Atoms:
Development of the
Atomic Theory
1
Ancient Belief

Ancient Greeks believed that all things
were made up of earth, wind, fire, and
water.
2
Democritus

460 BC - Greek philosopher proposes the existence
of the atom
said that all matter was composed of tiny particles
that he called atomos. Atomos is Greek for “not to
be divided”.
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His theory:
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all atoms are small hard particles
made of a single material formed into different
shapes and sizes
always moving, and that they form different
materials by joining together
3
John Dalton
 1803 - British chemist; elements combined in specific

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proportions to form compounds.
Rediscovered Democritus’ theory and added four
parts.
His theory:
 all substances are made of atoms that cannot be
created, divided, or destroyed
 atoms join with other atoms to make new
substances called compounds
 Atoms of different elements are different
 atoms of the same element are exactly alike, and
atoms of different elements are different in mass
and size (elements)
4
Edward Frankland

1852 - English chemist developed the
valence theory

His theory:
 every
atom has a fixed number of bonds
(chemical links) that it can form
 for the atom to be stable, all of these
bonds must be used.
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J.J. Thomson

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1897 - English chemist and physicist discovered 1st
subatomic particles
His theory: used a cathode-ray tube
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negatively charged particles called electrons and
positively charged matter
Assumed since there was a negatively charged
particle there must be a positively charged
particle: the proton but he did not know how they
were arranged
created a model to describe the atom as a sphere
filled with positive matter with negative particles
mixed in
Referred to it as the plum pudding model
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Cathode Ray Tube
7
Ernest Rutherford
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1912 - New Zealand physicist
Uses the gold-foil experiment and discovers the
nucleus
Said the atom was mainly a solid core at the center
(nucleus)
His theory:
 small, dense, positively charged particle present
in nucleus called a proton
 electrons travel around the nucleus, but their
exact places cannot be described
8
Niels Bohr

1913 - Danish physicist discovered
energy levels
It is the form we know today

His theory:

 electrons
travel around the nucleus in
definite paths and fixed distances
 electrons can jump from one level to a
path in another level
 Is called the planetary model
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Erwin Shrodinger

1924 - Austrian physicist developed
the electron cloud model

His theory:
electrons exact path cannot be predicted
 regions, referred to as the electron cloud,
are areas where electrons can likely be
found.

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James Chadwick

1932 - English physicist discovered
neutrons

His theory:
 neutrons
have no electrical charge
 neutrons have a mass nearly equal to the
mass of a proton
 unit of measurement for subatomic
particles is the atomic mass unit (amu)
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Demitiri Mendeleev

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1869-Russian chemist
The creator of the first Periodic Table of Elements
Predicted the properties of the elements yet to be
discovered
His periodic table is based on atomic mass.
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Henry Moseley
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1915- English scientist that developed the periodic
table that we use today
Sorted the chemical elements of the periodic table
of the elements in a logical order based on their
physics- on their atomic number (how many protons
an element has)
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Modern Theory of the Atom
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Atoms are composed of three main
subatomic particles: the electron, the
proton, and the neutron.
Most of the mass of the atom is concentrated
in the nucleus of the atom.
The protons and neutrons are located within
the nucleus while the electrons exist outside
of the nucleus.
In stable atoms, the number of protons is
equal to the number of electrons.
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Modern Theory of the Atom
Cont.

The type of atom is determined by the
number of protons it has.

The number of protons in an atom is equal to
the atomic number

The sum of the number of protons and
neutrons in a particular atom is called the
atomic mass
Valence electrons are the outermost
electrons and are where bonding takes
place
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Isotopes
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An element that has the same number
of protons but different number of
neutrons.
Example: C12, C14
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Chemical Symbol

The shorthand way of writing an element. 1st
Letter is always capitalized; the second and
third letters are lower-cased.
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Proton (p+)-Positively charged particle in the atom
and is found in the nucleus. It has a mass 1840
times greater than an electron. A proton has a
charge of +1 and a relative mass of 1amu.
Neutron (no)-Neutral particle in the atom found in
the nucleus. It has a mass nearly equal to that of a
proton, but it carries no electrical charge. A
neutron has 0 (zero) charge and a relative mass of 1
amu.
Electron (e-)-Negatively charged particle in the
atom found on the outside of the nucleus (the y
surround the nucleus in an electron cloud). Its mass
is 1/1840 that of a hydrogen atom. An electron as a
charge of -1 and has a relative mass of 0 (zero).
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Figuring out the Neutron
Number of any element

Subtract the proton number from the
atomic mass number. To calculate
mass number, you add the proton
number plus the neutron number.

Mass#=Proton # + Neutron #

Neutron #=Mass# - Proton#
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Element/
Ion
Atomic
Number
Atomic Mass
Mass
Number
Protons
Neutrons
Electrons
1
1.008
1
1
0
1
1H+
1
1
1.008
1
1
0
0
6 C
12
6
12.011
12
6
6
6
17
35.453
35
17
18
18
1H
1
17
Cl-
35
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Atoms
Isotope
Atomic
Number
Mass
Number
Number
of Protons
32.065
16
Number
of
Neutron
Number
of
Electrons
24
20
Zn-64
9
11
10
22.990
21
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Isotopes and average atomic massElements come in a variety of isotopes,
meaning they are made up of atoms
with the same atomic number but
different atomic masses. These atoms
differ in the number of neutrons. The
average atomic mass is the weighted
average of all the isotopes of an
element.
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Calculating Average Atomic Mass of an
Element
Example: A sample of cesium is 75% 133Cs, 20% 132
Cs, and 5% 134Cs. What is the average atomic
mass? (1st, change all percentages to decimals)
Answer: .75 x 133=99.75
.20 x 132=26.4
.05 x 134= 6.7____
132.85
133 a.m.u. (atomic mass units)
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
Radioactivity-the process by which some elements
spontaneously emit radiation.

Radiation is the rays and particles emitted by a
radioactive material. There are 3 types of natural
radiation:
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a-alpha radiation
b-beta radiation
g-gamma radiation
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Alpha radiation consists of particles containing two
protons and two neutrons. Alpha particles are
identical to a helium nucleus. The symbol is a or
4HeThey are large but do not penetrate. The
relative mass is 4 and the charge is +2.
Beta radiation consists of fast-moving electrons.
The symbol for a beta particle is B or 0B or 0e. They
are small and fairly penetrating. The relative mass
is 0 and the charge is -1.
Gamma radiation (or gamma rays) is not particles
but extremely high-energy electromagnetic
radiation. They are very penetrating and
dangerous. They have no mass and no charge.
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The neutron to proton ratio (n0:p+)
determines the stability of a nucleus.
Unstable nuclei will continue to decay
until a stable ration is achieved.
Nuclear equations show the changes
involved with the atomic number and
the mass number using nuclear
symbols.
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
The total mass numbers of the
reactants must equal the total mass
numbers of the products. The total
atomic numbers of the reactants must
equal the total atomic numbers of the
products.
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

Nuclear equations show the changes
involved with the atomic number and
the mass number using nuclear
symbols.
The total mass numbers of the
reactants must equal the total mass
numbers of the products. The total
atomic numbers of the reactants must
equal the total atomic numbers of the
products.
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