IX Atomic Structure (Discovery of a fundamental particle protons)

According to Goldstein , Atoms are electrically neutral in nature, should necessarily possess positively charged particles to balance the negatively charged electrons.So, The presence of positive charged particles in the atom has been predicted by Goldstein.

Goldstein’s Experiment: Goldstein repeated the cathode ray experiment by using a perforated
cathode. On applying a high voltage under low pressure, he observed a faint red glow on the wall behind the cathode.
Since these rays originated from the anode, they were called anode rays or canal rays or positive rays. 
Anode rays were found as a stream of positively charged particles.
When hydrogen gas is taken in a discharge tube, the positively charged particles obtained from hydrogen gas are called PROTONS.
Each of these protons is produced when one electron is removed from one hydrogen atom.

H                             --------->           H⁺                 +                 e

                                                  Anode rays                cathode rays

Thus, proton can be defined as hydrogen ion (H+)
Properties of anode rays
1. Anode rays travel in straight lines
2. Anode rays consist of material particles since they rotate the light paddle wheel placed in their path.
3. Anode rays are deflected by electric and magnetic field since they deflect towards negatively charged plate. This shows that anode rays consist of positively charged particles.
4. The properties of anode rays depend upon the nature of gas taken in the discharge tube.

5. The mass of the particle is the same as the atomic mass of the gas inside the discharge tube.

The study of the properties of the fundamental particles, like electron and proton, led to the conception of various atom models.

Atom model is the description of depicting the arrangement of various fundamental particles inside the atom.
Thomson’s atomic model: 
According to J.J. Thomson Limitation of Thomson model :Thomson’s model could successfully explain the electrical neutrality of atom. However, it failed to explain how the positively charged particles are shielded from the negatively charged electrons without getting neutralized.

• The nucleus is made up of protons and neutrons. These are bound together by a strong nuclear force.

• Electrons and protons carry equal but opposite charges. In a neutral atom, the number of electrons is the same as the number of protons.

• Electrons orbit the nucleus at certain fixed levels called shells.

1. An atom consists of a positively charged sphere and electrons are embedded in that sphere.

2. The positive and negative charges are equal in magnitude hence the atom as a whole is electrically neutral. 

Thomson’s model of atom is popularly known as plum pudding or apple pie model

Neutron: The neutral fundamental particle present in the atom is called neutron.

The mass of a neutron is 1.00898 amu or 1.67495 X 10^-27kg.  It has no charge.

Discovery of neutrons: Chadwick discovered neutrons during the bombardment of a thin layer of ₄Be⁹ with α-particles.

In 1932, James Chadwick bombarded beryllium (Be) with alpha aprticles. He allowed the radiation emitted by beryllium to incident on a paraffin wax. It was found that protons were shot out form the paraffin wax. People began to look for what was in the "beryllium radiations"

This discovery of neutron marked the beginning of current theories of nuclear structure. Immediately, the neutron-proton model (the Rutherford-Bohr model) of the nucleus was adopted:

AOMIC NUMBER, MASS NUMBER, ISOTOPES, ISOBARS, ATOMIC WEIGHT

Atomic number (Z):The number of protons or the number of electrons in an atom is called atomic number. It is represented by 'Z'.

Mass number(A): The total number of protons and neutrons in an atom is called mass number. It is denoted by 'A'.

A = no. of protons + no. of neutrons

A = Z + no. of neutrons no. of neutrons = A - Z

Isotope: Isotopes are the atoms of an element with same atomic number but differ in their mass numbers i.e., the isotopes of an element have same number of protons but differ in the number of neutrons.

E.g. Hydrogen has three isotopes : 

Hydrogen(₁H¹), 

Deuterium (₁H²) and 

Tritium (₁H³).

They have same number of protons (= 1) but the numbers of neutrons are 1, 2 and 3 respectively.

 Isobars : The atoms of different elements with same mass number but different atomic numbers are called “Isobars”.

E.g. ₆C¹⁴, ₇N¹⁴

Atomic weight: The atomic weight of an element is the average weight of all the isotopes of that element. It is usually expressed as gram atomic weight. 

