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The Atomic Model


The Atomic Model


Introduction to atoms

Everything is made up of atoms. Atoms are the smallest

unit of any element and cannot be seen.

Atoms of different elements can combine to make new

substances. Like this, we can make

all of the materials on the planet!

Atoms have a unique structure.

They are made up of a core

nucleus which contains

protons and neutrons.

The nucleus is surrounded

by electrons which are in shells.

nucleus

electron shell


Atomic zoom


How big are atoms?

Atoms have a diameter of about 1 × 10–10 metres (m), or

0.1 nanometres (nm), which is far too small to be seen with

the naked eye.

Within atoms are smaller, subatomic particles called

neutrons, protons and electrons.

The nucleus, which

contains neutrons and

protons, is 1/10,000 the

size of the atom and has

a diameter of about

0.00001nm.


Size of particles


Charge of atoms

If we can’t see atoms, how do we know so much about them?

Protons are positively charged. The number of protons an

atom has can be referred to as its atomic number. You can

see that on the periodic table, elements are arranged in

order of their atomic number.

Neutrons are neutral and

do not have a charge.

If the number of protons and electrons are the same, there

will be no overall charge for the atom. It is said to be neutral.

electron

proton

neutron

Electrons are

negatively charged.


The Atomic Model


Who discovered the atom?


John Dalton

John Dalton was an English chemist who lived in 19th century.

Dalton did a lot of scientific experiments involving gases.

His experiments led to him to think about what gases are

made up of.

The ancient Greeks had already come up with a theory that

everything is made of very small units, called atoms, which

cannot be broken down any further.

Dalton went on to do many experiments which showed that

when atoms of different elements combine to form chemical

compounds, they are always in whole numbers.

For example 2 oxygen atoms will

always react with 1 carbon atom

to make carbon dioxide, CO2.


Dalton’s theory

Although there were some errors in Dalton’s work, which

were later corrected, most of his theory was correct.

Dalton theorised that:

Dalton also discovered colour blindness!

elements are made of extremely small particles, called

atoms, so small in fact that we cannot see them

atoms of a given element are identical in size and mass,

and atoms of different elements differ in size and mass

atoms cannot be subdivided, created, or destroyed

atoms of different elements combine to form

chemical compounds.


JJ Thomson

Atoms were then thought to be solid,

indivisible spheres, but Thomson

found that the electron had a very

small mass. He thought that about

2,000 electrons must be needed to

make one hydrogen atom.

At the end of the 19th century, English physicist Sir Joseph

John Thomson, carried out an experiment where he

passed a high voltage beam through a low pressure gas.

He found that the beam would always

be attracted to a positive charge, and

therefore must be negative itself.

He had discovered the electron!


Plum pudding model

Thomson then concluded that

there must be fewer electrons,

which are spread out and

mixed with positive charges,

like plums in a pudding. This

new model of the atom was

therefore called the plum

pudding model.

Thomson’s work earned him the Nobel Prize in 1906.

A hydrogen atom made entirely of electrons would be

negatively charged. As this was not observed, he knew that

that could not be correct.

negatively-

charged

plums

positively-charged

pudding


Ernest Rutherford

Geiger and Marsden fired

positively-charged alpha

particles at a sheet of gold

foil, which is made up of

gold atoms. They expected

the alpha particles to pass

through the gold atoms.

Ernest Rutherford was a student of JJ Thomson.

Rutherford designed an experiment to test out the

plum pudding model theory.

The experiment was conducted by his

assistants, Geiger and Marsden.

gold

nucleus

alpha particle


Naming the proton

They concluded that, for

this to occur, there must be

a small core of positive

charge that repels the

alpha particles. They had

discovered the nucleus!

Why is there no

deflection here?

What actually happened in Rutherford’s scattering

experiment was that some of the alpha particles were

deflected and some were reflected straight back .

gold

nucleus

alpha

particle

alpha particle

deflected

Rutherford named the

positive charges “protons”,

from the Greek word

“protos” meaning “first”.


Nuclear model

New experimental evidence may lead to a scientific

model being changed or replaced. Rutherford’s scattering

experiment led to a new model of the atom.

Rutherford replaced the plum

pudding model with a new

model of the atom as a core

of positive charge in the

nucleus surrounded by

negatively-charged electrons.

He concluded that the nucleus

was very small.

This new model of the atom was called the nuclear model.


Niels Bohr

Other scientists thought Bohr’s idea was unacceptable,

until later experimental data supported his claim.

Bohr won the Nobel prize for his work in 1922.

In 1914, Danish scientist Niels Bohr further adapted the

nuclear model.

Bohr suggested that electrons

are not just orbiting the nucleus

in a random fashion, but are in

fixed energy levels.

This can be compared to planets

orbiting the sun. They have a fixed

path that they follow, just like the

electrons around the nucleus.

electron

energy levels

(shells) electron

nucleus


Sir James Chadwick

Chadwick won the Nobel prize

in 1935 for his discovery.

A few years later in 1932, English physicist, James Chadwick

carried out a series of experiments involving radiation.

Chadwick discovered that the

nucleus not only contained

positively-charged protons,

but also small particles with

the same mass as protons

but no charge. He called

these particles neutrons.


The Atomic Model


Glossary


True or false?


Multiple-choice quiz

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The Atomic Modeltodhigh.com/clickandbuilds/WordPress/wp-content/uplo… · · 2018-03-02Although there were some errors in Dalton’s work, ... passed a high voltage beam through