Atomic number

Definition

The atomic number is the number of protons in the nucleus of an atom. It uniquely identifies an element and determines its chemical properties.

5 Must Know Facts For Your Next Test

1. The atomic number is denoted by the symbol $Z$.
2. In a neutral atom, the atomic number also equals the number of electrons.
3. The periodic table is arranged in order of increasing atomic number.
4. The difference between isotopes of an element lies in their neutron numbers, not their atomic numbers.
5. A change in the atomic number means a change to a different element.

Review Questions

• What does the atomic number represent?
• How does the atomic number relate to the periodic table?
• If two atoms have different atomic numbers, are they isotopes or different elements?

Related terms

Mass Number:

The total number of protons and neutrons in an atom's nucleus.

Isotope:

Atoms of the same element with different numbers of neutrons and thus different mass numbers.

Proton:

A subatomic particle found in the nucleus with a positive electric charge equal in magnitude to that of an electron but opposite in sign.