Discovery of Electrons
1) In the beginning, the model of
an atom was a solid ball. This
atomic theory was published by
John Dalton in 1803 and was
subsequently met with great
interest by the scientist of the
time.
Dalton proposed that atoms
cannot be broken down into
smaller particles. This theory
was subsequently proven false
and experimental evidence was
obtained that showed that
atoms were not solid balls and
could be broken into subatomic
particles.
The old model of an
atom as a solid ball.
In 1897, through investigating the
cathode rays, J.J Thompson
discovered
the
sub-atomic
particles known as electrons
which are 1000 times smaller
than regular atoms. He showed
that an electron carries a negative
charge, and that atoms contained
electrons. He came up with a new
model, suggesting that an atom is
a positively charged sphere with
electrons on it.
Discovery Of Nucleus
Ernest Rutherford carried
out an experiment in 1910.
The experiment involved
shooting positively charged
atoms at a thin sheet of
gold foil. When the
particles come into contact
with the foil a small light is
produced and it can be
seen
through
a
microscope. During the
experiment, most of the
particles passed through
the foil, but few were
deflected.
Hence,
he
concluded that the atoms
of the gold foil were mostly
made up of empty spaces,
but there is a small object
in the atom that deflected
the particles. By using
mathematical calculations,
he concluded that the
small object is positively
charged. He discovered a
subatomic particle known
as proton.
Bohr’s model of an atom
Neil Bohr improved on the
Rutherford Model which is now
known as the Rutherford-Bohr
Model. Bohr discovered that
electrons orbit around the
nucleus in shells. The shells are
also known as ‘energy levels’. He
also discovered that electrons can
be moved to another shell if it is
hit by an electron or a proton of
light.
If you look closely, u will be able
to see that there is a ‘minus’ sign
on the electrons. This means that
the electrons are negatively
charged. The Rutherford-Bohr’s
model of an atom consists of the
nucleus, electrons and shells.
Differences between Dalton,
Thompson, Rutherford and Bohr’s
model
If you are confused about who
discovered what, do not worry as
this is a small conclusion on
Thompson, Rutherford and Bohr’s
model. Chronologically, Dalton
first discovered the atom and
suggested that it is a simple solid
ball. Then, Thompson proved that
Dalton was wrong as he
discovered that the atom also
contained negatively charged
subatomic particles known as
electrons. Rutherford found out
further that an atom does not
consist of electrons only. He
discovered that the atoms also
contained a positively charged
particle known as proton. Bohr
then further improved
Rutherford’s model and
discovered that there were shells
in which the electrons orbit
around the proton. The atom
structure that we are using now is
not the Bohr’s model of an atom.
but Rutherford-Bohr’s model of
an atom.
THE NEW MODEL OF AN ATOM
The new model of an atom
includes the three previously
discovered subatomic particles.
The proton, neutron and
electron. The nucleus of the atom
contains the protons and
neutrons and the nucleus is
surrounded by electrons which
travel at nearly the speed of light.
As the protons and neutrons are
both heavier than the electrons,
the nucleus is the densest part of
the atom. Thus explaining why
Rutherford observed that atoms
would sometimes deflect of the
foil. However, most of the atom is
empty space thus explaining why
most of the atoms Rutherford
shot at the foil went straight
through the foil.
The new model of an atom
with the nucleus and the
electrons surrounding it.
Download

Model of an Atom