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ATOMIC THEORY NOTES
ATOMIC THEORY

First attempts of defining atoms happened 2500
years ago.

Philosophers theorized if matter could be divided
indefinitely into smaller and smaller pieces, or
whether there was a piece so small that it could
not be divided. They called this smallest piece
atomos, which means indivisible. From this, we
get the word atom.
ARISTOTLE - 384 – 322 BC (ABOUT 2400 YEARS AGO)

Aristotle was a prominent
Greek
philosopher
who
described matter as composed
by different combinations of
water, fire, earth and air.

As he was very influential
during his time, his views
went undisputed for the next
2000 years, which delayed
the current atomic model’s
development.
AFTER ARISTOTLE

In Europe and the Middle East, a new brand of secretive
researcher appeared; they were called Alchemists, and were trying
to understand and experiment with matter while using mystical
thinking. They based their ideas on Aristotle’s view of matter.

Their main purpose was finding the philosopher's stone, a special
material which would be able to transform other matter such as
lead into gold.

They tried for more than a thousand years, but were unsuccessful.

By the 19th century, many people were doing experiments that led
them to question Aristotle’s four element theory.
JOHN DALTON 1766-1844

Suggested that particles which make up matter are
actually like small, hard spheres, different for each
different element. He brought back the idea of an atom
being the smallest particle of an element.
DALTON’S ATOMIC THEORY
•All matter are made up of small particles called atoms
•Atoms cannot be created, destroyed or divided into
smaller particles
•All atoms of the same element are equal in mass and
size, but they are different in mass and size from atoms
of another element
•Compounds are created when atoms of different
elements link together in definite proportions
J.J. THOMSON 1856-1940

In 1897, J.J. determined that currents were streams of
negatively charged particles, later called electrons. He found
that every substance he tested produced these particles.

Based on his experiments, he theorized that all atoms must have
these particles; he proposed that atoms were actually formed
by even smaller particles.

Thomson proposed the raisin bun model, where an atom would
be a positively charged ball with negative particles embedded in
it like raisins.
ERNEST RUTHERFORD 1871-1937

Designed an experiment to probe inside atoms.

He exposed a very thin sheet of gold to a stream of high speed,
heavy, positively charged particles, called alpha particles.

He noticed that when they were shot through the gold, the
particles
mostly
went
through,
but
some
bounced
unexpected directions. This was the discovery of the nucleus.
A
decade
established
later,
that
the
he
also
nucleus
must have at least two kinds of
particles: a positive proton, and
a neutral neutron.
in
NIELS BOHR 1885-1962

Bohr worked under Ernest Rutherford, and focused on the
regions around the nucleus, which was known to contain
electrons.

He proposed that the electrons were organized into specific
energy levels, or shells. The placement of each electron
would depend on how much energy they have. The more
energy an electron has, the closest they will be to the outer
shell.
WHAT IS AN ATOM?

An atom is the smallest particle of an element that retains the
properties of the element.

An atom is composed of subatomic particles: protons, electrons and
neutrons.

The nucleus is tiny, dense and composed of protons and neutrons. They
have much more mass than electrons, about 1800 times more.

Electrons are arranged around the nucleus in specific levels.

A neutral atom has the same number of protons and electrons.
ELECTRIC CHARGE

Electric charge comes in two types: positive and negative.
Protons are positive, and electrons are negative; since
positive and negative attract each other, protons and
electrons are attracted together.

Each proton counts as +1 and each electron counts as -1.

All neutral atoms have the same amount of protons and
electrons; this means that the charge will add up to zero.

Example: Oxygen has 8 electrons and 8 protons; the overall
charge is zero.
NUCLEUS

The nucleus has a positive charge because of the protons. For any
atom more complicated than Hydrogen, the nucleus must contain
neutrons. Neutrons keep the protons separated from each other.
The number of neutrons varies depending on how many are
necessary to keep the nucleus stable.
ELECTRONS

Electrons occupy special regions called energy levels, or shells, which
surround the nucleus.

The region occupied by electrons account for way over 99.99% of the
volume of the atom.

Each electron occupies the whole energy level at a time.
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