Note: The atomic number is a whole number whereas the atomic weight may be a fractional number.

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IX Atomic Structure ( a fundamental particles) Discovery of Electron

The first direct experimental evidence for the electrical nature of matter came from the experiments of Michale Faraday. He showed from his experiments that electricity is composed of particles called ‘atoms of electricity’.

Michale Faraday Studied the effect of electricity on solutions, coined term "electrolysis" as a splitting of molecules with electricity, developed laws of electrolysis.


In 1859,   J. Plucker Built one of the first gas discharge tubes ("cathode ray tube").


It was George Johnstone Stoney, an Irish Physicist who first proposed the word ‘electron‘ for atom of electricity’ in 1891.His contribution to research in this area laid the foundations for the eventual discovery of particles by J.J. Thomson in 1897.

In 1878, Sir William Crooke, while conducting an experiment using a discharge tube, found certain visible rays travelling between two metal electrodes. These rays are known as Crooke’s Rays or cathode rays. The discharge tube used in the experiment is now referred to as Crookes tube or more popularly as Cathode Ray Tube (CRT).

Cathode Ray Tube : It is a long glass tube filled with gas and sealed at both the ends. It consist of two metal plates (which act as electrodes) connected with high voltage. The electrode which is connected to the negative terminal of the battery is called the cathode (negative electrode). The electrode connected to the positive terminal is called the anode (positive electrode). There is a side tube which is connected to a pump. The pump is used to lower the pressure inside the discharge tube.


Discovery of electron

Later, J.J. Thomson also found that when a high voltage of 10,000 V was applied between the electrodes present in a partially evacuated cathode ray tube at a pressure of 0.01mm of mercury, a bright spot of light was formed on the screen coated with a fluorescent material placed at the other end of the tube.

The Fluorescent material coated on the screen started to glow because it was struck by the ray which originated from the cathode. Since these rays were emitted by the cathode, he named these rays as cathode rays. Later, he named it as electrons.


Properties of cathode rays
1.The cathode rays travel in straight lines.
Cathode rays fall on a small object which is placed in between the cathode and anode.A shadow which is of the same shape as the object is observed on the wall opposite to the cathode.
2.Cathode rays are made up of small particles that have mass and kinetic energy.
Cathode rays fall on a light paddle wheel which is placed between cathode and anode. The wheel starts rotating.

3.The cathode rays are negatively charged particles

Cathode rays are passed through an electric field. The cathode rays are deflected towards the positive plate of electric field.

4. Cathode rays are passed through a magnetic field . The deflection of the rays is perpendicular to the applied magnetic field.

The direction of deflection indicates that the cathode rays constitute negatively charged particles. These negatively charged particles are called electrons

Characteristics of cathode rays :
1) Cathode rays are not visible but their behavior can be observed with the help of a fluorescent or a phosphorescent.
2) These rays travel from cathode to anode.
3) These rays travel in straight lines in the absence of electric and magnetic fields.
4) However these rays are deflected towards the positive end in electric field. Hence it was concluded that the rays constitute negatively charged particles and are known as electrons.
These rays are also deflected in magnetic field.
5) The behavior and the properties of rays are independent of the nature of the cathode material and nature of the gas present in the cathode ray tube.
These facts conclude that the electrons are the negatively charged fundamental particles present in all the substances.

Charge to mass ratio of Electron (e/me): 

The charge to mass ratio of electron was calculated by J. J. Thomson. Its value is equal to 1.75882x10^11C.kg-1.

Charge on the Electron (e) : The charge on the electron was calculated by Millikan in oil drop experiment as 1.60 x 10^-19 coulombs.  Now the mass of an electron can be derived as follows:

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Class IX Chemistry E-Notes Chapter Atomic Structure

Dalton’s Atomic Theory

The important postulates of Dalton’s atomic theory are:

1. All elements are composed of atoms. Atom is too small so that it could not be divided into further simpler components.

