an atom containing protons and neutrons.4
Key Fact: The Atomic Number is what defines an element. If the number of protons changes, the element changes.
In a neutral atom: The number of protons is always equal to the number of electrons.
2. Mass Number
Definition: The Mass Number (also called Nucleon Number) is the total number of protons and neutrons in the nucleus of an atom.
Neutrons: Tiny particles found in the nucleus that have no electrical charge (they are neutral).14
The Mass Number is always a whole number because it is a count of particles.
Calculating Neutrons: To find the number of neutrons, you can subtract the atomic nnumber with the mass number
3. Isotopes
Definition: Isotopes are atoms of the same element that have the same Atomic Number but different Mass Numbers
Simply put: Isotopes have the same number of protons but a different number of neutrons.23
Properties: Because isotopes have the same number of electrons (since they are the same element), they have virtually the same chemical properties. They only differ slightly in their physical properties (like density).
5. Isobars
Definition: Isobars are atoms of different elements that have the same Mass Number but different Atomic Numbers