Isotopes

Atoms have two important properties, the atomic number and the mass number. The atomic number is the number of protons present in the nucleus. The mass number is the sum of protons and neutrons in the nucleus.

The atomic number is specific to an atom, each element has a fixed atomic number, thus the atomic number of Sodium is always 11, Mg is 12 and so on.
Two atoms which have the same number of protons but different number of neutrons, are called isotopes. Thus isotopes are atoms of the same element with different number of neutrons.

1. Isotopes of other elements may not have specific names.
2. Isotopes have the same chemical properties but different physical properties.
   (Physical properties: boiling point, melting point, atomic mass etc,.)
3. An element may exist in many isotopic forms, but their relative abundance in nature is not the same.
4. For example, carbon exists mainly as ¹²C₆ , ¹³C₆ , but their percentage in nature is 100 to 1.08 in that order.
5. Atoms which have same mass number and different atomic number are called isobars; they are atoms of different elements.

So far we learnt what are isotopes, but in what way are they are useful to us?
There are innumerable uses/applications. We will consider two important applications.

¹⁴C₆ Dating of a sample

Is it possible to find out the age of a biological sample, a piece of wood or a sample of any organism that was living at some point of time? (that is how long before it was living). Yes, by using a technique called ¹⁴C₆ dating studies. Here is how it is done.When an organism (even a human) is living, it is taking in carbon in the form of food, animals, in the form of starch and cellulose, plants in the form of CO₂ . The percentage of ¹⁴C and ¹²C in the atmosphere is fixed so the living organism also has the same ratio as in the atmosphere. ¹⁴C is radioactive. Let us know something about radio activity.

Certain elements or their isotopes emit what are called radio active rays from their nuclei. Such elements are radio active. This emission is constant and does not depend on any type of conditions physical or chemical. Heat chemical or physical state does not have any influence on this activity, it cannot be slowed down speeded up or stopped.

Since radio active rays are coming out of the nucleus, the nucleus will change and can become another element. So the atom after giving out radiation changes to some other atom or even element. The important point is that the time taken for a certain quantity of a radio active substance to disintegrate to half its initial value is called half life period and this is a constant which does not depend on the initial quantity. The process is said to follow first order kinetics, some chemical reactions also follow these kinetics.

What it means in simple terms is if the process starts with 100 gms and it takes 10 mts to become half of its initial value, for 50 gms to change to 25 it takes the same time of 10 mts. The half period here is 10 mts.