2. Matter
Anything that has mass and occupies space.
Matter is made up of tiny particles called atoms.
Atom
Atom is a smallest unit of matter. They
can participate in a chemical reaction.
3. Dalton’s Atomic Theory
In 1808 John Dalton proposed the theory
Main postulates of the theory:
1.Matter is made up of small indivisible particles called atoms.
2. It is considered as a atom occurs in spherical shape or a
indivisible particle of matter.
3. Atoms can neither be created nor be destroyed.
4. Atoms of different element have different properties.
5. All the atoms of a given element having same atomic
masses but having different properties.
4. Discovery of Electron
Sir J J Thomson was discovered electron by cathode ray
tube experiment.
Cathode ray tube is made up of glass containing two thin
pieces of metal, called electrodes, sealed in it. The electrical
discharge through the gases could be observed only at very
low pressures and at very high voltages. The pressure of
different gases could be adjusted by evacuation of the glass
tube. When sufficiently high voltage is applied across the
electrode, current starts flowing through a steam of particles
moving in the tube from negative electrode to the positive
electrode. These were called cathode rays or cathode ray
particles.
5. Drawbacks of Dalton’s Atomic Theory
1. The indivisibility of atom was proved wrong
because atom can be further divided into
electron, proton and neutrons.
2. Elements having same atomic number but
different atomic masses are called isotopes.
3. Elements having same atomic masses but
having different atomic numbers are called
isobars.
4.This theory fails to explain the existence of
allotropes.
6. Properties of Cathode Rays
1. Cathode rays travel in a straight line.
2. Cathode rays are made up of negatively
charged particles.
3. Cathode rays deflected towards magnet.
4. Cathode rays showing heating effect.
5. Cathode rays ionize the gases through which
they travel.
7. Multiple Choice questions:
1. Who discovered electron ?
(a) JJ Thomson
(b) James Chadwick
(c) Neil Bohr
2. To provide low pressure in the discharge tube we attach__
(a)heat
(b)vacuum pump
(c) electrodes
3. To discover electrons gas discharge tube is fitted with __
(a) two electrodes
(b) three electrodes
(c) four electrodes
4. Neutrally charged particles are protons.
(a) True
(b) False
5. Elements having same atomic number but different atomic mass are
called___
(a) Isotopes
(b) Isobars
