STRUCTURE OF ATOM

DAY 1
28.4.20
4 th period

Good Morning Boys,
Today we will start with new chapter i.e structure of atom.

Learning Outcomes
students will be able to

Learn the properties of electron ,proton and neutron.
know about the discoveries of subatomic particles
Describe Thomsons model

Note ; 1. Day 3 i.e 30.4.20 short assestment MCQ will be conducted on the topic of concentration of solution.

2.cathode ray experiment , discovery of electron, proton and neutron is for reading only. rest content to be noted in register.

Atom is the smallest indivisible particle of the matter. Atom is made of electron, proton and neutrons.

PARTICLE	ELECTRON	PROTON	NEUTRON
Discovery	Sir. J. J. Thomson (1869)	Goldstein (1886)	Chadwick (1932)
Nature of charge	Negative	Positive	Neutral
Amount of charge	1.6 x 10^-19Coloumb	1.6 x 10^-19Coloumb	0
Mass	9.11 x 10^-31kg	1.672614 x 10^-27kg	1.67492 x10^-27kg

Electrons were discovered using cathode ray discharge tube experiment.

Cathode ray discharge tube experiment:

A cathode ray discharge tube made of glass is taken with two electrodes. At very low pressure and high voltage,current starts flowing through a stream of particles moving in the tube from cathode to anode. These rays were called cathode rays. When a perforated anode was taken, the cathode rays struck the other end of the glass tube at the fluorescent coating and a bright spot on the coating was developed.

properties of cathode ray

a. Cathode rays consist of negatively charged electrons.
b. Cathode rays themselves are not visible but their behavior can be observed with help of fluorescent or phosphorescent materials.
c. In absence of electrical or magnetic field cathode rays travel in straight lines.
In presence of electrical or magnetic field, behaviour of cathode rays is similar to that shown by electrons
e. The characteristics of the cathode rays do not depend upon the material of the electrodes and the nature of the gas present in the cathode ray tube.

Charge to mass ratio of an electron was determined by Thomson. The charge to mass ratio of an electron is 1.758820 x 10¹¹. C kg^-1

Charge on an electron was determined by R A Millikan by using an oil dropexperiment. The value of the charge on an electron is -1.6 x 10^-19C.

The mass on an electron was determined by combining the results of Thomson’s experiment and Millikan’s oil drop experiment. The mass of an electron was determined to be 9.1094 x 10^-31kg.

Discovery of protons and canal rays:

Modified cathode ray tube experiment was carried out which led to the discovery of protons.

Characteristics of positively charged particles:

a. Charge to mass ratio of particles depends on gas from which these originated.
b)The positively charged particles depend upon the nature of gas present in the cathode ray discharge tube
c. Some of the positively charged particles carry a multiple of fundamental of electrical charge.
d. Behaviour of positively charged particles in electrical or magnetic field is opposite to that observed for cathode rays

Neutrons were discovered by James Chadwick by bombarding a thin sheet of beryllium by α- particles. They are electrically neutral particles having a mass slightly greater than that of the protons.

Atomic Number:

Equal to the total number protons present in the nucleus or total number of electrons present the neutral atom.
Represented by Z.

Mass Number:

Mass number of an element = number of protons + number of neutrons
Denoted by A.

Isotopes:

These are the atoms of the same element having the same atomic number but different mass number.
For example:₁H¹,₁H²,₁H³

Isobars:

These are the atoms of different elements having the same mass number but different atomic number.
For example: ₁₈Ar⁴⁰ , ₂₀Ca⁴⁰

Thomson model of atom:

atom is considered as a uniform positively charged sphere with radius about 10^-10m, in which electrons are is uniformly distributed.
Electrons are embedded in such a manner to give most stable electrostatic arrangement.
Mass of atom is assumed to be uniformly distributed in atom.
Also known as plum pudding raisin pudding or watermelon model.

Drawback of model

1.could not explain certain experimental results like the scattering of α- particles by thin metal foils.
2. couldnot explain the positioning of nucleus.

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