OVERVIEW
In 1896, Marie Curie started her Research in Radioactivity. Nucleus of the atom stays bound due to strong Nuclear force. But some unstable nucleus disintegrate spontaneously and shows radioactivity. There are 3 types of radioactive decay. This discovery has revolutionised Science in many ways.
The story begins in 1896. After a series of experiments, Henri Becquerel (French physicist) found that uranium emits some unique kind of radiation & that too spontaneously. By spontaneously, I mean that it needs no sunlight or no other source of energy to ignite the reaction. He named them “Uranic rays”.
Obvious question Arises,
Where is this Energy Coming From?
A lot of research was required for that.
This discovery by Henri Becquerel caught attention of Marie Curie and she was extremely electrified to start working on it.
Marie curie shared Nobel Prize in Physics,1903 for her extraordinary contribution in understanding Radioactivity along with Pierre curie and Henri Becquerel
She soon discovered and presented two Important conclusions:
- It was not limited to Uranium. Even Thorium showed the same behaviour.
- The strength of radiation depends solely on elements quantity. No matter how you treat it or mix it with other chemicals, the radiation’s strength varied just with element’s quantity.This is it : More the element more emission of radiation.
Marie Curie coined the term “Radioactivity” for this behaviour of atom. And thus elements that exhibit this radioactivity (i.e emit energy spontaneously) are called radioactive elements.
These two conclusions took us a step closer (actually a big step closer) to the real nature of matter.
We got a hint that some elements emits energy due something deep inside their atom.
Because, this behaviour is not dependent on chemical properties (which comes from the electrons orbiting the nucleus.)
So, it was due to the Nucleus. The energy is coming out of nucleus. But how? It was a great mystery of that time.
In this process, something is coming out of nucleus i.e disintegration of nucleus is happening. One nucleus is changing into other. As you know, Nucleus is key to atom’s identity. So if anything happens to that it will turn one element to other.
This is the concern:
Some nucleus are stable and do not disintegrate spontaneously.
While some are unstable. They disintegrate and show radioactivity.
But, what makes the nucleus unstable?
Actually the correct thing is to think why nucleus is stable at first place? Shouldn’t the protons repel each other ( due to electromagnetic force). This way the nucleus would just fall apart.
Strong Nuclear Force
is the Saviour.
One of the four fundamental forces, strong nuclear force , is responsible for binding the nucleus together.
Strong Nuclear Force: The strong force is ‘felt’ between nucleons (protons and neutrons) inside of the nucleus of an atom. This force is strong enough that it overcomes the repulsive force between the two positively charged protons, allowing protons and neutrons to stick together in an unimaginably small space.
So, why and when is the nucleus unstable?
Nucleus becomes unstable under 2 conditions.
- No. of Protons > No. of neutrons
- No. of Protons << No. of neutrons
In both the cases, nucleus becomes unstable and the element shows radioactivity.
But, how does Nucleus instability relates to radioactivity?
Here’s how. When the the nucleus is unstable it disintegrates or decays. This results in emission of energy. And it decays or releases energy spontaneously. That is called radioactivity.
This is how radioactivity related to nucleus instability.
Okay, Then What Comes Out Of Nucleus?
The process is called Radioactive Decay.
A radioactive element can undergo 3 types of decay :
Alpha decay: The parent nucleus release an alpha particle (helium nucleus i.e 2 protons and 2 neutrons) and turns into daughter nucleus i.e the two radioactive element turns into element two places lower in the periodic table. (Eg: Uranium to Thorium)
Beta Decay: There are two types of beta decay.
- β – deacy : This occurs when there are too many neutrons in the nucleus for it to be stable. Thus, Neutron gets converted into a proton and an electron. As a result, one neutron decreases while one proton increases The former element changes into an isotope of element one place higher in Periodic Table.
- β + deacy : This occurs when there are too many protons in the nucleus for it to be stable. Thus, proton gets converted into neutron and an electron. As a result, one proton decreases while one neutron increases The former element changes into an isotope of element one place higher in Periodic Table.
Gamma Decay: This is bit different from above two. When a nucleus undergo gamma decay, it doesn’t emit particles but emit Radiation. A high energy, extremely short wavelength electromagnetic radiation called, Gamma Rays are radiated.
If one element changes into other, how do we know if that first one have existed at all ?
This depends on how quickly the atom decays. The Rate of Radioactive Decay is defined by a term called “Half-Life”
Half-Life : the Time taken for a quantity to reduce to half its initial value.
For example, You have 100 g of cobalt-60 (whose half-life is 5.26 years). After 5.26 years, you will have 50g left ; after 10.52 years 25 g will be left; after 15.78 years 12.5 g if cobalt-60 will be and the cycle goes on…
The take away is even after hundreds of years trace quantities of radioactive element will still be present. That is how we know the existence of radioactive elements. (They are present in extremely less quantities now)
How do we know if a element is Radioactive?
The Answer is 82. All elements with atomic number more than 82 are radioactive. This graph can be extremely useful to know if an element is Radioactive or not (you must have seen this in your high school physics textbook!)
Are elements below 82 not radioactive at all?
No, there are some isotopes which fulfill the conditions of nucleus to be unstable.
For example , carbon 14. There are many elements below 82 whose isotopes are Radioactive .
This discovery of radioactivity has revolutionized not just physics but the whole Science (especially the medical physics). there are tons of uses of radioactivity and still continues to open up new Research opportunities.