Atomic mass

Atomic mass of an element is the relative mass as compared with an atom of carbon 12 and it is expressed in AMU.


AMU is the Atomic mass unit and it is defined as the mass equal to 1 by 12th of the mass of carbon 12 isotope.

1 amu = 1.66056x 10-24

Molecular mass

It is the sum of atomic masses of various atoms present in a molecule. It is obtained by simply addition of atomic masses.

Formula unit mass

It may be defined as the sum of the masses of iron is present in a formula unit of an ionic molecule.

Equivalent mass or equivalent weight

Equivalent mass or weight of a molecule is Obtained by dividing the molecular mass by valancy or the number of electrons exchanged .

Equivalent mass = molecular mass / valency or the number of electrons exchanged.

Characteristics of an atom

Atomic number

Atomic number is the very basic term which defines the characteristics of an element like electronic configuration, valance see and physical properties.

Atomic number is the number of protons present in an atom.

The number of protons is always equal to the number of electrons.

So the atomic number in a neutral atom is equal to the number of protons in the nucleus equal to the number of electrons in an atom.

Mass number

It is the sum of the number of protons and the number of neutrons present inside the nucleus or simply it is the sum of the number of nucleons present in an atom it is denoted by A.

For example atomic number of oxygen is 8

It has 8 protons and 8 neutrons

So its mass = 8+8

Mass of oxygen = 16

Different atomic species


( greek word isos+ topos; isos means same and topos means place)

These are the atoms of the same element with the same atomic number but different atomic mass.

They have all the chemical and physical properties similar to each other but the only difference is related to the atomic mass.

For example carbon is having three isotopes with mass number, 12, 13, 14.

Add to your knowledge

Hydrogen is the only element Having different names for its isotopes.

Polonium is the element which has maximum number of isotopes.


Isos+baros; greek – isos means equal and baros means weight.

Atoms of elements having same atomic mass But different atomic number.

For example, argon ,potassium  and calcium.

I saw tones these are the atoms having same number of neutrons and Different atomic number and mass number.

For example phosphorus and silicon both have 16 neutrons.

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