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Atomic Number vs. Mass Number: What's the Difference?

Edited by Aimie Carlson || By Janet White || Published on January 22, 2024
Atomic number is the count of protons in an atom's nucleus, while mass number is the total of protons and neutrons.

Key Differences

The atomic number, found on the periodic table, identifies the element by counting the protons in the nucleus. The mass number represents the sum of protons and neutrons, indicating the atom's mass.
Elements are arranged by atomic number, reflecting the number of protons, which defines the element's identity. Mass number, however, can vary in isotopes of the same element due to differing neutron counts.
Atomic number determines the chemical properties of an element, as it defines electron configuration. The mass number affects the atomic mass but doesn't significantly alter chemical properties.
A change in atomic number means a change in the element itself, while a change in mass number represents different isotopes.
Atomic number is always a whole number and unique for each element. Mass number also is a whole number but can vary within the same element.

Comparison Chart


Count of protons in nucleus
Total of protons and neutrons

Variation in Isotopes

Constant for each element
Varies in different isotopes


Element identity
Atomic mass

Effect on Chemical Properties

Defines chemical properties
Does not alter chemical properties

Representation on Periodic Table

Directly listed
Not directly listed

Atomic Number and Mass Number Definitions

Atomic Number

Atomic number determines the element's chemical properties.
Carbon's atomic number of 6 defines its unique properties.

Mass Number

Mass number varies in different isotopes of the same element.
Uranium-238 has a mass number of 238.

Atomic Number

Atomic number is the number of protons in an atom's nucleus.
The atomic number of hydrogen is 1.

Mass Number

Mass number is not listed on the periodic table.
The mass number of isotopes must be calculated or referenced.

Atomic Number

Atomic number equals the number of electrons in a neutral atom.
Neon, with an atomic number of 10, has 10 electrons.

Mass Number

Mass number is always a whole number.
An iron atom with 26 protons and 30 neutrons has a mass number of 56.

Atomic Number

It identifies the position of an element on the periodic table.
Helium has an atomic number of 2, placing it second on the table.

Mass Number

It indicates the total mass of the nucleus.
A carbon atom with mass number 12 has 6 protons and 6 neutrons.

Atomic Number

The atomic number is unique to each element.
Oxygen's atomic number, 8, is exclusive to it.

Mass Number

Mass number is the sum of protons and neutrons in an atom.
The mass number of helium-4 is 4.


What is mass number?

The mass number is the total number of protons and neutrons in an atom.

Can the atomic number change for an element?

No, the atomic number is constant for each element.

What is atomic number?

The atomic number is the count of protons in an atom's nucleus.

How is mass number calculated?

By adding the number of protons and neutrons in an atom.

Can two elements have the same atomic number?

No, each element has a unique atomic number.

Are atomic number and mass number the same?

No, they represent different properties of an atom.

Can mass number be a decimal?

No, mass number is always a whole number.

Is atomic number shown on the periodic table?

Yes, atomic number is directly listed on the periodic table.

Does the mass number change in isotopes?

Yes, different isotopes of the same element have different mass numbers.

Can isotopes of an element have the same mass number?

No, isotopes differ in mass number due to varying neutron counts.

Why is atomic number important in chemistry?

It identifies elements and determines their chemical behavior.

Can atomic number be zero?

No, atomic number starts from 1 (hydrogen) upwards.

Does mass number vary in chemical reactions?

No, chemical reactions involve electrons and don't change mass number.

What determines an element's position on the periodic table?

The atomic number determines an element's position.

Can an element have multiple mass numbers?

Yes, due to the existence of different isotopes.

What happens to the atomic number during radioactive decay?

It can change, as radioactive decay may involve proton emission or capture.

Does mass number affect an element's chemical properties?

No, mass number does not significantly alter chemical properties.

Is the mass number equal to the atomic mass?

Not exactly, atomic mass is the average mass considering all isotopes.

Is the atomic number equal to the number of electrons?

Yes, in a neutral atom, the atomic number equals the electron count.

Does changing the number of neutrons change the atomic number?

No, changing neutrons affects the mass number, not the atomic number.
About Author
Written by
Janet White
Janet White has been an esteemed writer and blogger for Difference Wiki. Holding a Master's degree in Science and Medical Journalism from the prestigious Boston University, she has consistently demonstrated her expertise and passion for her field. When she's not immersed in her work, Janet relishes her time exercising, delving into a good book, and cherishing moments with friends and family.
Edited by
Aimie Carlson
Aimie Carlson, holding a master's degree in English literature, is a fervent English language enthusiast. She lends her writing talents to Difference Wiki, a prominent website that specializes in comparisons, offering readers insightful analyses that both captivate and inform.

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