Transcript The atomic theory

Development of the atomic
theory.
What is an atom ?
An atom is the smallest particle into which
an element can be divided into and still be
the same substance.
Atoms make up elements; elements
combine to form compounds.
What is a theory ?
The atomic theory has been around for
over 2000 year.
A theory is an explanation for a wide range
of hypotheses and observations that have
been supported by testing.
Let us travel back in time.
Ancient Greece.
Greek philosopher named Democritus.
Imagined cutting a silver coin.
Proposed that you would eventually end
up with an uncuttable particle.
Democritus proposes the atom.
He called that particle an atom
From the Greek word atomos :meaning
indivisible.
He claimed that atoms are always moving,
they form different materials by joining
together.
Atomic theory and Experiments
Scientists had learned that elements
combine in specific proportions to form
compounds.
John Dalton was a British chemist and a
school teacher.
He developed his atomic theory from
observations gathered from many
experiments.
His theory stated :
All elements are made of atoms which are
small particles that cannot be divided or
destroyed.
Atoms of the same elements are exactly
alike. Atoms of different elements are
different.
Atoms join with other atoms to make new
compounds (substances).
Where do we go from here?
The atomic theory has changed as scientists
have discovered new information about the
atom. It was found that atoms can be divided
into even smaller parts, called subparticles.
Thomson ,a British scientist, improved Dalton’s
theory by using a cathode-ray tube.
Rutherford, another British scientist, investigated
the structure of the atom by using the Gold foil
experiment.
Thomson’s cathode-ray Experiment
There are two types of electric charges: Positive and Negative.
Objects with opposite charges attract each other.
Objects with the same charge push each other away.
From his experiment he concluded that
particles exist in every type of atom and
that they are identical.
The negatively charged particles found in
all atoms are now called electrons .
Rutherford’s Gold foil Experiment.
The shooting Gallery:
Rutherford’s Experiment.
Rutherford proposed that in the center of the
atom is a tiny, extremely dense, positively
charged region called the nucleus .
Most of the atom mass is concentrated here.
The nucleus is positively charged.
The nucleus consists of protons (Positively
charged) and neutrons (No charge).
Few models have been proposed to describe
possible structures of an atom.
What is a Model ?
A model is a representation of an
object or a system.
A model presents a picture of
what a theory explains.
Model showing parts of an atom.
The modern theory.
According to the current theory, there are
regions inside the atom where electrons
are likely to be found.
These regions are called electron clouds.