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In the early nineteenth century, an

English chemist Dalton stated clearly

his ideas regarding atoms. He was the

first person to give out atomic theory

in 1803. This theory is known asDalton's Atomic Theory.

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DALTON'S ATOMIC THEORYDALTON'S ATOMIC THEORY

1. Elements are made up of tinyparticles calledatoms.

2. Atoms are indivisible.

3. Atoms can neither be creatednor destroyed.

4. Atoms of the same elementare alike in their mass and

chemical properties.

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DALTON'S ATOMIC THEORYDALTON'S ATOMIC THEORY[Con.]

5. Atoms of different element differ in their

mass and chemical properties.

6. Atoms of one element cannot be converted

into atoms of another element

7. Atoms of different elements always combine

in simple, definite and integral ratios to form

compound atoms.

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According to Dalton atom is thesmallest, indivisible particle of an

element that takes part in a chemicalreaction.

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1. According to Dalton ,what is an atom ?

2. What are the properties of an atomaccording to Dalton ?

3. What are the postulates of Dalton'satomic theory ?

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Cathode rays, discovery of electrons

Discovered during dischargetube experiments for studyingpassage of electricity through

gases at low pressures

The credit of studying cathode raysgoes to Sir.J.J.Thomson, an English

physicist.

A glass tube with two metallicplates.

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Discharge tube

ANODE One plate is connected to

positive terminal of high voltagepower supply

CATHODE The other to negativeterminal.

The tube is filled with any gas.

When the pressure of the gas islowered to about0.01mm ofmercury and a high voltage

(10000volts) is applied, electricity

begins to flow and light is emitted.

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Cathode Rays

If the pressure is further reduced to about10-4 mm of mercury, theemission of light stops ,but the wall of the glass tub begins to glow

with a faint greenish light.

This greenish light is due to the bombardment of certain rays whichare liberated at the cathode .These rays are known as "Cathode Rays"

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Cathode rays originate from cathode.

Cathode rays travel in straight line.

Cathode rays are deflectedbyelectric field.

Cathode rays are deflectedby magnetic field.

These rays consist ofmaterial particles.

Cathode rays consistnegatively chargedparticlescalled "electron".

Cathode rays: properties

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2. Cathode rays travel instraight lines.

When the object isplaced in the path ofcathode rays shadow of

the objectis formed.

Properties: Cathode rays

1. Cathode rays originatefrom cathode.

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3. Cathode rays are deflected by electric

field .

4. Cathode rays aredeflected by magneticfield.

This shows that cathoderays are consist of

charged particles.

PROPERTIES OFPROPERTIES OF

CATHODE RAYSCATHODE RAYS

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PROPERTIES OF CATHODERAYS

5. Apaddle wheelrotates when placed in

the path of cathode

rays.

This shows thatcathode rays are

consist of particles

which causemechanical motion.

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Cathode rays consistnegativelycharged particles called"electrons".

Conclusion: Electrons arenegatively chargedandcommon constituents of atoms

of all elements.

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TEST FOR YOU

FILL IN THE BLANKS.

Cathode rays travel in_________ lines.

Cathode rays are discovered by ______. Cathode rays are consists of

______charged particles.

In an atom negatively charged particlesare called __________.

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ANODE RAYS ,DISCOVERY

OF PROTONS

In 1886, Goldstein didexperiments with discharge

tubes containing aperforatedcathode, shows that some

rays are moving in theopposite direction of the

cathode rays. These rays arecalled as 'anode rays'.

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Properties of Anode rays

Anode rays travel in straight line in theopposite direction of cathode rays.

Anode are made up ofpositively chargedparticles.

Rutherfordnamed these positivelycharged particles areprotons.

Conclusion:Protons are positivelycharged, and common constituents ofatoms of all elements.

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RUTHERFORDS ALPHA RAY

SCATTERING Around 1911, E. Ruther fordAround 1911, E. Ruther ford

and his students performed aand his students performed aseries of experiments usingseries of experiments using

alpha rays.alpha rays.

A powerful beam of alphaA powerful beam of alphaparticles from a radioactiveparticles from a radioactive

source is made to strike asource is made to strike agold foil.gold foil.

In order to examine the alphaIn order to examine the alphaparticles zinc sulphide screenparticles zinc sulphide screen

is used. When an alphais used. When an alphaparticle strikes the zincparticle strikes the zinc

sulphide screen a flash ofsulphide screen a flash oflight is seen.light is seen.

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OBSERVATIONS OF ALPHA RAYS

SCATERING EXPERMINT

1. Most of the a-particles passed

through the metal foil with out

any change in their path.

2. A few of the a-particles were

deflected through small angles.

3. A very small number of the a-particles were deflected through

such large angles that they

almost retraced their original

path.

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RUTHERFORD'S MODEL OF

ATOM

Rutherford proposed what is known as the

nuclear model of the atom. According to this

model

An atom has a central nucleus surrounded by

electrons.

Electrons are far away from the nucleus.

The central nucleus is positively charged

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RUTHERFORD'S MODEL OF

ATOM

Total number of positive chargeson the nucleus is equal to the

number of electrons.

Almost the entire mass of the atomis concentrated in the nucleus.

The volume of the nucleus is verysmall compared to the volume of

the atom.

Electrons are not stationary. Theyrevolve round the nucleus at

extremely high speed.

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Name the following

1. Positively charged particle in an Atom.

2. Central part of the Atom.3. Anode rays experiment was conducted by.

4. Electrons are faraway from.

5. Electric charge of the nucleus.

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DISCOVERY OF NEUTRONS

In 1932,James Chadwickdiscovered that when a

thin sheet ofberyllium orboron is bombarded by

the alpha particles,

neutrons are emitted.The neutrons have almostthe same mass as that as

protons.

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Thus protons, electrons andneutrons are the fundamental

particles of an atom.

Protons are positively charged.

Electrons are negatively

charged.

Neutrons are electrically neutral.

Proton

Neutron

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Atomic number-The number of unitpositive charges carried by thenucleus of an element is calledAtomic number of the element.

Atomic no.of an element

=No.of unit positive charges in

the nucleus

=No.of protons present in

the nucleus

=No.of electrons present out sidethe nucleus.

Atom of potassium

Electrons 19

Protons 19

Neutrons 20

Atomic number of

potassium is 19

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Atomic Mass:- Atomic mass of an element is

numerically equal to the sum of the number of

protons and the neutrons present in the nucleus.

Atomicmass= No.of protons+ No.of neutrons.

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RUTHERFORD-BOHR MODEL

OF ATOM

Electrons revolve around the nucleus in definiteorbits. These are called Stationary states.

Each stationary state is associated with a

definite quantity of energy. Hence thesestationary states are also called Energy levels.

As long as electrons are moving in thesestationary states , they do not lose or gain

energy.

Energy is lost or gained by an electronwhenever it jumps from one energy level to

another .

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Answer the followings.

1.Name the fundamental particles of an atom.

2.What is an atomic mass?

3.What is atomic number?

4.What are neutrons?

5.Name the particles present in the nucleus.