Atoms and Nuclei

Class XII/PHYSICS

Introduction

(1) In 1895, Jean Perin proved that electrons (known as cathode rays that time) are negatively charged.

(2) Shortly thereafter, J. J. Thomson determined charge ( e ) to mass ( m ) ratio of electrons to be of the order of

1011. This meant

that of electron is of the order of 10- 11 and that the mass of electron is very small.

(3) In 1909, Millikan measured the magnitude of the charge of an electron.

(4) Studies on X-rays, discovered in 1895, resulted in the discovery of radioactivity.

(5) Rutherford’s experiments on radioactivity proved emission of a-particles besides electrons in radioactive

radiations. Thus one

more particle was discovered.

(6) In the 19th century, scientists were trying to measure wavelengths of radiations emitted by different gases

filled in discharge

tubes using the diffraction grating discovered by Henry Rowland. These wavelengths were found to be

discrete and dependent

on the type of gas filled in the discharge tube.

(7) Same time Max Planck presented the photon theory and showed that the black body radiation is discrete.

Einstein explained

photoelectric effect using photon theory of light for which he received the Nobel prize.

(8) In 1902, J.J.Thomson proposed the first simple and most primitive model, which considered an atom to be a

sphere of

uniform positive charge into which the negatively charged electrons were embedded. This model was like a

plum pudding

dotted with raisins and was thus called as 'plum pudding' model

of the atom.

With this model he tried to explain the observed spectra of

elements.

According to him, the vibrations of electrons would give rise to

electromagnetic energy radiated. He could explain with some

success the

process like chemical reaction, radioactivity etc.

This model had many drawbacks. The chief among them are:

(a) Since the weight of an electron is about a thousandth part of a hydrogen atom, it would mean that a

single atom, especially of the heavier elements, would contain many thousand electrons. But J.J. Thomson

himself found that the number of electrons in an atom cannot be greatly different from the atomic weight.

(b) According to this model, hydrogen can give rise to only one spectral line, contrary to the observed fact of

several lines.

(c) This model could not explain the large angle scattering of alpha particles by thin metal foils.

Geiger-Marsden’s experiment and Rutherford’s model of atom

The schematic diagram of Geiger-Marsden’s experimental arrangement is

shown in the figure.

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------SRHSS/2010/ ATOMS AND NUCLEI Page 1 of 3 Prepared by: Sandhya.K

Class XII/PHYSICS

S is 83Bi214 source emitting a-particles of energy 5.5 MeV placed inside a thick block of lead. α-particles

coming out of the slit in the block were scattered when incident on a thin foil (F) of gold of thickness 2.1 x10 -

7 m. Scintillations were observed on a screen E of zinc sulphide mounted on a microscope M. The whole

arrangement was enclosed in a cylinder having thick walls. The cylinder was mounted on a thick disc which

could be rotated by the arrangement shown by T. Source S and foil F were kept steady on the base but could

be rotated around microscope M. Whenever any charged particle strikes at any point on

ZnS, a bright spot is formed at that point. The observations made in this scattering

experiment were as follows:

Most of the α-particles particles pass through the metal and are undeflected.

Some of the α-particles are deflected through small angles.

Only a very few of them are deflected through as much as 90o or even larger

angles.

The results of the scattering experiment could not explain the Thomson's atomic model. Hence, Rutherford

concluded that:

As most of the α-particles passed undeflected, most of the space inside the atom is empty or hollow.

Some that deflected with large angles show that heavy positively charged body must be present

inside the body of the atom, which repelled the like charge of the α-particles. This heavy, positively

charged body was named as nucleus.

The number of heavy positively charged particles that undergo deflection is very small. The volume

occupied by the nucleus is very small compared to the volume of the atom.

When heavy particles like the α-particles get deflected, the nucleus of the atom must also have an

appreciable mass.

Rutherford's nuclear model

On the basis of the scattering experiment, Rutherford described the structure of the atom as:

An atom consists of a positively charged nucleus surrounded by electrons that move around it. The

positive charge of the nucleus is due to the protons.

Electrons and neutrons are held together by Coulombic force of attraction.

The effective volume of the nucleus is extremely small as compared to the effective volume of the atom.

The volume occupied by the nucleus is about 10-12 times the volume of the atom.

The entire mass of the atom is concentrated at the nucleus.

Since each atom is electrically neutral, the number of positive charges in the nucleus of an atom is equal

to the number of electrons in it.

. The volume occupied by the nucleus is about 10-12 times the volume of the atom.

The entire mass of the atom is concentrated at the nucleus.

Since each atom is electrically neutral, the number of positive charges in the nucleus of an atom is equal

to the number of electrons in it.

Rutherford's model suffered some drawback, as it could not explain the

stability of the atom in-spite of the revolving electrons around the nucleus.

The moving electrons should emit radiations and lose energy. They should


Rutherford's model of an

atom

Class XII/PHYSICSslow down, gradually move towards the nucleus, follow a spiral path and then fall into the nucleus. This would

make the atom collapse and hence unstable, which is not true.


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Atoms and Nuclei