What Is The Difference Between Atomic Number And Atomic Mass?

Atomic number and atomic mass are both numbers associated with atomic structure. Atomic number is important as it is the criteria used to arrange elements in the periodic table, a tool used extensively by chemists to study the properties of the elements.

Atomic number is also called proton number and is the number of protons in the nucleus of an atom. It is also equal to the number of electrons that orbit the nucleus. Electrons are the negatively charged particles in an atom.

The relative atomic mass of an atom is the mass of that atom calculated relative to one-twelfth of the mass of an atom of Carbon-12. It is sometimes referred to as the atomic weight of an atom but this is the incorrect term – mass should always be used. Relative atomic mass refers to the added numbers of protons and neutrons in the atom as electrons have only negligible mass. So, for example, the relative atomic mass of oxygen is 16, the atomic number of oxygen is 8; it has 8 protons and 8 neutrons and 8 electrons.

The atomic number is the number of PROTONS in an atom.  Example - Carbon has 6.

The atomic mass is the number of PROTONS AND NUETRONS in an atom.  Example:  Carbon has 6 protons and 6 nuetrons so mass is 12!

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1. Number of protons present in the nucleus of an atom is called the atomic number of an element. It is denoted by Z. Combined number of protons and neutrons present in the nucleus of an atom is called the mass number of an element. It is denoted by A. The atomic and mass numbers of an atom X are often shown A X. For example 4He means


2.                                                                 Z                           2   


3. that the atomic number of helium is 2 and the mass number is 4.

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