DALTON'S ATOMIC STRUCTURE

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Transcript DALTON'S ATOMIC STRUCTURE

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 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 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: properties
• 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".
Properties: Cathode rays
1. Cathode rays originate from
cathode.
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 OF
CATHODE RAYS
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 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
and his students performed a
series of experiments using
alpha rays.
A powerful beam of alpha
particles from a radioactive
source is made to strike a
gold foil.
In order to examine the alpha
particles zinc sulphide screen
is used. When an alpha
particle strikes the zinc
sulphide screen a flash of
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.
Proton
• Protons are positively charged.
• Electrons are negatively
charged.
• Neutrons are electrically neutral.
Neutron
Atom of potassium
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.
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.