Atomic Mass and Atomic Mass Unit

Atomic Mass and Atomic Mass Unit

Mass refers to the mass of a neutral atom. Thus, this includes the masses of corresponding nucleons, the masses of the orbital electrons and the mass equivalent to their binding energy. Being too small, the masses of atoms, nucleus and fundamental particles (electrons, protons neutrons etc.) are expressed in a unit called atomic mass unit, abbreviated as amu or u. 1 amu is defined as 1/12 of the mass of 6C12 atom. As 12 g of 6C12 contains 6.023 X 1023 atoms of 6C12 ,so mass of 6C12 atom is

12/(6.023 X 1023 )g

By definition,

1 amu = 1/12 (12/(6.023 X 1023)g

=1/(6.023 X 1023)g

=1.663 X 1024 g

1 amu = 1.66 X 10-27 kg

On atomic scale, mass of protons, neutrons and electrons are,

m p = 1.672622*10-27 kg = 1.007276u

m n = 1.674927*10-27  kg = 1.008665u

me = 9.10938* 10-31kg = 0.000548580u 

Atomic Mass Unit in Terms of Energy

According to Einstein's mass energy relation E = mc2, the energy equivalent to 1 amu ( 1.66 X 10-27) is given by,

E = 1.66 X 10-27X (3 X 108)2 J = 1.49 X 10-10 J

But 1.6 X 10-19 J = 1 eV, so

E = ( 1.49 X 1010)/(1.6 X 10-19 ) eV

= 0.931 X 10 9 eV

= 931 MeV

\ 

It can be proved that the energy equivalent of the mass of an electron, proton and neutron are respectively given by

me = 0.511 MeV

m p = 938.279MeV

m n = 939.573 MeV

Types of Nuclei

1.Isotopes

Isotopes are the nuclei having the same atomic number Z but different mass number A. Isotopes of an element have identical chemical behavior ( due to same atomic number which determine chemical properties) and differ physically only in mass.

Isotopes of some elements are:

1. Hydrogen : ₁H¹, ₁H², ₁H³

2. Helium : ₂He³, ₂He⁴

3.Carbon : ₆C¹⁴, ₆C¹²

2.Isobars

Isobars are the nuclei having the same mass number but different atomic number. These are the nuclei of different elements having different physical and chemical properties. Some of the examples of isobars are :

1. ₁H³ and ₂He³

2. ₇N¹⁴ and ₆C¹⁴

3.₂₀Ca⁴⁰ and ₁₈Au⁴⁰

3. Isotones

Isotones are the nuclei having the same neutron number. Some examples of isotones are:

1. ₂He⁴ and ₁H³ (N = 2)

2. ₁H² and ₂He³ (N = 1)

3.₇N¹⁷, ₈O¹⁸ and ₉F¹⁹( N = 10)