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.

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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.

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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.

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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.

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CONCEPT OF ATOM

• According to Dalton atom is the smallest,
  indivisible particle of an element that takes
  part in a chemical reaction.

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Recall your memory

• 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 ?

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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.

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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.

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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"
  

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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".

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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.

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PROPERTIES OF CATHODE RAYS

• 3. Cathode rays are deflected by electric field
  .

• Cathode rays are deflected by magnetic field.
• This shows that cathode rays are consist of
  charged particles.

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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.
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Cathode rays consist negatively charged
particles called "electrons". Conclusion
Electrons are negatively charged and common
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 particles are
  called __________.

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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'.

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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.

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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.

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OBSERVATIONS OF ALPHA RAYS SCATERING EXPERMINT

• 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 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.

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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 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.

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CONSTITUENTS OF ATOM

• 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
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ATOMIC NUMBER
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
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ATOMIC MASS

• 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.

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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 .

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GET READY TO ANSWER

• 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.