The Atom, Atomic
Structure & Theories
ATOMS
• Are the smallest unit of matter that have all the
properties of an element.
• They composed of smaller subatomic particles as
protons, neutrons, and electrons.
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Atomic Theories
Democritus
• Greek philosopher proposes the
existence of the Atom.
All atoms:
• Are small hard particles
• Are made of a single material formed into different
shapes and sizes
• Are always moving, and they form different materials by
joining together
He called them atoma which is Greek for “indivisible”.
Atomic Theories
Aristotle
• He did not think there
was a limit to the number
of times matter could be
divided.
• He thought that all substances were
built up from only four elements:
Earth
Fire
Water
Air
Atomic Theories
John Dalton
•
Although the concept of the atom dates back to the
ideas of Democritus, the English meteorologist and
chemist John Dalton formulated the first modern
description of it as the fundamental building block of
chemical structures.
Solid Sphere Model or
Bowling Ball Model
Proposed by John Dalton
Atomic Theories
Dalton’s Atomic Theory
1. All matter is composed of small particles called atoms.
2. Atoms of a given element are identical in size, mass, and
other properties; atoms of different elements differ in
size, mass, and other properties.
3. Atoms cannot be sub-divided, created or destroyed.
4. Atoms of different elements combine in simple whole
number ratios to form a chemical compounds.
5. In chemical reactions atoms are combined separated or
rearranged.
Atomic Theories
Dalton’s Atomic Theory
However, Further studies conducted examined each postulate of
Dalton’s atomic theory as follows:
1.
2.
3.
4.
Atoms of elements today can be destroyed by artificial transmutation
called bombardment or nuclear fission.
With the existence of Isotopes, the second postulate is only partially
accepted. Isotopes are atoms of the same element having the same
atomic number but of different atomic masses.
Atoms consist of smaller particles such as protons, neutrons and
electrons. These particles can be extracted to exist in individual form
so that they are called Particles of matter.
It is true that they combine to form molecules, but they combine not
only in small whole number ratio but also in large whole number
ratio such as formation of organic compounds.
Atomic Theories
Joseph John Thompson
English chemist and physicist;
discovered 1st subatomic particles.
• Atoms contain negatively charged particles called electrons and
positively charged matter.
• 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 or Raisin Bun Model
Atomic Theories
Ernest Rutherford
New Zealand physicist, who conducted an
experiment about the bombardment
of gold foil with alpha particles. To account for the results of the
experiment, Rutherford proposed the following
1.
2.
3.
•
Atom consists of a large empty space
Atoms consists of very small but massive region, hence, the
particles that bounced back were presumed to have hit this
region. The Discovery of Nucleus.
Those which deflected approached the positive part, hence,
there was repulsion since the alpha particles were also positive.
Proposed the: Nuclear Model
Atomic Theories
Neils Bohr
Danish physicist who discovered
energy levels.
Concluded that electrons are located in planet-like orbits around the
nucleus in certain energy levels.
•Electrons travel around the nucleus in definite paths and fixed
distances.
•Electrons can jump from one level to a path in another level.
Atomic Theories
Erwin Shrodinger
Austrian physicist; developed
the electron cloud model.
His Theory:
•The exact path of electrons cannot be predicted.
•The region referred to as the electron cloud, is an area where
electrons can likely be found.
Electron Cloud Model
Proposed by Erwin Schrodinger
Electron Cloud Model
Proposed by Erwin Schrodinger
Atomic Theories
James Chadwick
English physicist; who discovered
neutrons
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).
Subatomic Particle
Symbol
Mass (g)
Mass (u)
Charge
Proton
p+
1.67252 x 10-24
1.0073
Positive (+)
1.0087
No Charge (0)
0.00055
Negative (-)
Neutron
Electron
nº
e-
1.67482 x 10-24
9.109 x 10-28
Atomic Number & Mass Number
ATOMIC NUMBER
• number of protons in a nucleus
• Each element has a characteristic atomic
number
• The number of electrons equals the
atomic number in a neutral atom
MASS NUMBER/ ATOMIC MASS
• Total number of protons and neutrons in
an atom
Atomic mass (A)
A = protons + neutrons
A = Atomic no. (Z) + neutrons
Atomic Number (Z)
Z = No. of Protons
Z = No. of Electron for
neutrally charged atoms
No. of Neutrons
=Mass Number – Atomic number
Exercise 1
Complete the following table:
Element
Atomic No.
Mass No.
A
17
35
B
108
C
D
E
No. of p+
No. of e-
No. of nº
92
146
47
82
125
27
Atomic mass (A)
A = protons + neutrons
A = Atomic no. (Z) + neutrons
No. of Neutrons
=Mass Number – Atomic number
14
Atomic Number (Z)
Z = No. of Protons
Z = No. of Electron for
neutrally charged atoms