NUCLEI

• Rutherford alfa particle scattering gives ideas that the positive charge and all the mass of an atom is densely concentrated at central of the atom in very small space and that is known as nucleus.
• Nucleus is very small in comparison with the whole atom.
• Atom is 10⁴ times larger than the nucleus.
• The volume of atom is 10¹² times larger than the volume of nucleus.

Atomic masses

• The mass of an atom is very small in comparison with kilogram. Therefor a different unit is used to measure the mass of an atom is called atomic mass unit.
• 1 Amu is defined as the 1/12th of the mass of the Carbon-12 (¹²C) atom.
• 1 amu=1.660539*10²⁷ kg.

Isotopes

• If the two atom have same atomic number and difference in mass number is known as isotopes.
• Same atomic number means same elements that means many elements are found in different isotopes. 
• So the atomic mass of that element is the average weight of the isotopes that is found.

example:- Chlorine is found in two different isotopes one is ³⁵Cl and another one is  ³⁷Cl. The percentage of ³⁵Cl is 75.4 and ³⁷Cl  is 24.6 in nature.

So the average of atomic mass of chlorine calculated as:-

 

=(75.4* 34.98 +24.6* 36.98 )/100

=35.47 u

• The smallest atom hydrogen also have 3 isotopes.
1. Hydrogen: It have no neutron. (98.985% in nature) 
2. Deuterium: It have 1 neutron.
3. Tritium: It have 2 neutron.( Not present in nature because it is unstable . It is made in laboratories)

Atomic number

• It is the number of proton in atom.
• And we know that charge on electron and proton is equal.
• We also know that atoms are neutral that means number of electron and number of proton is equal in atom.
• Atomic number is denoted by Z.

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