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The Atom – Its Size

Atom – Its symbols

 Atomic mass of an element

(1) The mass of an atom is related to the number of protons, electrons, and neutrons it has. Atom of an element is extremely small and therefore it is not easy to weigh it. Even that, it is possible to determine the mass of one atom relative to another experimentally. For this, it is necessary to assign a value to the mass of one atom of a given element so that it can be taken as a standard value. An atom of carbon isotope (called carbon-12) was chosed for this purpose. Carbon-12 has six protons and six neutrons and has been assigned a mass of exactly 12 atomic mass unit (amu now known as u).
Thus one atomic mass unit (amu or u) is defined as a mass exactly equal to one twelfth (1/12th) of the mass of one carbon-12 atom.         

Mass of one atom of Carbon-12   =  12 u or 12 amu
or         1 u   =   mass of one Carbon-12 atom / 12

Mass of every other element is determined relative to this mass. For e.g. –
(a) atomic mass of nitrogen is 14 u i.e. one atom of nitrogen is 14 times heavier than 1/12 of a carbon-12 atom.
(b) Hydrogen atom is only 0.0840 times heavier than C-12 atoms.That means, on carbon-12 scale, atomic mass of hydrogen = 0.0840 × 12.00 u = 1.008 u.

(c) An oxygen atom is, on the average, 1.3333 times heavier than C-12 atom. Therefore
atomic mass of oxygen   = 1.3333 × 12.00 u = 16.0 u

Atomic mass of a few elements on C-12 scale is provided in Table.

Atomic Number –    Symbol –      Name –           Atomic mass (u)

             1                                  H              Hydrogen                   1.008

             2                                  He            Helium                       4

             6                                  C              Carbon                       12.01

             7                                  N              Nitrogen                    14

             8                                  O              Oxygen                      16

           11                                   Na            Sodium                     23

           12                                  Mg            Magnesium             24

            13                                  Al              Aluminium             27

            15                                  P               Phosphorus             31

            16                                  S               Sulfur                        32

            17                                 Cl               Chlorine                   35.5

            20                                Ca              Calcium                    40

            26                                Fe               Iron                           56

            29                                Cu              Copper                      63.5

It can be analyzed from the table that atomic mass is not a whole number. For example, atomic mass of carbon is not 12 u but 12.01 u. This is because most naturally occurring elements (including carbon) have more than one isotope. Therefore when we determine atomic mass of an element we generally measure or calculate average mass of the naturally occurring mixture of isotopes. For example – Chlorine contains two natural isotope Cl-35 and Cl-37 in the ratio of 3:1.

average atomic mass of chlorine =   35 x 3 + 37 x 1 /4

                                     = 35.5 u

Thus, ‘atomic mass’ of an element means average atomic mass of that element.

Quiz – Isotopes and Atomic mass

(3) The actual  or absolute mass of  one atomic mass unit is
1.66 x 10-24 gram or 1.66 x 10-27 Kg.
1 u  =  1.66 x 10-24 gram or 1.66 x 10-27 Kg
Example – The actual  or absolute mass of  one carbon atom is 12 u or
12 x 1.66 x 10-24  gram  or  1.99 x 10-23 gram.

  • Do you know how much atoms really weigh, in pounds or grams?

One gram is about 600,000,000,000,000,000,000,000 amu (that’s 600 sextillion, or a 6 followed by 23 zeros). A pound is just shy of 300 septillion amu–that is, 300,000,000,000,000,000,000,000,000.
(26 zeros there.) but we don’t normally use those units for measuring the mass of an atom.

  • Atomic mass is sometimes referred to as atomic weight. Although mass and weight are not the same thing, the terms “atomic mass” and “atomic weight” do have the same meaning.

Particle          Symbol       Charge           amu               grams

electron               e-                    -1              5.45 x 10-4        9.07 x 10-28

proton                 p                    +1                 1.00                1.67 x 10-24

neutron               n                     0                  1.00                1.67 x 10-24
An electron is so small (~0.05% of a proton) that its mass is ignored in normal chemical calculations.

Read next –

Molecules and Elements

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