ICSE Class 8 Chemistry Chapter 1 Structure of Atom

Structure of Atom

In This Chapter You Will Learn

Ancient views on atom

Sub-particles of atoms

Discovery of electrons, protons and neutrons

Atomic number, mass number, atomic mass

Arrangement of electrons in an atom

Valency, ions and chemical bonds

Radioactivity

Dalton's atomic theory

Atomic models

Atom and Its Structure

Atoms: Building Blocks of Matter

You are aware that anything which occupies space and has mass is matter. But do you know the smallest particle of matter is the atom. Atoms are extremely minute particles. They cannot be seen through the naked eye. However there are experimental proofs about the existence of atoms, and they can even be seen through very powerful electron microscopes.

In the ancient times, Indian and Greek philosophers were puzzled about the nature of matter. Gradually the idea developed that all matter must be made of some basic elements.

In the ancient times water, earth, fire, air and sky were thought to be the five fundamental elements. But we know now that an element is a pure substance made up of one kind of atoms and has definite set of properties.

Maharishi Kanad's Views of Atom

Kanad was a great Indian philosopher (600 BCE). According to him, "matter consisted of indestructible particles called paramanus (param means ultimate and anu means particle) (now called atoms)". A paramanu does not exist in free state, rather it combines with other paramanus to form a bigger particle called the anu (now known as a molecule). There are different types of paramanus. Each one of them exhibits specific properties.

The Greek philosopher Democritus (460 BC - 370 BC) called the paramanu as "atom", which comes from the Greek word atomos, meaning indivisible.

Dalton's Atomic Theory

In 1808, John Dalton, an English scientist, described the atom as the smallest particle exhibiting all the properties of a particular element. According to him, atoms could not be divided further into smaller particles.

The fundamentals of Dalton's atomic theory are:

1. Matter consists of very small and indivisible particles called atoms, which can neither be created nor can be destroyed.

2. The atoms of an element are alike in all respects but they differ from the atoms of other elements.

3. Atoms of an element combine in small numbers to form molecules of the element.

4. Atoms of one element combine with atoms of another element in simple numerical ratio to form molecules of compounds.

5. Atoms are the smallest units of matter that can take part in a chemical reaction.

Note: The latest research works about atoms have proved that most of Dalton's atomic theory is incorrect. But Dalton was right that atoms take part in chemical reactions.

Sub-Atomic (Fundamental) Particles of Atoms and Early Models of Matter

Studies and discoveries in the late nineteenth and the early twentieth centuries showed that atoms are divisible i.e. they are composed of still smaller particles. The three main particles present in an atom are electrons, proton and neutrons. These particles are also called fundamental particles or sub-atomic particles.

The existence of the sub-atomic particles was proved by the fact that an atom is electrically neutral but it can be made to gain a positive or a negative charge. This means that an atom must contain tiny particles each carrying either a positive or a negative charge. These opposing charges balance each other under ordinary conditions to make an atom electrically neutral.

Discovery of Electrons (e-)

Electrons were discovered in 1878 by William Crooks, a British scientist, who performed an experiment to study the phenomenon of electric discharge through gases.

He observed that when an electric current of high voltage was passed through a discharge tube (a glass tube sealed at both ends with metal plates) containing a gas at very low pressure (0-01 mm of mercury). Rays were emitted from the negative terminal called cathode. He called these rays as the 'cathode rays'.

The diagram shows a discharge tube in which electrons are flowing. The pressure is 0.01 mm of mercury. Cathode ray is shown going to a vacuum pump. The discharge tube has an induction coil underneath it. A glow is visible due to cathode ray on, removing anode at one side.

J.J. Thomson's work on cathode rays

J.J. Thomson, another British scientist, studied the characteristics and the constituents of the cathode rays and concluded that: Cathode rays consist of negatively charged particles (now called electrons), present in atoms of all the elements.

J.J. Thomson's Experiment

Electric field was applied in the path of cathode rays in the discharge tube. It was observed that cathode rays were deflected towards the positive plate of electric field. This showed that cathode rays were negatively charged.

When magnetic field was applied in the path of cathode rays, they were again deflected in a direction in which moving negative charge would be deflected.

