Physical Chemistry

Atomic Number Mass Number Atomic Mass Unit

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Atomic Number, Mass Number and Atomic Mass Unit are basic terms to have basic understanding about how to deal with term atom.

Atomic Number

The atomic number (represented by the letter Z) of an element is the number of protons in the nucleus of each atom of that element.

An atom can be classified as a particular element based solely on its atomic number.

For example, any atom with an atomic number of 8 (its nucleus contains 8 protons) is an oxygen atom, and any atom with a different number of protons would be a different element.

 The periodic table displays all of the known elements and is arranged in order of increasing atomic number.

 In this table, an element’s atomic number is indicated above the elemental symbol.

Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus.

Next on the table is helium, whose atoms have two protons in the nucleus.

Lithium atoms have three protons, beryllium atoms have four, and so on.

Mass Number

The mass number (represented by the letter A) is defined as the total number of protons and neutrons in an atom.

Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus.

 Its nucleus also contains two neutrons. Since 2+2=42+2=4, we know that the mass number of the helium atom is 4.

 Finally, the helium atom also contains two electrons, since the number of electrons must equal the number of protons.

 This example may lead you to believe that atoms have the same number of protons and neutrons, but a further examination of the table above will show that this is not the case.

Lithium, for example, has three protons and four neutrons, giving it a mass number of 7.

What is Relative atomic mass?

“The mass of one atom of an element compared with the mass of one atom of carbon taken as 12 is called relative atomic mass of the element.”


The mass of one atom of magnesium is two times the mass of one atom of carbon, therefore relative atomic mass of magnesium would be 24.

Atomic Mass Unit:

The relative atomic mass is expressed in atomic mass unit (amu). It is the 1/12th of the mass of one atom of 12C.

The relative atomic mass of C is 12.0000 amu whereas that of H is 1.008.

The masses of the atoms vary from 10-24 to 10-23 grams. Since such an extremely small mass cannot be measured by any balance, this is why relative atomic masses are used.

The relative atomic mass of some important elements are as follows:

Relative Atomic Mass of Hydrogen:  1.008 amu

    Relative Atomic Mass of Carbon:   12.0000 amu

    Relative Atomic Mass of Oxygen:  15.9994 amu

    Relative Atomic Mass of Cupper:   63.546 amu

    Relative Atomic Mass of Cesium:   132.905429 amu

  Relative Atomic Mass of Uranium:   238.0289 amu

        Relative Atomic Mass of Neon:   20.1797 amu

  Relative Atomic Mass of Chlorine:    35.453 amu

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