ICSE Class 8 Chemistry Chapter 04 The Structure of the Atom

The Structure of the Atom

As you know, everything is ultimately made of atoms, and atoms are made up of subatomic particles - electrons, protons and neutrons. Let us learn a little more about this idea that has brought about a revolution in science.

How the Idea of Atoms Emerged

The Views of Kanad

Way back in the sixth century BC, the Indian philosopher Kanad came forward with the following idea.

Matter is

- not continuous, and

- made up of tiny particles, named paramanus.

(In Sanskrit, param means final or ultimate, and anu means particle.)

Kanad further said that two or more paramanus combine to form bigger particles.

The Views of Democritus and Leukiposs

In the fifth century BC, the Greek philosophers Democritus and Leukiposs came up with a similar idea. They thought that on dividing a piece of a substance, one would ultimately get a particle that could not be divided further. They gave the name atomos (in Greek, atomos means indivisible) to these ultimate particles.

Dalton's Theory

The theories of Kanad as well as of Democritus and Leukiposs remained forgotten for more than two thousand years. But when experimental chemistry developed, it became necessary to explain the observed facts. In this connection, in 1803, an English chemist, John Dalton, put forward his atomic theory, which can be summarised as follows.

1. Elements are made up of very small particles of matter, called atoms (derived from the Greek word atomos).

2. Atoms are indivisible.

3. The atoms of an element have the same weight.

4. The atoms of different elements have different weights.

5. It is the atoms of elements that take part in a chemical reaction.

6. The atoms of an element combine in a simple numerical ratio with those of other element(s) to form a compound.

An atom is defined as the smallest part of an element that takes part in a chemical reaction.

The Subatomic Particles

In the late nineteenth century, however, it was proved that atoms are divisible. And later it was found that atoms are made up of subatomic (or fundamental) particles - electrons, protons and neutrons.

The Electron

Under ordinary conditions, gases are bad conductors of electricity. But a gas becomes a good conductor of electricity if

(i) the pressure of the gas is very low (say, 10 mm of mercury or lower), and

(ii) the voltage applied is very high (say, 10,000 V).

These conditions are achieved in what is called a discharge tube.

Cathode rays

A discharge tube (Figure 4.1) is a long glass tube, at the two ends of which are sealed two metal plates. These plates can be connected to a high-voltage source and are called electrodes. The electrode connected to the negative terminal of the source is called the cathode, and the one connected to the positive terminal is called the anode. There is also a side tube which can be connected to an exhaust pump, used for lowering the pressure of the gas inside the discharge tube.

When a high voltage is applied across the terminals, and the pressure inside the tube is 0.01-0.001 mm of mercury, the end of the tube opposite the cathode starts glowing. This phenomenon is called fluorescence. Investigations have shown that invisible rays, starting from the cathode, fall on the opposite wall of the tube, causing fluorescence. These rays were named cathode rays.

The characteristics of cathode rays

Sir J J Thomson and others found that cathode rays have the following characteristics.

1. Cathode rays originate at the cathode and travel in straight lines.

When an object is placed between the cathode and the anode, a shadow of the object falls on the wall opposite the cathode. A shadow can be formed only when the rays travel in straight lines.

2. Cathode rays are a stream of particles.

A light paddle wheel, placed in the path of the cathode rays, rotates. This shows that some particles strike the plates of the wheel.

3. The particles constituting cathode rays are negatively charged.

This is proved by the fact that the cathode rays bend towards the positive plate in the presence of an electric field.

4. The particles constituting cathode rays have a fixed charge to mass ratio (e/m). This ratio does not change with the gas, the electrodes and the kind of glass used for making the tube.

Thus, Sir J J Thomson concluded that the particles constituting cathode rays are a universal constituent of all atoms. He named these particles electrons in 1897.

The charge and mass of an electron

Absolute charge and mass The charge to mass ratio (e/m) of an electron was determined by J J Thomson to be 1.78 x 10^8 C/g (coulomb per gram). In 1908, R A Millikan determined the charge of an electron to be 1.6 x 10^-19 C. Thus, the mass of an electron can be calculated as follows.

\[e = 1.6 \times 10^{-19}C\]

\[\frac{e}{m} = 1.78 \times 10^8 C/g\]

\[m = \frac{e}{1.78 \times 10^8 C / g} = \frac{1.6 \times 10^{-19}C}{1.78 \times 10^8 C / g}\]

\[= 9.1 \times 10^{-28} g\]

\[= 9.1 \times 10^{-28} \times 10^{-3}kg\]

\[= 9.1 \times 10^{-31} kg\]

Relative charge and mass The charge on an electron is taken as the unit of negative charge. So it is said to have a charge of -1 unit. The mass of an electron is about 1/1840 that of a hydrogen atom and so it is treated as negligible.

The Proton

An atom is electrically neutral. But the electrons present in it are negatively charged particles. Hence, the atom must also contain some positively charged particles so that the overall charge on it becomes zero. These particles should be found in the discharge tube itself, when cathode rays are formed.

Goldstein repeated the cathode-ray experiment using a perforated cathode (Figure 4.5). He observed that there was a glow on the wall opposite the anode. So, some rays must be travelling in the direction opposite that of the cathode rays, i.e., from the anode towards the cathode. These rays were called anode rays or canal rays (as they moved through the perforations, or canals, in the cathode). It was found that these rays contained positively charged particles, and so J J Thomson called them positive rays.

The characteristics of anode rays

The characteristics of anode rays were found by carrying out experiments similar to those with cathode rays. The following characteristics distinguish anode rays from cathode rays.

1. Anode rays are a stream of positively charged particles (because they bend towards the negative plate in an electric field). The charge on a particle which is part of an anode ray is the same (1.6 x 10^-19 C) as that on an electron, but opposite in sign.

2. The charge to mass ratio (e/m) of the particles constituting anode rays is not

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