How did Ernest Rutherford discover the nucleusHello friends, welcome to our blog, today we are going to discuss how did Sir Rutherford discovered the nucleus. In 1911, Rutherford performed alpha
(α) ray scattering experiment to know about the position of fundamental particles electron, proton in an atom. The alpha(α) particles were obtained form the radioactive element radium enclosed in the box of lead. After passing through a slit, the particles in the form of rays were made to strike against the very thin foil(4*10-5 cm thick) of heavy metal like gold. These particle got scattered and were made to fall on circular screen coated with zinc sulphide(ZnS). They produce flashes of light (also known as scintillation) on screen this help in identifying these particles.
From the experiment, Rutherford made the following observations
1. Most of the alpha particles (99%) passed through gold foil undeflected.
2. Some of these particles be deflected by small angles.
3. A very few alpha particles (one out of 20,000 particles) suffered major deflections by more than 90° and even came back in same direction.
Conclusion from the observationThe scattering experiment was performed on the gold foil consisting of atoms of gold closely packed in a space and arranged in layers. Rutherford was of the opinion that
1. Most of the space inside the atom is expected to be empty. The electrons are negligible mass was supposed be present in this space.
2. As a few of alpha particles suffered from minor deflections and are very few even major deflections, this means that they must have come across some obstruction in their path which must be very small(nucleus contains charge that repel positive charge alpha particle).
Rutherford regarded this(nucleus) small, heavy and positively charged portion inside the atom as a nucleus.All the positive charge protons was supposed to be present in the nucleus and they also accounted for its mass.Some of the space around the nucleus is called extra nuclear portion and all the electrons were supposed to be present in it.
Rutherford further stated the electron present in extra nuclear portion did not offer any objection to alpha particles and therefore most of them could pass undeflected.
Rutherford Model of atomIn the light of alpha Ray scattering experiment, Rutherford gave the following picture of an atom.
1. An atom consists of two parts nucleus and extra nuclear portion
2.Nucleus is present in the centre of atom. It is massive positively charged also extremely small in size. The radius of nucleus is about 10-15 m. while the atom is about 10-10 m. The size of nucleus is very small as compared to that of atom.
3. Positive charge of the nucleus is due to protons in the atoms. Since the atoms differ in number of protons therefore, the magnitude of positive charge on nucleus differ from atom to atom.
4. Extra nuclear portion is a space around the nucleus in which all the electrons are present.
5. Total positive charge of the nucleus is equal to the total net charge of electrons and the atom as a whole electrically neutral.
6. Electrons in the extra nuclear portion are not stationery but are revolving around the nucleus at a very highest speed in a circular path called orbits. The revolving electrons have certain centrifugal force acting away from the nucleus which balance the force of attraction directed towards nucleus.
The model of an atom is similar to the solar system where the nucleus is like a sun and electrons may be compared to the planets. The electrons are therefore, called as planetary electrons.