NCERT Class 9 Science Exploration Chapter 08 Journey Inside the Atom PDF Download

Chapter 8: Journey Inside the Atom

Everything that you see or observe, or feel around is matter. You have learnt that matter consists of tiny particles called atoms. Take a closer look at the picture given above. What do you observe? Do you notice that both living beings, like us, and non-living things, like a house, are ultimately composed of atoms? These atoms are so tiny that they cannot be seen with the naked eye.

You may be wondering - Is an atom truly the smallest unit of matter, or can it be divided even further?

Scientists, too, have been exploring whether atoms are divisible. If so, what are their constituents and how are these arranged? Let us examine how the concept of atoms emerged and how it has been evolved since then.

Think It Over

• Are atoms the smallest indivisible particles?
• Why do electrons not fall into the nucleus even though they are attracted to protons in it?
• Why did scientists keep modifying atomic models?

8.1 Rediscovering the Roots of Atomic Theory

Let us embark on a journey that takes us back more than 2,000 years, to the intellectual landscapes of ancient India and ancient Greece. In these distant yet remarkably parallel civilisations, profound thinkers, such as Acharya Kanada in India, and Leucippus and Democritus in Greece, pondered over the same fundamental question that has continued to inspire human inquiry across centuries - What is everything made up of?

Acharya Kanada suggested that if matter (dravya) is divided repeatedly, you will reach a stage where you would encounter the smallest particles that can no longer be divided. He called these particles parmanus. His ideas are recorded in the Sanskrit text Vaisesika Sutras. A parmanu is infinitely small and cannot be perceived by the senses. Combinations of these forms dyads (groups of two parmanus) and triads (groups of three parmanus), and so on. It is out of these combinations that the whole of the material universe, including the bodies of living beings, is created. However, this description does not specify the proportions in which parmanus combine to form different substances.

The Greek philosophers Leucippus and Democritus also proposed a similar idea. They called these indivisible particles atomos (in Greek, atomos means indivisible).

You must remember that the concept of 'atom' originated as an imaginary idea rather than from experimental observations.

Teacher's Note

Notice that ancient thinkers in India and Greece arrived at similar ideas about matter being made of tiny indivisible particles, without any experiments or modern equipment. This shows how powerful logical thinking can be, but also why we need experiments to test and refine our ideas.

Many centuries later, in 1808, John Dalton proposed his atomic theory. It was based on scientific experiments of that time. He proposed that all matter is composed of indivisible particles called atoms. That is, the atoms are the fundamental building blocks of matter that cannot be broken down into smaller parts. Dalton's atomic theory was the first scientific description of how matter is made. It became the starting point for the current understanding of atomic structure.

You may be wondering how Dalton's idea evolved into a model for the structure of the atom.

Following Dalton's theory, scientists were curious to know the answers of the questions:

• What are atoms made up of?
• What would atoms look like if we could see them?
• What makes the atoms of one element different from the atoms of another element?

8.2 A Short Historical Journey Through Atomic Models

More than a hundred years ago, scientists tried to imagine what atoms might look like by proposing simple models. As new experiments were performed and new evidence came to light, these models were changed and improved. Although we now know that the early models were not fully correct, they are still important because they show how science moves forward - one step at a time, driven by curiosity, questioning, and experimentation.

Until the late 19th century, atoms were thought to be the smallest, indivisible units of matter. However, scientists discovered that certain elements emit invisible energy and particles called radiation, a phenomenon known as radioactivity. This showed that atoms must be composed of smaller particles, proving that they were not indivisible as previously believed.

In 1897, J. J. Thomson studied the conduction of electric current through gases at a very low pressure. He used a glass tube with two electrodes and applied a high voltage. He observed rays moving from the cathode (negative electrode) to the anode (positive electrode) (Fig. 8.1). These were called cathode rays. By studying these cathode rays in electric and magnetic fields, he concluded that they are streams of negatively charged particles, with a much smaller mass than atoms. These particles, later called electrons, were emitted from atoms, indicating that atoms are composed of smaller subatomic components.

[Figure 8.1: A line diagram of cathode ray tube, See in your textbook]

It was found that the nature of cathode rays was independent of the material of the cathode and the gas filled in the cathode ray tube. It showed that electrons are a fundamental component of all atoms, present in every element. The charge of an electron (\( -1.602 \times 10^{-19} \) C) is taken as \( -1 \) as a matter of convention and convenience.

8.2.1 Thomson's model of an atom

When J. J. Thomson discovered tiny negatively charged particles called electrons, he faced a puzzle - atoms are neutral, so where is the positive charge present? To solve this, Thomson proposed the atom to be a sphere of positive charge with electrons distributed throughout it (Fig. 8.2). This model was compared to a pudding with plums embedded in it, called the plum pudding model. A more familiar picture would be that of a watermelon (Fig. 8.3), where the red pulp represents the positively charged matter, and the seeds represent electrons distributed throughout the atom. This simple picture, though later replaced, was the first genuine attempt to describe how the atom's positive and negative charges stay balanced.

[Figure 8.2: Thomson's model of an atom, See in your textbook]

[Figure 8.3: Watermelon, See in your textbook]

Meet a Scientist

J. J. Thomson's most significant discovery was that of the electron, the first subatomic particle to be identified and a part of every atom. He received the Nobel Prize in Physics in 1906 for his studies of the electrical conductivity of gases. This research led him to discover electrons. As the head of the famous Cavendish Laboratory in Cambridge, he guided and inspired many scientists, including Ernest Rutherford.

Teacher's Note

When you read that Thomson won the Nobel Prize in 1906 for discovering the electron, remember that his discovery in 1897 took nearly a decade to be formally recognized. In the same laboratory, his student Rutherford later discovered the nucleus, which contradicted Thomson's model - this is how science works, with each generation building on and challenging the work of the previous one.

Key Points

• Ancient philosophers like Acharya Kanada and Democritus imagined that matter is made of indivisible particles, but Dalton was the first to base this idea on experiments.
• J. J. Thomson discovered electrons by studying cathode rays, proving that atoms are made of smaller particles, not indivisible units.
• Thomson's plum pudding model explained how electrons fit inside atoms, with a positively charged sphere containing negatively charged electrons throughout.

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