Nucleus CLASS 12 JEE MAINS

• The nucleus is the tiny, dense central part of an atom.

• It contains almost all the mass of the atom, even though it occupies a very small volume compared to the whole atom.

• Discovered by Ernest Rutherford in 1911 through the gold foil experiment.

Constituents of a Nucleus

The nucleus is made of two types of particles:

1. Protons

   • Positively charged particles.

   • Number of protons = atomic number (Z) of the element.

   • Determines the chemical identity of the element.

2. Neutrons

   • Neutral particles (no charge).

   • Number of neutrons = mass number (A) – atomic number (Z).

   • Help in stabilizing the nucleus by reducing the repulsion between protons.

Characteristics of Nuclei

1. Size:

   • Extremely small (typical radius ~ 1 femtometer = 10−1510^{-15}10−15 m).

   • Compared to an atom, the nucleus is like a pea in a football stadium.

2. Mass:

   • Most of the atom’s mass is in the nucleus because protons and neutrons are much heavier than electrons.

3. Charge:

   • Positive due to protons.

   • Number of protons = nuclear charge.

4. Density:

   • Nuclei are extremely dense, about 101410^{14}1014 times denser than water.

5. Binding Energy:

   • The nucleus is held together by the strong nuclear force, which is much stronger than the electric repulsion between protons.

Terminology

1. Atomic Number (Z): Number of protons in the nucleus.

2. Mass Number (A): Total number of protons + neutrons.

3. Isotopes: Atoms with same Z but different A (same protons, different neutrons).

4. Isobars: Atoms with same A but different Z.

5. Isotones: Atoms with same number of neutrons but different protons.

Key Points to Remember

• Nucleus is positively charged, dense, and massive.

• Protons determine identity, neutrons help stability.

• Electrons orbit the nucleus and occupy almost all the volume of the atom, but contribute little to its mass.