A brief slide about the history of atoms and how they are discovered.

1. History of atom

2. Greek Philosopher Democritus
In 460 BC the greek philosopher, Democritus,followed ideas from his teacher
Leucippus to develop the notion of atoms.He reasoned that if you break a piece of
matter into two,and then break it again and again,how many breaks will you have
to make before you can break it no further?
Democritus thought that it ended at some point,
and it is the smallest possible bit of matter.
He called these basic matter particles atoms
(from the greek word atomos, meaning indivisible)
but he was largely ignored by other thinkers at that time.
So no one believed him.

3. John Dalton
It was not until the early 1800s that people began to again question about what is
matter is made out of.The English chemist John Dalton(1766-1844) performed
experiments with different chemicals that showed that matters seems to be made of
elementary and tiny particles (atoms).He came up with an atomic theory that stated the
following.
● Elements are made out of tiny particles called atoms .
● All atoms of an element are identical and they have the same mass.
● Atoms of a given element are different from those of any other elements.
● Atoms of one element can combine with atoms of other elements to form chemical
compounds.
● Any particular compound always has the same relative numbers of types of
atom.

4. ● Atoms can’t be created or destroyed by any chemical.
● A chemical reaction simply changes the way atoms are grouped together.
Dalton’s symbols

5. Joseph John Thomson
In 1897, the english physicist Joseph John Thomson(1856-1940) discovered the
electron and in 1906 he suggested a model of the atom.This he assumed was a
sphere of positive matter in which electron are stationed by electrostatic
forces.Thomson knew that electron had a negative charge and thought that matter
must have a positive charge.He therefore assumed the basic shape of an atom to
be a spherical object containing a number of electrons confined in a homogenous
jelly like, but relatively massive positive charge distribution. The total positive and
negative charges would cancel each other out, since atoms are normally
electrically neutral.

6. Thomson’s atomic model looked like raisins stuck in and on the surface of a lump
of pudding.This is sometimes known as the ‘plum pudding’ or ‘raisin bread’ model.