This proved that cathode rays contained negatively charged particles called electrons.

Properties of electrons

1. Electrons are an integral part of all atoms.

2. Its properties are independent of the nature of the gas in the discharge tube.

3. An electron has a definite mass and it carries a definite electric charge.

4. The mass of an electron has been found to be 1/1837 the mass of a hydrogen atom (9.108 x 10^-28 g)

5. Its charge is one (1) unit negative charge, i.e. 1.602 x 10^-19 coulombs.

An electron is denoted by the symbol _-1e^0. The superscript 0 represents its mass and the subscript -1 represents its electrical charge.

Discovery of Protons (p+)

The presence of the negatively charged electrons in an atom suggests that it must contain positively charged particles as well, otherwise an atom would not be electrically neutral. These positively charged particles were discovered by E. Goldstein, a German scientist, while he was performing an experiment with a discharge tube fitted with a cathode with small holes to allow passage of positive rays (called as perforated cathode) (Fig 1.3). A ray, which was just the opposite to the cathode ray in all respects, was emitted from the anode. This ray was named the anode ray. The anode ray consisted of the positively charged particles (now called protons).

The diagram shows anode, cathode rays, and positive rays. It shows a perforated cathode in the middle.

Properties of protons

1. The mass of a proton was calculated as being equal to the mass of an atom of hydrogen, i.e. 1.672 x 10^-24 g.

2. The positive charge on a proton equalled the negative charge on an electron, i.e. 1.602 x 10^-19 coulombs.

Further experiments proved that all elements are composed of electrons and protons. However, no two elements contain the same number of protons in their respective nuclei. For example, the atoms of hydrogen, helium, lithium, carbon, nitrogen and oxygen contain 1, 2, 3, 6, 7 and 8 protons respectively. Since an atom is electrically neutral, the number of electrons in an atom is equal to the number of protons in that atom.

Protons are denoted as _+1p^1, where the superscript 1 represents 1 amu mass and the subscript +1 represents one unit positive charge.

Thomson's Model of the Atom

Now the question arose as to how protons and electrons were arranged in an atom. The first model for an atom was worked out by J.J. Thomson. It is known as the Plum Pudding Model (Fig. 1.4).

According to this model an atom is a uniform positively charged sphere in which electrons are embedded just like dry fruits in a spherical pudding.

Therefore it is known as Plum Pudding Model.

Since the total positive charge of the atom was equal to the total negative charge of its electrons, it followed that an atom would become negatively charged if it gained electrons and positively charged if it lost electrons. However, this model failed to explain many experimental observations about atoms. Thomson's model was not accepted.

Discovery of the Nucleus

In 1911, Lord Rutherford, a scientist from New Zealand, conducted an experiment in order to find the arrangement of electrons and protons in an atom. This experiment led to the discovery of a small, positively charged nucleus in the centre of the atom.

Rutherford's alpha particles scattering experiment

Rutherford bombarded a thin sheet of gold (of 0.00004 cm thickness) with alpha particles in an evacuated chamber.

The diagram shows a beam of alpha rays hitting a thin gold foil. Most alpha particles pass straight through undeflected, some particles are deflected at strong deflection angles, and a small number of particles show small deflection.

Following were his observations:

Most of the alpha particles passed straight through the foil without any deflection from their path.

A small fraction of them were deflected from their original path by small angles.

Only a few particles bounced back.

On the basis of above observations Rutherford made following conclusions:

Most of the space in an atom was empty because alpha particles went straight.

There was a heavy positively charged mass in the atom which caused deflection of a small fraction of alpha particles.

The positively charged mass is very small and is centrally located because only few particles bounced back.

Based on his experiment Rutherford suggested a model for the structure of the atom which is known as Rutherford's Atomic Model.

Alpha particles are positively charged particles with two units of positive charge and four units of mass. They are formed by the removal of two electrons from Helium.

Teacher's Note

Understanding atomic structure helps us comprehend why materials have different properties - from the flexibility of rubber to the hardness of diamonds. This knowledge is essential for explaining everyday phenomena like why salt dissolves in water.

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