Inside the Atom: Charge, Mass, and Isotopes

Manual: General · Subject: Nuclear Physics

Learn how atomic number, mass number, and isotopes describe different nuclei.

Counting Particles in a Nucleus

Atomic number

The atomic number $Z$ is the number of protons in the nucleus. It determines which element the atom is: hydrogen has $Z=1$ , helium has $Z=2$ , carbon has $Z=6$ , and so on.

Mass number

The mass number $A$ is the total number of protons and neutrons in the nucleus, so $A = Z + N$ , where $N$ is the number of neutrons.

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Isotopes

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons.

Examples of isotopes

Isotope	Protons $Z$	Neutrons $N$	Mass number $A$
Carbon-12	6	6	12
Carbon-13	6	7	13
Carbon-14	6	8	14

Useful notation

Symbol

Meaning

^A_ZX

Nuclear notation for an element

X

Z

Number of protons

A

Total number of nucleons

N

Number of neutrons

In the notation $^{14}_{6}\text{C}$ , how many protons are in the nucleus?

6 8 14 20

If a nucleus has $Z=8$ and $A=16$ , how many neutrons does it have?

What makes two atoms isotopes of the same element?

Same number of neutrons Same number of protons Same mass number Same chemical symbol and mass

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Inside the Atom: Charge, Mass, and Isotopes

Counting Particles in a Nucleus

Atomic number

Mass number