Composition and Size of Nucleus, Atomic Masses, Isotopes and Isobars

An atom is the smallest possible entity of matter, which cannot be further divided into smaller particles. Atoms have all the characteristic chemical behaviour unique to a particular chemical element. If we look into the composition of an atom, it is found that atoms consist of three subatomic particles known as protons and neutrons, surrounded by electrons that revolve around the nucleus. One can call the combination of proton and neutron nucleons, and these are found at the centre of an atom. This small dense region situated at the central area of an atom that contains protons and neutrons is known as the atomic nucleus. 

The protons and neutrons are a combination of elementary particles, which we can call quarks. Quark is an elementary particle that undergoes different combinations to give a group of subatomic particles known as hadrons, where protons and neutrons are the most stable form.

The strong force or interaction between protons and neutrons, which holds them together, is known as nuclear forces.

Now let us understand what protons and neutrons are:

Protons: The discovery of protons is the combined result of individual work by Eugen Goldstein and Ernest Rutherford. It is one of the subatomic particles that make up the atomic nucleus having one positive electric charge(+1) and a mass of 1 AMU(atomic mass unit) equal to 1.67×10-27 Kg. 

Neutrons:  Neutron was discovered by physicist James Chadwick in 1932. These are subatomic particles with zero charge and a mass slightly greater than that of protons (1 AMU). The mass of a neutron is numerically taken as 1.67×10-27 Kg.

Size of nucleus

The size of a nucleus was found experimentally by scientist Ernest Rutherford. He conducted a scattering experiment with the help of radioactive substances emitting alpha particles using gold foil. In this experiment, he passed a very high beam of alpha particles emitted by radioactive substances through a very thin (100 nm thickness) sheet of gold foil. He placed a zinc sulphide screen that exhibits fluorescence around the gold foil in order to determine the angle of deflection.

From this experiment, he observed that:

1. Most of the alpha particles pass through gold foil undeflected. 

2. Some of the alpha particles underwent deflection with a very small angle.

3. Very few alpha particles get deflected nearly by 180⁰. 

From the above observation, Rutherford concluded some interesting facts:

1. Most of the spaces of an atom are empty as alpha particles pass without any deflection.

2. Almost all the mass of an atom is located in only one part of the atom, which is the central area called the atomic nucleus. It is found that the atomic nucleus is positively charged as the alpha particles are opposed by the gold atoms.

3. The size of an atomic nucleus is very small, which is about 10-15 m compared to the total volume or size of an atom. 

From this experiment, it is found that the radius of the nucleus is very small compared to that of an atom. It is about 104 times smaller than the radius of an atom. The size of a nucleus is found to vary directly with mass number A. If the radius of the nucleus is R, then                          

  R = R0A1/3 where R0 =1.2×10-15                

Atomic mass 

Atomic mass can be defined as the mass of a single atom, which is the sum of protons and the number of neutrons present in an atom. It can also be defined as the mass of 1/12 of the mass of carbon-12, which is expressed in AMU (atomic mass unit). 1 AMU corresponds to 1.660539040×10-24 grams.

For example, atomic mass of carbon = number of protons+number of neutrons  

= 6+6

= 12 amu

What is an isotope?

Federick Soddy was the first to propose the term isotope in 1913. Isotopes are groups of two or more elements or atoms that have an equal number of protons but different numbers of neutrons.

Properties of isotopes: 

1. Isotopes showing radioactivity are known as radioisotopes. Isotopes which are not radioactive are known as stable isotopes. 

2.  All isotopes of an element have a similar electronic structure since they have an equal number of electrons. 

3. All isotopes show similar chemical behaviour but different physical behaviour.

Example: Isotopes of Chlorine

Chlorine has two isotopes:

1. 37Cl: It has 17 protons and18 neutrons. 

2. 35Cl: It has 17 protons,20 neutrons.

What is an isobar?

Scientist Alfred Walter Stewart suggested the name isobar in 1918. Any group of nuclear species of atoms having an equal number of nucleons but different numbers of protons. 

Isobars are found to have different chemical behaviour as well as different physical properties.

Example: Ce76, Se 76 

Ce76 and Se 76 have the same mass number of 76 but contain different atomic numbers.

Conclusion

The nucleus consists of protons and neutrons situated at the centre of the atom. Most of the mass of an atom is in the atomic nucleus, which occupies a very small area of the total volume of the atom. Neutrons are neutral subatomic particles and have a mass of one atomic unit. Isotopes are two or more collections of elements that have an equal number of protons but a different mass number.