Discovery of Nucleus

Discovery of Nucleus

Thomson’s model of atoms is a conceptual representation like many other models in science. Scientists test scientific models by doing experiments to find out if they were wrong. The model proposed by Thomson was conceptual. Scientists were eager to test it by doing an experiment. How would you test if the model is correct or wrong? They are so small that even a powerful microscope is useless in peering inside an atom.

In 1905, Ernest Rutherford along with his scholars Hans Geiger and Ernest Marsden came up with an interesting idea to test the omson’s model. In omson’s model recall that the charges are symmetrically distributed. Suppose you shoot a highly energetic positively charged particle smaller than an atom, to collide at an atom, what do you expect? As the incoming particle is positive, it should be repelled by the positive atom. is is because you know that ˝like charges repel each other.˝ If according to plum pudding model, the positive charge of atoms is evenly distributed; it should be very small at each point inside the atom. But as the energy of the incoming particle is higher than the repulsion at the point of contact, the particle should overcome the repulsion and penetrate the atom.

Once it is inside the atom, the positively charged particle is repulsed on all sides with the same force. Assuming that atom is a uniformly positively charged mass with random moving electrons, the particle should come out of the other end of the atom almost unde ected. Some of the electrons inside the atom could attract the positively charged particle and make small change in the path. Therefore it can be predicted that deviation if any, be less than a small fraction of a degree and is negligible.

1. Rutherford’ s – ray scattering ex periment

Alpha particles are positively charged it possess adequate energy to overcome the repulsive force of positive charge, if the charge is evenly distributed in an atom. As you probably know, according to Coulomb’s law, the less concentrated a sphere of electric charge is, the weaker is its electric field at its surface.

Atoms are so small that you cannot pick them one by one to be kept as a target and shoot alpha particles. Gold as you may know is a highly malleable metal and can be made in to a very thin layer.

They arranged an experimental set up. A natural radioactive source that emitted highly energetic alpha particles was chosen. The source was kept inside a lead box with a small hole in it. Alpha particles came out of the source in all directions. Those particles which hit the walls of the box were absorbed by it. Only those alpha particles that were emitted in the direction of the hole could escape. These rays of alpha particles followed a straight line.

A thin gold foil, about 400 atoms thick, was kept on the path of the alpha particle. They also kept a circular screen coated with zinc sulphide surrounding the foil. When an alpha particle hit the screen, it would produce uorescence glow in the point where they struck the screen. From the point on the screen, one can infer the path taken by the alpha particle after penetrating the gold foil. The whole set up was kept inside a vacuum glass chamber, to avoid alpha particles from interacting and getting scattered by air molecules.

The experiments were repeated for reproducibility. Each time when the experiment was conducted, they computed and tabulated the angle of the rays of alpha particle after it hits the gold foil. They observed the following.

(I) Most of the fast moving α-particles passed straight through the gold foil.

(II) Some α particles were deflected by small angles and a few by large angles.

(III) Surprisingly very few α particles completely rebounded.

The experiments showed that most of the alpha particles behaved as expected, but there was a small discrepancy. Out of every 2000 particles that got scattered, just one was de ected by a full 180°. at is, they simply retraced their path after hitting the gold foil. You know that change of direction is possible only if a strong enough force acted against the direction of the motion of the particle.

Based on the plum pudding model of the atom, it was assumed that there was nothing dense or heavy enough inside the gold atoms to de ect the massive alpha particles from their paths. However, what Rutherford actually observed did not match his prediction. These observations indicated that a new model is needed to account for the evidences gathered in the experiment.

Rebound of alpha particle was impossible under the Thomson model. The alpha particle could have been deflected at 180° only if the positive charge was concentrated at a point rather than dispersed throughout the atom. If all the positive charge of the atom was concentrated at a small area inside the atom, only then, the electrostatic repulsion would be strong enough to bounce them back at 180°.

Now two observational evidence were before Rutherford and his team

1)    Most of the particles passed are not deviated as there was no obstruction to their path: This should imply that most part of the atom is empty

2)    Some alpha particle was deflected right back; implying that the positive charge should be concentrated at the centre of atom.

To be sure that their findings were really correct, the team performed the same type of experiments with many other materials including gases between the period 1908 and 1913.

Based upon these evidences, Rutherford rejected the omson’s idea and proposed that all the positive charges are concentrated in the central region of the atom called ‘nucleus’, and electrons orbit the nucleus at a distance. Further he stated that in between the nucleus and electron inside an atom there existed a void. is came to be called as planetary model of atom.

2. Rutherford’ s model of an atom- salient features

i.            Atom has a very small nucleus at the centre.

ii.            There is large empty space around the nucleus.

iii.            Entire mass of an atom is concentrated in a very small positively charged region which is called the nucleus.

iv.            Electrons are distributed in the vacant space around the nucleus.

v.            The electrons move in circular paths around the nucleus.

3. Limitations in Rutherford’ s model

Although the model suggested by Rutherford went beyond the one by omson and explained the behaviour of alpha particles, it also le a few questions unanswered. Planets can go around the Sun under the gravitational attraction. But negatively charged electron should be attracted by the positively charged nucleus, since opposite charges attract. But it does not happen that way.

It was shown by Clark Maxwell that a charged body moving under the in uence of attractive force loses energy continuously in the form of electromagnetic radiation. us unlike a planet the electron is a charged body and it emits radiations while revolving around the nucleus. As a result, the electron should lose energy at every turn and move closer and closer to the nucleus following a spiral path consequently the orbit will become smaller and smaller and nally the electron will fall into the nucleus. In other words, the atom should collapse. However, this never happens and atoms are stable.

Thus the stability of the atom could not be explained by Rutherford Model. There were also a few more objections to his model. This led on to more research and evolving better models of atomic structure.

4. Bohr’ s model of an atom

A new model of atom was needed because Rutherford model could not explain the stability of atom. Neils Bohr developed a successful model of hydrogen atom. In order to justify the stability of an atom Neils Bohr made some improvements on Rutherford’s model. The main postulates are:

I.            In atoms, electrons revolve around the nucleus in certain special or permissible orbits known as discrete orbits or shells or energy levels

II.            While revolving in these discrete orbits the electrons do not radiate energy.

III.            The circular orbits are numbered as 1, 2, 3, 4,… or designated as K, L, M, N, shells. These numbers are referred to as principal quantum numbers (n).

IV.            K shell (n=1) is closest to the nucleus and is associated with lowest energy. L, M, N, …. etc are the next higher energy levels. As the distance from the nucleus increases the energy of the shells also increases.

V.            The energy of each orbit or shell is a xed quantity and the energy is quantized.

VI.            As the distance from the nucleus increases, the size of the orbits also increases.

VII.            Maximum number of electrons that can be accommodated in an energy level is given by 2n2 where n is the principal quantum number of the orbit.

VIII.            When an electron absorbs energy, it jumps from lower energy level to higher energy level.

IX.            When an electron returns from higher energy level to lower energy level, it gives o energy.

5. Limitations of Bohr’ s model

Many arguments were raised against Bohr’s model of an atom. One main limitation was that his model was applicable only to Hydrogen. It could not be extended to multi electron atoms. Hence more research and deeper study of atoms became necessary. A detailed study of these aspects will be done in higher classes.

Orbit or shell:

Orbit is defined as the path by which electrons revolve around the nucleus.

Illustration:

The number of electrons in the first orbit (K)(n = 1);2 x 1² = 2

The number of electrons in the second orbit (L) (n = 2); 2 x 2² = 8