Maharashtra Board Class 12 Physics Chapter 15 Structure of Atoms and Nuclei PDF Download

15. Structure of Atoms and Nuclei

Greek philosophers Leucippus (-370 BC) and Democritus (460 – 370 BC) were the first scientists to propose, in the 5th century BC, that matter is made of indivisible parts called atoms. Dalton (1766-1844) gave his atomic theory in early nineteenth century. According to his theory (i) matter is made up of indestructible particles, (ii) atoms of a given element are identical and (iii) atoms can combine with other atoms to form new substances. That atoms were indestructible was shown to be wrong by the experiments of J. J. Thomson (1856-1940) who discovered electrons in 1887. He then proceeded to give his atomic model which had some deficiencies and was later improved upon by Ernest Rutherford (1871-1937) and Niels Bohr (1885-1962). We will discuss these different models in this Chapter. You have already studied about atoms and nuclei in XIth Std. in chemistry. This chapter will enable you to consolidate your concepts in this subject.

We will learn that, an atom contains a tiny nucleus whose size (radius) is about 100000 times smaller than the size of an atom. The nucleus contains all the positive charge of the atom and also 99.9% of its mass. In this Chapter we will also study properties of the nucleus, the forces that keep it intact, its radioactive decays and about the energy that can be obtained from it.

15.1. Introduction

Greek philosophers Leucippus (-370 BC) and Democritus (460 – 370 BC) were the first scientists to propose, in the 5th century BC, that matter is made of indivisible parts called atoms. Dalton (1766-1844) gave his atomic theory in early nineteenth century. According to his theory (i) matter is made up of indestructible particles, (ii) atoms of a given element are identical and (iii) atoms can combine with other atoms to form new substances. That atoms were indestructible was shown to be wrong by the experiments of J. J. Thomson (1856-1940) who discovered electrons in 1887. He then proceeded to give his atomic model which had some deficiencies and was later improved upon by Ernest Rutherford (1871-1937) and Niels Bohr (1885-1962). We will discuss these different models in this Chapter. You have already studied about atoms and nuclei in XIth Std. in chemistry. This chapter will enable you to consolidate your concepts in this subject.

We will learn that, an atom contains a tiny nucleus whose size (radius) is about 100000 times smaller than the size of an atom. The nucleus contains all the positive charge of the atom and also 99.9% of its mass. In this Chapter we will also study properties of the nucleus, the forces that keep it intact, its radioactive decays and about the energy that can be obtained from it.

15.2. Thomson's Atomic Model

Thomson performed several experiments with glass vacuum tube wherein a voltage was applied between two electrodes inside an evacuated tube. The cathode was seen to emit rays which produced a glow when they struck the glass behind the anode. By studying the properties of these rays, he concluded that the rays are made up of negatively charged particles which he called electrons. This demonstrated that atoms are not indestructible. They contain electrons which are emitted by the cathode.

Thomson proposed his model of an atom in 1903. According to this model an atom is a sphere having a uniform positive charge in which electrons are embedded. This model is referred to as Plum-pudding model. The total positive charge is equal to the total negative charge of electrons in the atom, rendering it electrically neutral. As the whole solid sphere is uniformly positively charged, the positive charge cannot come out and only the negatively charged electrons which are small, can be emitted. The model also explained the formation of ions and ionic compounds. However, further experiments on structure of atoms which are described below, showed the distribution of charges to be very different than what was proposed in Thomson's model.

Teacher's Note

Thomson discovered electrons using a vacuum tube. This is like how TV screens work. The electrons are the tiny negative particles that make pictures on the screen.

Exam Trick

Remember: Thomson's plum-pudding model = positive charge spread everywhere like a pudding, with electrons like plums inside. This model was wrong because the nucleus is very small and hard, not soft like pudding.

Points to Remember

Thomson found that atoms have electrons inside them.
Electrons are negatively charged.
Thomson said atoms are like plums in a pudding.
This model was later proved wrong.

15.3 Geiger-Marsden Experiment

In order to understand the structure of atoms, Rutherford suggested an experiment for scattering of alpha particles by atoms. Alpha particles are helium nuclei and are positively charged (having charge of two protons). The experiment was performed by his colleagues Geiger (1882-1945) and Marsden (1889-1970) between 1908 and 1913. A sketch of the experimental set up is shown in Fig.15.1. Alpha particles from a source were collimated, i.e., focused into a narrow beam, and were made to fall on a gold foil. The scattered particles produced scintillations on the surrounding screen. The scintillations could be observed through a microscope which could be moved to cover different angles with respect to the incident beam. It was found that most alpha particles passed straight through the foil while a few were deflected (scattered) through various scattering angles. A typical scattering angle is shown by T in the figure. Only about 0.14% of the incident alpha particles were scattered through angles larger than 0.1°. Even out of these, most were deflected through very small angles. About one alpha particle in 8000 was deflected through angle larger than 90° and a fewer still were deflected through angles as large as 180°.

Teacher's Note

The Geiger-Marsden experiment is like throwing stones at a thin metal sheet. Most stones pass through, but a few bounce back. This tells us that the metal is mostly empty space with tiny hard bits inside.

Exam Trick

Remember: Most alpha particles pass straight through = atom is mostly empty. Few particles bounce back = there is a tiny hard nucleus in the center of the atom.

Points to Remember

Alpha particles are helium nuclei that are positively charged.
Most alpha particles passed straight through the gold foil.
A few alpha particles bounced back.
This showed that atoms have a small, hard nucleus inside.
Most of the atom is empty space.

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