CBSE Class 9 Science Structure of Atoms Exam Notes

CBSE Class 9 Science Structure of Atoms Exam Notes. Please refer to the examination notes which you can use for preparing and revising for exams. These notes will help you to revise the concepts quickly and get good marks.

Introduction

Atoms are made up of three subatomic particles electrons, Protons and neutrons. Electron has negative charge, proton has positive charge, whereas neutron has no charge, it is neutral. Protons and neutrons are present in a small nucleus at the centre of the atom. Electrons are outside the nucleus. The atoms of different elements differ in the number of electrons, protons and neutrons.

Cathode ray

A discharge tube is long glass tube. This tube is fitted with metal electrodes on either end across which high voltage can be applied. The electrode which is connected to the negative terminal of the power source is called cathode while the electrode which is connected to the positive terminal is called anode. The tube is also connected to a vacuum pump for controlling the pressure of gas inside the discharge tube. The gas pressure is reduced to about 10–2 atmospheres and a potential difference of about 10000 volts is applied to the electrodes, an electric current flows and at the same time light is emitted by the gas. The fluorescence was caused due to the bombardment of the walls of the tube by rays emanating from cathode. These rays were called cathode rays.

 Properties of cathode rays

* Cathode rays always travel in straight line.

* Cathode rays consist of material particles and possess energy, hence they can produce mechanical effects.

* Cathode rays consist of negatively charged particles.

* Cathode rays can penetrate through thin metalic sheets.

* Cathode rays ionize the gas through which they travel.

* Cathode rays heat up the object on which they fall. When they strike an object, a part of the kinetic energy is transferred to the object resulting in a rise in temperature.

* Cathode rays produce a green fluorescence on glass surface.

* When cathode rays fall on certain metals like copper, X-rays are produced. X-rays are not deflected by any electrical or magnetic fields but they pass through the opaque material and are only stopped by solid objects like bones.

* The mass of electrons is very small compared to the mass of an atom. The same type of negatively charged particles are formed even if different gases are taken in the discharge tube or different metals are used as cathode.

The pressure in the tube is decreased, it was observed that in addition to cathode rays, a new kind of rays are also found which came through the perforations (holes) of the cathode. These rays travelled in opposite direction of the cathode rays and passed through the holes of the cathode and struck the other end of the discharge tube. When these radiations struck the end of the tube P, the fluorescent radiations were also produced. These rays were called canal rays because they are passed through the holes or canals in the cathode. These were also called anode rays because they move from the anode. It was found that the

anode rays consist of positively charged particles. Therefore, these rays were also called positive rays.

◊ Properties of Anode Rays

♦ Anode rays travel in straight lines.

♦ Anode rays consist of material particles.

♦ Anode rays are deflected by electric field towards negatively charged plate. This indicates that they are positively charged.

♦ Anode rays are deflected by magnetic field. The direction of deflection indicates that they are positively charged.

♦ The charge to mass ratio of the particles in the anode rays was determined by W. Wien by using Thomson’s technique. Charge to mass ratio of the particles in the anode rays depends upon nature of the gas taken in the discharged tube.

♦ The nature of anode rays depend on the nature of the gas taken in discharge tube.

 Electron

Electron was discovered by J.J. Thomson. Cathode rays consist of small, negatively charged particles called electrons. The electron is a negatively charged particle found in the atoms of all the elements. The electrons are located outside the nucleus in an atom. An electron is usually represented by the symbol e–

◊ Characteristics of an electron

♦ Mass of an electron :

The mass of an electron is about of the mass of hydrogen atom. Since the mass of a hydrogen atom is 1 u. The absolute mass of an electron is, however, 9 × 10^–28 gram.

♦ Charge of an electron :

The absolute charge on an electron is 1.6 × 10^–19 coulomb of negative charge. The relative charge of an electron is, – 1

  Proton

Proton was discovered by Goldstein. The proton is a positively charged particle found in the atoms of all the elements. The protons are located in the nucleus of an atom. Only hydrogen atom contains one proton in

its nucleus, atoms of all other elements contain more than one proton. A proton is usually represented by

the symbol P+

◊ Characteristics of a proton

♦ Mass of proton : The mass of proton has been found to be equal to 1.67 × 10^–27 kg. This is almost equal to that of an atom of hydrogen. Since the mass of a hydrogen atom is 1 a.m.u., therefore, the relative mass of a proton is also 1 a.m.u.

♦Charge of proton : The charge of proton is equal and opposite to the charge of an electron. The value of charge on proton is 1.602 × 10–19 coulomb of positive charge.

 Neutron

In 1932, Chadwick discovered the fundamental particle neutron. The neutron is a neutral particle found in the nucleus of an atom. The subatomic particle not present in a hydrogen atom is neutron. A hydrogen atom contains only one proton and one electron. A neutron is represented by the symbol n

◊Characteristics of a Neutron

♦ Mass of a neutron : The mass of neutron is equal to the mass of a proton. The relative mass of a neutron is

1♦. The absolute mass of a neutron is 1.6 × 10^–27 kg.

♦ Charge of an neutron : Neutron has no charge. It is electrically neutral.

Comparision between Proton, Neutron and Electron:

 Structure of atom

After the discovery of electron and proton, the scientists started thinking of arranging these particles in an atom. J.J. Thomson was the first scientist to propose a model for the structure of atom. Mainly there are three model preposed.

