Atomic Physics

Atomic number of an atom (Z) = Number of protons present in the nucleus

Number of neutrons In nucleus-N

Mass number’ A’ – Z + N

Consider Helium atom,

Mass number He Thus helium has 2 protons and 2 neutrons. Atomic number

• The atomic number and mass number is used for notations because this represents an atom uniquely.
•  Atoms containing the same number of protons but different number of neutrons in their nude are known as Isotopes. The atom in that case is unstable. Isotopes of some elements a unstable. These are also termed as radioactive isotopes. Radium, Thorium and Uranium some of the radioactive elements. This instability occurs because there are too many or too for neutrons in the nucleus. When this situation exists, the nucleus produces radiations. (To radio means to spread out rays or tiny particles).
• These radiations include emission of alpha (α), beta (β), gamma (Y) particles/ rays.

Alpha (α) particles :

• It is made up of two protons and two neutrons, hence carry positive charge. Streams of these protons and neutrons can travel a few centimeters in air at about 3 x 10 cm/ sec. These are not very strong. They cannot penetrate paper or skin. However, they may be dangerous when inhaled. 

Beta (β) particles :

• They are more penetrating than a particles. They consist of electrons. These can travel a few meters. These can pass through skin, but will be stopped by a thin sheet of glass or metal. 

Gamma (Y) rays :

• These are not particles but electromagnetic waves which are very penetrating compared to others and is hazardous to health. They can pass easily right through human body and may be stopped by thick shields of lead or concrete.
•  A final form of radiation which should be mentioned is Neutron radiation which is used in nuclear power stations. A neutron on colliding with an atom Interacts with the nucleus and produces following effects : (1) Elastic collision, (II) Inelastic collision, (III) Absorption, (IV) Fission and (iv) are important in case of nuclear power plant.

Elastic collision :

• When a neutron collides with the nucleus of an atom, a part of the initial kinetic energy of the neutron is transferred to the nucleus and causes displacement of nucleus, thus changing structural properties of a material. The neutron slows down after elastic collision. This effect is utilized in nuclear reactor for slowing down the neutron. (Moderators are used that produce elastic collision).

Fission :

• This type of reaction is possible only with heavy nucleus such as Uranium 92235U,92235U.

• In general, heavy nucleus is more unstable. If a slow moving neutron hits this unstable nucleus, the neutron is absorbed and the nucleus splits into two smaller, more stable nuclei and two or three neutrons are usually released, and lot of heat energy is released. Actually heat is generated because these freed neutrons and split nuclei move at a very great speed and collide with another atom. During collision their kinetic energy is released in the form of heat. This energy is about 85% of the energy released from fission. Further, these neutrons strike another nucleus causing it to split and release two or three more neutrons and lot of heat energy is released. This chain or cycle will continue and enormous amount of energy will be liberated. This process is known as Chain Reaction. (Refer Figure).

Mass defect :
Fission can be stated in terms of mass defect. When slow neutron hits a heavy nucleus it splits up into two nearly equal parts with huge amount of energy. This energy release is due to defect i.e. the mass of two parts is not equal to mass of initial heavy atom. The deficit converted into heat energy or the mass defect is converted into heat energy.