2. Atom cannot be destroyed or produced.

3. Atoms of an element are similar in all respects. They have same mass and properties.

4. Atoms of different elements combine in a definite simple ratio to produce compounds.

Mass Number
The total number of the protons and neutrons present in the nucleus of an atom is called mass number. The protons and neutrons together are called nucleon. Hence it is also known as nucleon number. It is denoted by A. the number of neutrons present in the nucleus of an atom is rperesented by N.

Discovery of Electron

A discharge tube is a glass tube. It has two electrode, a source of electric current and a vacuum pump.

(Diagram)

Sir William Crooks (1895 performed experiments by passing electric current through gas in the discharge tube at very low pressure. He observed that at 10-4 (-4 is power to 10) atmosphere pressure, shining rays are emitted from cathode. These rays were named cathode rays. Cathode rays are material particles as they have mass and momentum.

Properties of Cathode Rays

The properties of these particles are given below:

1. These particles are emitted from cathode surface and move in straight line.

2. The temperature of the object rises on which they fall.

3. They produce shadow of opaque object placed in their path.

4. These particles are deflected in electric and magnetic fields.

5. These particles are deflected towards positive plate of electric field.

Discovery of Proton

Gold Stein (1886) observed that in addition to the cathode rays, another type of rays were present in the discharge tube. These rays travel in a direction opposite to cathode rays. These rays were named positive rays. By using perforated cathode in the discharge tube the properties of these rays can be studied. Positive rays are also composed of metered particles. The positive rays are not emitted from anode. They are produced by the ionization of residual gas molecules in the discharge tube. When cathode rays strike with gas molecule, electrons are removed and positive particles are produced.

Properties of Positive Rays

1. They are deflected towards negative plate of electric field. Therefore these rays carry positive charge.

2. The mass of positive rays is equal to the mass of the gas enclosed in the discharge tube.

3. The minimum mass of positive particles is equal to the mass of hydrogen ion (H+). These positive ions are called Protons.

4. The charge on proton is equal to +1.602×10^-19 Coulomb. (-19 is power of 10)

Natural Radioactivity

The phenomenon in which certain elements emit radiation which can cause fogging of photographic plate is called natural radioactivity. The elements which omit these rays are called radioactive elements like Uranium, Thorium, Radium etc. There are about 40 radioactive elements.                                     

Henri Bequrel (1896) discovered radioactivity.Madam Curei also has valuable contribution in this field. In natural radioactivity nuclei of elements are broken and element converted to other elements. Natural radioactivity is nuclear property of the elements.

Alpha Rays

1. They are helium nuclei. They are doubly positively charged, He2+.

2. They move with speed equal to the 1/10th of the velocity of the light.

3. They cannot pass through thick-metal foil.

4. They are very good ionizer of a gas.

5. They affect the photographic plate.

Beta Rays

1. They are negatively charged.

2. They move with the speed equal to the velocity of light.

3. They can pass through a few millimeter thick metal sheets.

4. They are good ionizer of a gas.

5. They can affect the photographic plate.

Gamma Rays

1. They are electromagnetic radiations.

2. They travel with speed equal to velocity of light.

3. They carry no charge.

4. They have high penetration power than alpha and beta rays.

5. They are weak ionizer of gas.

Rutherford Experiment and Discovery of Nucleus

Lord Rutherford (1911) and his coworkers performed an experiment. They bombarded a very thin, gold fail with Alpha particles from a radioactive source. They observed that most of the particles passed straight through the foil undeflected. But a few particles were deflected at different angles. One out of 4000 Alpha particles was deflected at an angle greater than 150.

Following conclusions were drawn from the Rutherford’s Alpha Particles scattering experiment.

1. The fact that majority of the particles went through the foil undeflected shows that most of the space occupied by an atom is empty.

2. The deflection of a few particles over a wide angle of 150 degrees shows that these particles strike with heavy body having positive charge.

