Inorganic chemistry

What is Atomic number and mass number?

Examples of atomic number and mass number

Atomic number :atomic number and mass number

Concept of electron, proton and neutron:

In order to understand the basic concept of atomic number and mass number we have knowledge about electron, proton and neutrons.

Electron :

Generally we say that the electrons carry negative charge and are subatomic particles.Electrons revolve around the shells of radii in different energy levels. The electrons are represented by small letter “e”


The protons have positive charge and are denoted by symbol small “p”.Rutherford discovered these subatomic particles.

Neutrons :

A neutron is taken its name due to the fact of having no electrical charge. It is also subatomic particle which is present in nucleus of every atom. James Chadwick dicover the neutrons.These are denoted by symbol “n”.


Atomic number or proton number:

We can say that every element has a specific number through which it shows that how many protons are there in one atom. So we can define atomic number as:

It is generally equal to the number of protons found in nucleus of elements.

Symbol Of atomic number :

Atomic number is represented by symbol “Z”.This symbol” Z” comes from German language and stands for” Zahl” meaning number in German.

Explanation of atomic number :

The atomic number of an element is equal to the number of protons present in the nucleus. As all atoms of an element have the same number of protons in their nuclei, they have the same atomic number. Hence each element has a specific atomic number as its identification number.

Examples Of atomic number

Hydrogen :hydrogen

All hydrogen atoms have one proton in their nuclei so their atomic number is Z=1.


All atoms in carbon have 6 protons, so their atomic number is Z=6.

Oxygen :

All atoms in oxygen have 8 protons in their nuclei, so it’s atomic number is Z=8.

Sulphur :

Sulphur atoms have 16 protons in their nuclei, so it’s atomic number is Z=16.

Mass number :

We say that generally mass number consists of masses of 3 subatomic particles.

Definition :

The collection  of number of protons and neutrons found in the nucleus of an atom is called mass number.

Symbol :

Mass number is represented by a symbol “A”.This symbol comes from German word atomic weight.

Formula :

Mass number is calculated by using a formula :

A= Z+n

Where n= number of neutrons

Each proton and neutron has one unit of mass.

Examples :

Hydrogen :

Hydrogen atom has one proton and zero number of neutrons in its nucleus. It’s mass number is

A= Z+n

A= 1+0=1

Hence, the mass number of hydrogen is 1.

Carbon:Atomic number of carbon

Carbon atom has 6 protons and 6 neutrons, hence it’s mass number is

A= Z+n

A= 6+6=12

Hence,  the mass number of carbon is 12.

Some elements along with their Atomic number and mass number.


Number of protons is 7 and number of neutrons is 7. So it’s atomic number is 7 and mass number is 14.

Oxygen :

8 protons, 8 neutrons

Atomic number 8, mass number 16.

Sodium :

11 protons, 12 neutrons

Atomic number 11, mass number 23.

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