(A) Thomson’s model of an atom

(B) Rutherford model of an atom

(C) Bohr’s model of an atom

(A) Thomson’s model of an atom

Thomson was the first to propose a detailed model of the atom. He proposed that an atom consists of a uniform sphere of positive electricity in which the electrons are distributed more or less uniformly. The negative and the positive charge are equal in magnitude. Thus, the atom as a whole is electrically neutral. This model of atom is known as the “Plum pudding model”.

(B) Rutherford model of an atom

Rutherford and his co-workers made a fundamental contribution in understanding the structure of the atom and establishing the presence of a small nucleus in the atom. They performed a number of experiments known as scattering experiments. They took very thin sheets of gold foil (only 4 × 10–5 cm thick) and bombarded it with a stream of alpha (a) particles. The alpha particles are positively charged helium ions (He2+) which carry two units of positive charge and mass four times that of an atom of hydrogen (i.e., mass of helium ions = 4 a.m.u.). These are emitted from radioactive elements such as radium.

◊ Following important observations :

♦ Most of the fast moving a-particles passed straight through the gold foil without any deflection from their original path.

♦ Some of the a-particles were deflected from their path through small angles.

♦ Very few (about 1 in 12,000) did not pass through the foil at all but suffered large deflections (more than

90º) or even came back in the direction from which they came.

◊The main conclusions of Rutherford’s experiment :

♦ Most of the space inside the atom is empty. Therefore, most of the a-particles went through the gold foil without deflecting from their path.

♦ There is a positive tiny part in the atom in its centre, which deflects or repels the a-particles. This must be containing the whole mass of the atom. Moreover, this mass must be occupying a very small space within the atom because only a few a-particles suffered large deflections. This positively charged heavy mass in the centre of the atom is called nucleus. The a-particle get deflected from their normal path when they came close to nucleus due to force of repulsion (similar charges).

Ø The deflections of the a-particles to large angles indicate that the a-particles has direct collision with the positively charged nucleus.

It was observed that the volume of the nucleus is very small as compared to the total volume of the atom. The radius of the atom is of the order of 10–10 m, while the radius of the nucleus has been estimated to be of the order of 10–15 m. This means that the size of the nucleus is extermely small i.e., about 105 times less than the size of the atom.

◊ Rutherford’s Atomic model : The main features of Rutherford’s model of an atom are :

♦ The atom consists of a positively charged centre called the nucleus.

♦ Most of the mass of the atom is concentrated in the nucleus.

♦ The volume of the nucleus is very small as compared to the total volume of the atom.

♦ The nucleus is surrounded by the negatively charged electrons. the electrons balance the positive charge of the nucleus. Therefore, the number of electrons in an atom is equal to the number of protons in it.

♦ The magnitude of the positive charge on the nucleus is different for different atoms.

♦ The electrons are not stationary but they are revolving around the nucleus at very high speeds like planets revolving around the sun. As a result, the electrons are also known as planetary electrons.

◊ Drawbacks of Rutherford’s model :

Rutherford model failed in view of electromagnetic theory given by Maxwell. According to this theory a charged particle when accelerated emits energy in the form of electromagnetic radiation. According to Rutherford’s model, electrons are revolving around the nucleus. This means, electrons would be in a state of acceleration all the time. Since electrons are charged particles, therefore, electron revolving in an orbit should continuously emit radiations. As a result of this, it would slow down and would no longer be able to withstand the attractive force of the nucleus. Hence, it would move closer and closer to the nucleus and would finally fall in the nucleus by following a spiral path. This means atom would collapse. But actually we know atom is stable. Thus, Rutherford’s model failed to explain stability of atoms.

(C) Bohr’s model of an atom

In 1912, Niels Bohr put forward a model to explain the structure of an atom. The main postulates of the model are:

♦ An atom is made up of three particles : electons, protons and neutrons. Electrons have negative charge, protons have positive charge whereas neutrons have no charge, they are neutral. Due to the presence of equal number negative electrons and positive protons, the atom on the whole is electrically neutral.

♦The protons and neutrons are located in a small nucleus at the centre of the atom. Due to the presence of protons, nucleus is positively charged.

♦ The electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells. The energy levels or shells are represented in two ways: either by the numbers 1, 2, 3, 4, 5 and 6 or by the letters K, L, M, N, O and P. The energy levels are counted from the centre outwards.

♦ There is a limit to the number of electrons which each energy level can hold. For example, the first energy level (or K shell) can hold a maximum of 2 electrons; second energy level (or L shell) can hold a maximum of

8 electrons; third energy level (or M shell) can hold a maximum of 18 electrons and fourth energy level (or

N shell) can hold a maximum of 32 electrons.

♦ Each energy level (or shell) is associated with a fixed amount of energy, the shell nearest to the nucleus having minimum energy and the shell farthest from the nucleus having the maximum energy.

♦ There is no change in the energy of electrons as long as they keep revolving in the same energy level, and the atom remains stable. The change in the energy of an electron takes place only when it jumps from a lower energy level to a higher energy level or when it comes down from a higher energy level to a lower energy level. When an electron gains energy, it jumps from a lower energy level to a higher energy level, and when an electron comes down from a higher energy level to a lower energy level, it loses energy.

Please click on below link to download pdf file for CBSE Class 9 Science Structure of Atoms Exam Notes.