3. The heavy positively charged central part of the atom is called nucleus.

4. Nearly all of the mass of atom is concentrated in the nucleus.

5. The size of the nucleus is very small as compared with the size of atom.

Defects of Rutherford Model

Rutherford model of an atom resembles our solar system. It has following defects:

1. According to classical electromagnetic theory, electron being charged body will emit energy continuously. Thus the orbit of the revolving electron becomes smaller and smaller until it would fall into the nucleus and atomic structure would collapse.

2. If revolving electron emits energy continuously then there should be a continuous spectrum but a line spectrum is obtained.

Bohr’s Atomic Model

Neil Bohr (1913) presented a model of atom which has removed the defects of Rutherford Model. This model was developed for hydrogen atom which has only proton in the nucleus and one electron is revolving around it.

Postulates of Bohr’s Atomic Model

The main postulates of Bohr’s Model are given below:

1. Electrons revolve around the nucleus in a fixed orbit.

2. As long as electron revolves in a fixed orbit it does not emit and absorb energy. Hence energy of electron remains constant.

3. The orbit nearest to the nucleus is the first orbit and has lowest energy. When an electron absorbs energy it jumps from lower energy orbit to higher energy orbit. Energy is emitted in the form of radiations, when an electron jumps from higher energy orbit to lower energy orbit. The unit of energy emitted in the form of radiations is called quantum. It explains the formation of atomic spectrum.

4. The change in energy is related with the quantum of radiation by the equation :

E2 – E1 = hv where,

E1 = Energy of first orbit      

E2 = Energy of the second orbit      

 h = Planck’s constant

v = Frequency of radiation

Atomic Number

The number of protons present in the nucleus of an atom is called atomic number or proton number. 

It is denoted by z. The proton in the nucleus of an atom is equal to number of electrons revolving around its nucleus.


Mass Number  =       No of Protons  + No of neutrons
                    A =               Z              +        N
Isotopes

The atoms of same elements which have same atomic number but different mas number are called Isotopes. 

The number of protons present in the nucleus of an atom remains the same but number of neutrons may differ.

Isotopes of Different Elements

Isotopes of Hydrogen
Hydrogen has three isotopes:
1. Ordinary Hydrogen or Protium, H.
2. Heavy Hydrogen or Deutrium, D.
3. Radioactive Hydrogen or Tritium, T.

Protium
Ordinary naturally occurring hydrogen contains the largest percentage of protium. It is denoted by symbol H. It has one proton in its nucleus and one electron revolve around the nucleus.
Number of Protons = 1
Number of Electrons = 1
Number of Neutrons = 0
Atomic Number = 1
Mass Number = 1
Deutrium
Deutrium is called heavy hydrogen. The percentage of deutrium in naturally occuring hydrogen is about 0.0015%. It has one proton and one neutron in its nucleus. It has one electron revolving around its nucleus. It is denoted by symbol D.
Number of Proton = 1
Number of Electron = 1
Number of Neutrons = 1
Atomic Number = 1
Mass Number = 2

Tritium

Radioactive hydrogen is called tritium. It is denoted by symbol T. The number of tritium isotope is one in ten millions. It has one proton and 2 neutrons in its nucleus. It has one electron revolving around its nucleus.
Number of Proton = 1
Number of Electron = 1
Number of Neutron = 2
Atomic Number = 1
Mass Number = 3

Download: Class IX Chemistry Test paper Chapter 4: Structure of the Atom 

1. Plum pudding Model of atom was discovered by _______________

2. Combining capacity of an atom is called _______________

3. Alpha - particle scattering experiment of Rutherford led to discovery of _________

4.  __________ are atoms having the same mass number but different atomic number.

5. The charge on the electron is found to be ________ coulombs.

6. What do you understand by valency of an element? What is valency of boron?

7. State the valencies of the following elements

(a) Magnesium

(b) Phosphorous

(c) Argon

(d) Flurine

8. Draw the atomic diagram of calcium.

9.Calculate the mass of an electron.

10.Name two applications of radioisotopes.

11.What is mean by valency of an atom?

12.What is octel rule?

13.Define radioactivity.

14. List the features of Rutherford's nuclear model of atom.

15. What are the postulates of Bohr Model of an atom?
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