Modern Physics - Part II

Radioactive transformation
Radioactive transformation by emission of radiation such as , and - rays leads to nuclear transformation, as these radiations are emitted from the unstable nucleus. But before we see how the transformations occur, let us first recapitulate a bit about the atomic nucleus.

An atom is made up of three basic particles : positively charged protons, negatively charged electrons and neutral neutrons. Neutrons and protons make up the nucleus of an atom. The electrons circulate around the nucleus. 

Mass of a neutron is nearly equal to a mass of a proton. But an electron mass is 1800 times smaller than the proton mass. In an atom, the number of electrons is equal to the number of protons. Thus an atom as such is always electrically neutral.

Number of protons in an atom is known as the atomic number of the atom. Letter Z represents atomic number. Z = number of protons. Each element in the periodic table of elements has different Z.

For example : Hydrogen (H)  has 1 proton. Therefore its atomic number Z = 1

Calcium (Ca) has 20 protons. Therefore its atomic number Z = 20

An atom of Gold (Au) has 79 protons. Therefore its atomic number Z = 79.

Number of neutrons in an atom is denoted by N. N = number of neutrons.

Mass number is defined as the total number of protons and neutrons present in an atom. Mass number is denoted as A.

A =  Z + N

For example : Hydrogen (H)  has 1 proton and no neutrons.
Therefore its atomic number Z = 1, N = 0, A = 1.

Calcium (Ca)  has 20 protons and 20 neutrons.
Therefore its atomic number Z = 20, N=20 and A = 40.

An atom of Gold (Au) has 79 protons and 118 neutrons. 
Therefore its atomic number Z = 79, N = 118 and A = 197.

It has to be remembered that atomic mass and atomic weights are not exactly the same. Atomic weight takes into account the actual weight of an atom, including the weight of the electrons, their binding energies, etc.

We have now seen that there are three numbers which describe an atom : Z, N and A. The number of electrons is same as the number of protons Z, hence no extra number to show the electrons is required while representing an atom.

An atom  X is represented as    ^A_ZX_N  , ^A_ZX  or  as _ZX^A .

Even though the number N is not specifically written sometimes, from the values of A and Z, N can be calculated N = A – Z.

Examples :  Hydrogen is represented as ¹₁H,  Calcium is written as ⁴⁰₂₀Ca, Gold is written as ¹⁹⁷₇₉Au.

Atoms of an element having the same atomic number Z but different atomic masses A, are called isotopes of each other. Isotopes differ in their neutron numbers only. Chemically isotopes are identical. The chemical symbol for isotopes is same as that of its atom, since they have the same Z.

Example  1: Hydrogen has two isotopes deuterium and tritium.
Hydrogen : Z = 1, N = 0 , A = 1, represented as ¹₁H
Deuterium : Z = 1, N = 1, A = 2, represented as ²₁H
Tritium : Z = 1, N = 2, A = 3, represented as ³₁H.

             Isotopes of Hydrogen

Example 2 : Carbon has two isotopes. The number of protons in carbon is 6. The two isotopes have neutron numbers 6 and 8 and are called carbon-12 and carbon-14 respectively.

Example 3 : The number of protons in an uranium atom is 92. But some of the uranium atoms have N= 143 and some have N = 146. Thus there are two isotopes of Uranium. They are represented as  ²³⁵₉₂U  and  ²³⁸₉₂U.

When a radioactive transformation occurs, the nucleons rearrange themselves inside the nucleus. The original nucleus is called the parent nucleus and the product of the radioactive disintegration is called the daughter nucleus.

The rules or laws of radioactivity are as follows :
1. When an - particle is emitted, the atomic number Z reduces by 2 and the neutron number N reduces by 2 (atomic mass A reduces by 4). This is because an - particle is nothing but doubly ionized helium atom (or helium nucleus).

  ^A_ZX_N                ^A-4_Z-2Y_N-2    + ⁴₂He

 ^A_ZX_N is the parent nucleus and ^A-4_Z-2Y_N-2 is the daughter nucleus.

2. When the parent nucleus ^A_ZX_N  emits a    or an electron, the mass number A remains the same, but the atomic number Z increases by one to Z + 1.

  ^A_ZX_N               ^A_Z+1Y_N-1   + e^-

When the parent nucleus ^A_ZX_N   emits a  or a positron, the mass number A remains the same, but the atomic number Z decreases by one to Z - 1.

  ^A_ZX_N                ^A_Z-1Y_N+1   + e⁺

3. When a parent nucleus ^A_ZX_N emits a - radiation, the  atomic mass number A and the proton number Z as well as the neutron number N remains unchanged. The nucleons jump from higher excited levels to lower level and emit a - radiation only.

  ^A_ZX_N                ^A_ZX_N     +             

Properties of  - particles :

- particles are nuclei of He. Emission of - particles reduces the mass number by 4 and atomic number by 2 of the emitting parent nucleus. - particles are positively charged particles. Their charge is 2e. They produce intense ionization of the surrounding material through which they pass. They are not very penetrating, as their mass is high. Being charged particles, they are deflected by both electric and magnetic fields.

Properties of  - particles :

- particles are made of two types : - particles are electrons. They are negatively charged. - particles are positrons. They are positively charged. Emission of - particles leads to change in the atomic number Z, but the atomic mass number A remains unchanged. - particles are generally emitted with high energies. Since the mass of the - particles is small, their ionizing power while passing through any matter is relatively small. The penetrating power of - particles is larger than that observed for the - particles. But the penetration is less than that seen for the -rays. Since - particles are charged particles, they can be deflected by electric and magnetic fields.

Properties of  -rays :

-rays are photons or electromagnetic waves. Their wavelength is very small ~ 10-10m. Emission of a -ray does not change any of the A or Z or N numbers. -rays are uncharged and have high energies. -rays have very high penetrating power, higher than both the alpha rays and the beta rays. -rays have very low ionization power. Since they are uncharged, -rays cannot be deflected by either electric or magnetic fields.

Next        Main        Previous