1. Prepared By: Shakil Raiman

2.  Atoms are made up of electrons, protons and neutrons.  The diameter of the nucleus is about 10,000 times smaller than the diameter of the atoms.

3.  Check the video.

4.  Mass Number: Total number of protons and neutrons present in an atom is called it’s mass number. It is denoted by A.  Atomic Number: The number of protons present in an atom is called it’s atomic number. It is denoted by Z.

5.  Number of neutrons = no of nucleons – number of protons  = A - Z

6.  Atoms of the same element having same number of protons but different number of neutrons are called isotopes.

8.  Isotopes have different mass and different stability.  The protons are held in the nucleus by the nuclear force. This force is very strong and acts over a very small distance. It is strong enough to hole the nucleus together against the electric force repelling the protons away from each other as protons carry positive charge and like charges repel.  The presence of neutrons in the nucleus affects the balance between these forces.

9.  Too many or too few neutrons will make the nucleus unstable.  An unstable nucleus will eventually decay.  When the nucleus of an atom decays it gives out energy and may also give out alpha or beta particles.

10.  When unstable nuclei decay they give out ionising radiation. Ionising radiations causes atoms to gain or lose electric charge forming ions.  Unstable nuclei decay at random. This means that it is not possible to predict which unstable nucleus in a piece of radioactive material will decay or when the decay will happen.

11.  There are three types of ionising radiation:  They are: alpha(α), beta(β) and gamma (  ) radiation.

12.  Alpha particles are helium nuclei.  The relative charge is +2.  It has relatively high mass of mass number 4.  Alpha particle has short range (a few cm) in air and can be stopped by a piece of paper.  They have short range because they interact with atoms along their paths, causing ions to form. This means that they rapidly give up the energy.

13.  Beta particles are very fast-moving electrons.  The relative charge is -1.  Beta particle has longer range (a few m) in air and can be stopped by a piece of 2mm Al sheet.  They have longer range because they interact with atoms along their paths less frequently, causing less ions to form. This means that they slowly give up the energy.

14.  Electromagnetic waves with very short wavelength.  It is emitted as packets of energy called photons.  It has no mass and no charge.  It has very less ionising power.  They are extremely penetrating.  Longest range (few hundred metres)

16.  Wish you all very good luck and excellent result.