Chapter 3 Section 1 Notes
Atomic Structure and History of the Atomic Theory
Do theories in science always stay
the same?
No, theories are changed or replaced as new
discoveries are made.
 Theories of the atom (Atomic Theory) have
changed many times over the past couple
thousand years based on new knowledge
that has been gained through experimenting.
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
What are Atoms?
Atoms are tiny units that determine the
properties of matter
 Atoms are EVERYWHERE and make up
EVERYTHING around you!

Democritus (460-370 BC)


In the 4th Century B.C.,
the Greek philosopher
Democritus suggested
that the universe was
made of indivisible
units that he called
atoms.
“Atom” comes from the
Greek word atomos,
which means “unable
to be cut or divided”
Democritus (460-370 BC)


Democritus concluded
that there was a limit to
how far matter could be
divided; eventually, you
would end up with a piece
of matter that could not be
cut.
What did people think of
Democritus? People did
not believe Democritus
because he had no
evidence to support his
theory.
John Dalton (1766-1844)


In 1808, two thousand
years after Democritus,
an English school
teacher named John
Dalton proposed a
revised atomic theory.
His theory was
developed based on
scientific evidence and
some parts of his
theory still hold true
today!
John Dalton (1766-1844)

John Dalton proposed 3 new ideas about
the atom:
1. Every element is made of tiny, unique
particles called atoms that cannot be
subdivided.
2. Atoms of the same element are exactly
alike.
3. Atoms of different elements can join to
form molecules.
Evidence for Dalton
Unlike Democritus, Dalton based his theory
on experimental evidence.
 Today, Dalton’s theory is considered the
foundation for the modern atomic theory.


Some parts of this theory turned out to be
correct, but his theory could not explain all of the
experimental evidence.
J.J. Thomson (1856-1940)

In 1897, J.J. Thomson, a British scientist,
performed an experiment that suggested that
atoms were not indivisible.
J.J. Thomson (1856-1940)

What did Thomson discover?

While investigating with electricity and cathode
ray tubes, Thomson discovered that atoms are
made of smaller, negatively charged particles
called electrons.
The stream of
electrons is
attracted to
positively
charged plate.
Thomson’s Atomic Theory
In his model, electrons are spread
throughout an atom, just like blueberries are
spread throughout a muffin.
 His model is called the plum-pudding model,
which was a dessert popular in his time.

In this model,
electrons are
scattered
throughout the
atom.
Ernest Rutherford (1871-1937)
Was a student of J.J. Thomson but
disagreed with the “Plum Pudding Model”.
 Devised an experiment to investigate the
structure of positive and negative charges in
the atom.

What did Rutherford Propose?
Rutherford proposed that most of the mass
of the atom was in the atom’s center.
 An interactive model of Rutherford’s Gold-foil
Experiment:
http://micro.magnet.fsu.edu/electromag/java/rut
herford

Rutherford’s Gold-Foil Experiment

What did most of the alpha particles shot at the
gold foil do?


What was the surprising behavior of a few of the
particles?


Most of the particles traveled straight through the gold
foil.
A few of the particles were deflected and some even
bounced back.
http://cwx.prenhall.com/petrucci/medialib/media_po
rtfolio/text_images/006_RUTHERFORD.MOV
Rutherford’s Atomic Model

What did Rutherford
discover?


Rutherford discovered
the nucleus, which is the
dense core of an atom.
What is Rutherford’s
Atomic Model?

Negatively charged
electrons orbit the
nucleus like planets
orbit the sun.
Niels Bohr (1885-1962)
In 1913, the Danish scientist Bohr suggested
that electrons in an atom move in set paths
around the nucleus like planets orbiting the
sun.
 In his model, every electron has a certain
energy level that is determined by its path
around the nucleus.

Modern Atomic Theory (1925)
This is the theory that we currently believe.
 The modern atomic theory believes that
electrons behave more like waves on a
vibrating string.

Do atoms really look like this?
Well…..
It is a much easier way to fit it on paper!
 If a nucleus were as big as a marble, the
whole atom would be the size of a football
stadium! The nucleus is very, very small in
comparison to the atom.

So, can atoms be subdivided?
Both Democritus in the 4th century and later
Dalton in the 19th century believed that the
atom was the smallest particle and could not
be subdivided.
 We now know that this is NOT TRUE!!!!the atom can be divided into subatomic
particles

What’s in an atom?

An atom has 3
subatomic particles in
it:
 Protons: have a
positive charge and
are located in the
nucleus
 Neutrons: have no
charge and are
located in the
nucleus

Electrons: have a
negative charge and
are located orbiting
around outside the
nucleus
Parts of an Atom
Particle
Location
Mass
Charge
Proton
Nucleus
1.67 x 10-27 kg +1
Neutron
Nucleus
1.67 x 10-27 kg 0
Electron
Outside the 9.11 x 10-31 kg -1
nucleus
Overall Charge of an Atom
 The
overall charge of an atom is
ZERO. Atoms have equal numbers
of protons and electrons, and their
charges exactly cancel.
The nucleus

What two particles are located in the
nucleus?


Protons and Neutrons
What is the overall charge of the nucleus
ONLY?

The nucleus has a positive charge because it is
made of protons, which are positive, and
neutrons, which have no charge.
Energy Levels
According to Modern Atomic Theory, it is
nearly impossible to determine the exact
location of an electron.
 1st Energy Level: can hold up to 2 e 2nd Energy Level: can hold up to 8 e 3rd Energy Level: can hold up to 18 e

*Note: Each lower level needs to be filled before
going to the next energy level.
Orbitals
An orbital is a region in an atom where there
is a high probability of finding an electron.
 There are 4 orbitals:

s orbital
 p orbital
 d orbital
 f orbital

 *These
energy.
orbitals are in order from lowest to highest
Valence Electrons

A Valence Electron is an electron that is
located in the outer most energy level.