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The
Atom
The
History of
a model
The structure of the matter and the study of atoms
really begins with Greek philosophers. They believed:
Matter was made up of some material
with structure
 All matter was formed from 4 basic
elements
1. Air
3. Water
2. Fire 4. Earth

Greek philosopher Democritus
Matter was made up of atoms-from the
word “atomos” which means
indestructible.
 Atoms were very tiny particles
 Between atoms was empty space, he
called the void.
 There are different kinds of atoms
depending upon the type of matter

From Democritus' time, around 400
BC, to 1700 AD not much happened.
This time was called the Dark Ages
and only the Alchemists were doing
much and what they did didn't
directly improve upon atomic theory
Age of Enlightenment
A revolution in scientific study came
about and atomic theory advanced
rapidly.
 In 1782 Lavoisier discovered the Law of
Conservation of Matter.
 Joseph Proust discovered that H2O is
always 11% hydrogen and 89%
oxygen. (Explanation… )

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Proust also found that many
compounds also had consistent
compositions by mass and had
therefore discovered the Law of Definite
Proportions
John Dalton
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1.
2.
3.
4.
Dalton was the first to actually propose an
atomic theory based on many experiments.
His was the first workable atomic theory.
It stated that:
All matter was made up of atoms.
Atoms are indestructible.
Each element has its own atom and that different
elements had different types of atoms.
Atoms of the same element are identical.
JJ Thomson
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discovered that elements had a negatively
charged particle, and since atoms were
neutral, there must be an opposite positive
charge.
He had discovered electrons.
He proposed a new atomic theory that
included the charges.
His was the Raisin Bun/Plum Pudding model.
Crooke’s Tube:
Crooke’s Tube:
Millikan
Calculated the charge on an electron
 Oil drop experiment (1909)
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In 1886, it was discovered that the
Crooke's Tube also emitted positive
particles.
Ernest Rutherford

1.
2.
3.
4.
Due to the discovery of radiation Rutherford
was able to discover that atoms were mostly
empty space with a massive positive but tiny
nucleus. He proposed a new updated
planetary model where:
Electrons orbited a nucleus like planets
around the sun.
The nucleus was tiny but massive.
The nucleus is positively charged.
Most of the atom is empty space.
Rutherford was credited with
discovering the nucleus and the proton.
 The number of protons in an atom is
the atom's atomic number. The atomic
number is identified by the symbol Z.

Gold Foil Experiment
Rutherford atom
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In 1910 JJ Thomson discovered that even
atoms from the same element varied in mass.
He called atoms from the same element with
varying masses isotopes.
The average of all the known isotopes of an
element give the element its average atomic
mass.
Elements on the periodic table have decimals
in their masses for this reason (and because
they are all based on Carbon-12)
Chadwick

In 1930 Chadwick discovered the last
subatomic particle; the neutron.
Neils Bohr
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worked with Rutherford agreed with Rutherford's
model but could not explain the physics of the
model—why didn't the electron's spin into the
nucleus?
Bohr began studies with electromagnetic spectra of
elements.
electromagnetic radiation, E.R., has two important
properties.
1.
wavelength
2.
frequency
The higher the frequency, the shorter the
wavelength.
Gamma Rays are the most energetic, radio waves
are the least where as visible light fits in the middle.
Bohr
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Ran electricity through gaseous versions of elements,
thus an electromagnetic spectrum was produced-unique to each element.
Believed that these spectra coincided with energy
levels of e-. He could never predict any elemental
spectrums beyond hydrogen though.
Excited (electrified) electrons gain energy and jump
up to a higher energy level or orbit.
When they give up this exact amount of energy they
return to their normal or ground state.
e- can skip over many energy levels if given
enough energy and they can skip many on their
way back too.
Modern Model - Charge Cloud Model
Today's model uses predicted
(calculated) locations for electrons.
 These possible locations are
represented by clouds, the darker the
clouds, the higher the “probable”
location of an electron.

Energy levels can only hold a certain
number of electrons

a. 1st energy level holds 2 e
b. 2nd energy level holds 8 e
c. 3rd energy level holds 18 e
d. and so on.
 The maximum number of outer
electrons is 8
 The Octet Rule
 The outer electrons are called valence
electrons and are the electrons involved
in chemical reactions.
