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.

•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

DALTON’S ATOMIC THEORY

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 in

unexpected directions. This was the discovery of the nucleus.

A decade later, he also

established that the nucleus

must have at least two kinds

of particles: a positive proton,

and a neutral neutron.

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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