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 as Dalton's Atomic Theory.

DALTON'S ATOMIC THEORYDALTON'S ATOMIC THEORY

1. Elements are made up of tiny particles called “atoms”.

2. Atoms are indivisible.

3. Atoms can neither be created nor destroyed.

4. Atoms of the same element are alike in their mass and

chemical properties.

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.

According to Dalton atom is the smallest, indivisible particle of an

element that takes part in a chemical reaction.

1. According to Dalton ,what is an atom ?

2. What are the properties of an atom according to Dalton ?

3. What are the postulates of Dalton's atomic theory ?

Cathode rays, discovery of electrons

Discovered during discharge tube experiments for studying passage of electricity through

gases at low pressures

The credit of studying cathode rays goes to Sir.J.J.Thomson, an

English physicist.

A glass tube with two metallic plates.

Discharge tube

ANODE One plate is connected to positive terminal of high voltage

power supply

CATHODE The other to negative terminal.

The tube is filled with any gas.

When the pressure of the gas is lowered to about 0.01mm of mercury and a high voltage

(10000volts) is applied, electricity begins to flow and light is

emitted.

Cathode Rays

If the pressure is further reduced to about 10-4 mm of mercury, the emission 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 which are liberated at the

cathode .These rays are known as "Cathode Rays"

• Cathode rays originate from cathode. • Cathode rays travel in straight line.

• Cathode rays are deflected by electric field. • Cathode rays are deflected by magnetic

field. • These rays consist of material particles. • Cathode rays consist negatively charged

particles called "electron".

Cathode rays: properties

2. Cathode rays travel in straight lines.

When the object is placed in the path of cathode rays shadow of the object is formed.

Properties: Cathode rays

1. Cathode rays originate from cathode.

3. Cathode rays are deflected

by electric field .

4. Cathode rays are deflected by magnetic field.

This shows that cathode rays are consist of charged particles.

PROPERTIES OF PROPERTIES OF CATHODE RAYSCATHODE RAYS

PROPERTIES OF CATHODE RAYS

5. A paddle wheel rotates when placed in

the path of cathode rays.

This shows that cathode rays are

consist of particles which cause

mechanical motion.

Cathode rays consist negatively charged particles called "electrons".

Conclusion: Electrons are negatively charged and common constituents of atoms of all elements.

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 particles are called __________.

ANODE RAYS ,DISCOVERY OF PROTONS

In 1886, Goldstein did experiments with discharge

tubes containing a perforated cathode, shows that some

rays are moving in the opposite direction of the

cathode rays. These rays are called as 'anode rays'.

Properties of Anode rays

• Anode rays travel in straight line in the opposite direction of cathode rays.

• Anode are made up of positively charged particles.• Rutherford named these positively charged particles

are protons.

Conclusion: Protons are positively charged, and common constituents of atoms of all elements.

RUTHERFORDS ALPHA RAY SCATTERING

Around 1911, E. Ruther ford Around 1911, E. Ruther ford and his students performed a and his students performed a series of experiments using series of experiments using

alpha rays.alpha rays. A powerful beam of alpha A powerful beam of alpha

particles from a radioactive particles from a radioactive source is made to strike a source is made to strike a

gold foil. gold foil. In order to examine the alpha In order to examine the alpha particles zinc sulphide screen particles zinc sulphide screen

is used. When an alpha is used. When an alpha particle strikes the zinc particle strikes the zinc

sulphide screen a flash of sulphide screen a flash of light is seen.light is seen.

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.

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

RUTHERFORD'S MODEL OF ATOM

• Total number of positive charges on the nucleus is equal to the

number of electrons.

• Almost the entire mass of the atom is concentrated in the

nucleus.

• The volume of the nucleus is very small compared to the volume of

the atom.

• Electrons are not stationary. They revolve round the nucleus

at extremely high speed.

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.

DISCOVERY OF NEUTRONS

In 1932, James Chadwick

discovered that when a thin sheet

of beryllium or boron is

bombarded by the alpha particles,

neutrons are emitted. The

neutrons have almost the same mass as that as

protons.

• Thus protons, electrons and neutrons are the fundamental

particles of an atom.

• Protons are positively charged.

• Electrons are negatively charged.

• Neutrons are electrically neutral.

Proton

Neutron

Atomic number-The number of unit positive charges carried by the nucleus of an element is called Atomic 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 side the nucleus.

Atom of potassium

Electrons 19

Protons 19

Neutrons 20

Atomic number of potassium is 19

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.

Atomic mass= No.of protons+ No.of neutrons.

RUTHERFORD-BOHR MODEL OF ATOM

• Electrons revolve around the nucleus in definite orbits. These are called Stationary

states.

• Each stationary state is associated with a definite quantity of energy. Hence these

stationary states are also called Energy levels.

• As long as electrons are moving in these stationary states , they do not lose or gain

energy.

• Energy is lost or gained by an electron whenever it jumps from one energy level to

another .

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.