3. Atomic Theory Definition
• Atomic theory is the idea that matter is made up of little units called
atoms. When the ancient Greek philosopher Democritus came up
with the idea in the 5th century BC, is was originally meant to refer
to indivisible units. As of 1897, the British scientist J.J. Thomson
discovered that atoms are in fact made up of smaller particles.
Today atomic theory refers to matter being made up of units that are
indivisible only some of the time. Exceptions include plasmas such
as fire, other ionic arrangements such as those found in the
body, radioactive materials, and many more.
• The word "atom" (from the ancient Greek
adjective atomos, 'indivisible'[) was applied to the basic particle that
constituted a chemical element, because the chemists of the era
believed that these were the fundamental particles of matter.
4. Development of the atomic theory
The idea that everything is made up of a few simple
parts originated during the 400's B.C. in the philosophy of atomism.
Atomism was founded by the Greek philosopher Leucippus, but his
disciple, Democritus developed the philosophy more fully.
Democritus gave his basic particle the name atom, which means
uncuttable. He imagined atoms as small, hard particles, all composed of
the same substance but of different sizes and shapes.
During the 300's B.C., a Greek philosopher named Epicurus
incorporated Democritus' ideas about atoms into his philosophy. About
50 B.C., the Roman philosopher and poet Lucretius presented the
fundamental principles of atomism in his long poem, On the Nature of
5. Development of the atomic theory
During the Middle Ages, the idea of atoms was
largely ignored. This neglect resulted partly from
the fact that atomism had been rejected by
Aristotle, an ancient Greek philosopher whose
theories dominated medieval philosophy and
The idea that atoms form the basic units of all matter did
During the 1500's and 1600's, such founders of modern science as
Francis Bacon and Isaac Newton of England and Galileo of Italy
believed in atoms. But those scientists could add little more to the
atomic theory than Democritus had described.
7. Democritus’ Life and Contribution
Ancient Greek philosopher born in Abdera, Thrace, Greece. He was
an influential pre-Socratic philosopher and pupil of Leucippus, who
formulated an atomic theory for the cosmos. Democritus is
recognized as the earliest proponent of the concept of atomism.
Leucippus, the founder of the atomism, was the greatest influence
The theory of Democritus and Leucippus held that everything is
composed of "atoms", which are physically, but not
geometrically, indivisible; that between atoms lies empty space; that
atoms are indestructible; have always been, and always will be, in
motion; that there are an infinite number of atoms, and kinds of
atoms, which differ in shape, and size.
Of the mass of atoms, Democritus said "The more any indivisible
exceeds, the heavier it is." But their exact position on weight of
atoms is disputed.
Democritus, along with Leucippus and Epicurus, proposed the
earliest views on the shapes and connectivity of atoms. They
reasoned that the solidness of the material corresponded to the
shape of the atoms involved.
8. The birth of the modern atomic theory
In 1750, Rudjer Boscovich, a scientist born in what is now
Croatia, suggested that Democritus might have been wrong in
believing that atoms are "uncuttable." Boscovich thought that atoms
contain smaller parts, which in turn contain still smaller parts, and so
forth down to the fundamental building blocks of matter. He felt that
these building blocks must be geometric points with no size at all.
Today, most atomic physicists accept a modern form of this idea.
The development of the atomic theory advanced greatly when
chemistry became an exact science during the late 1700's.
Chemists discovered that they could combine elements to form
compounds only in certain fixed proportions according to mass. In
1803, a British chemist named John Dalton developed an atomic
theory to explain this discovery.
9. John Dalton’s Life And Contribution
• Around 1803, Dalton developed an atomic theory to explain the ratios in
which elements combine to form compounds. It was the cornerstone for
modern atomic theory.
• 5 main points of Dalton’s atomic theory:
• 1. Elements are made of extremely 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
• 3. Atoms cannot be subdivided, created, or destroyed.
• 4. Atoms of different elements combine in simple whole-number ratios to
form chemical compounds.
• 5. In chemical reactions, atoms are combined, separated, or rearranged.
• - He also developed an assumption based on the faith of nature’s simplicity
that when atoms combine in only one ratio, it must be presumed to be
binary one, unless some cause appear to the contrary.
• - Limitations included that nowadays, Dalton’s second and third points to the
atomic theory and proven wrong.
10. John Dalton’s Life And Contribution
There were three fundamental laws established
by Dalton and other scientists of his time to
support the atomic theory. These laws are the:
• a) Law of Conservation of Mass
The law of conservation of mass states that in a chemical reaction, matter is
neither created nor destroyed, or, more accurately, there is no detectable
change in mass during an ordinary chemical reaction.
• b) Law of Definite Proportions
The law of definite proportions states that different samples of any pure
compound contain the same elements in the same proportions by mass.
• c) The Law of Multiple Proportions
The law of multiple proportions states that the mass of one element that can
combine with a fixed mass of another element are in a ratio of small whole
11. J.J Thomson’s Life And Contribution
In 1896 , he took the cathode ray experiments a step further by
ﬁrstly improving Perrin’s version to more clearly prove cathode rays
do carry negative charges. With this , Thomson then went on to
discover the electron through his demonstration of cathode rays
responding to electrode ﬁelds just as negatively charged particles
Thomson had ﬁgured out a way to determine the charge of the mass
by using both an electric and magnetic ﬁeld.
Used mutually perpendicular electric and magnetic ﬁelds to
determine the speed of cathode rays. Then with only one ﬁeld
turned on, he measured the deﬂection of rays. These deﬂections
depended on magnitude of ﬁeld, length of path in the ﬁeld, and the
speed, mass, and charge of cathode-ray particles.
With calculation, he found reasonably consistent values for the
charge-mass ratio, which allowed him to conclude that all cathode
rays consist of identical particles with exactly the same negative
1897 - English chemist and physicist; discovered 1st subatomic particles
12. J.J Thomson’s Life And Contribution
• 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
13. Ernest Rutherford’s Life And Contribution
Small, dense, positively charged particle present in nucleus called a
Electrons travel around the nucleus, but their
exact places cannot be described.
1912 - New Zealand physicist discovered the nucleus
By 1909, he had shown that some radioactive elements, such as
radium and thorium, emitted positively charged helium ions, which are
also known as alpha particles and when passed through a thing sheet
of mica, a beam of alpha particles will spread out.
He had a technique that allowed him and his assistants, Hans Geiger
and Ernest Marsden, to measure the proportion of alpha particles
scattered at different angles from various materials. They would
produce a pencil-shaped beam of alpha particles and position a thin
sheet of gold foil at a right angle to the beam. Then they would use a
screen coated with zinc sulﬁde, which would detect the scattered
particles by letting off faint ﬂashes of light visible with a microscope. By
moving the screen and microscope around the foil, they were able to
measure the rates at which alpha particles appear at various angles.
Eventually, they concluded the positive charge in a gold atom must be
concentrated in an incredibly tiny volume, so most of gold was actually
14. Ernest Rutherford’s Life And Contribution
When using aluminum foil instead of gold, they proved that the
positive charge and most of the mass of an atom are contained in a
radius less than 10 -14
Discovered nucleus and disproved the raisin-bun model
Lead to planetary model of atom consisting of electrons orbiting the
nucleus of anatom and there being an electrostatic attraction
between positive nucleus and negative electrons, which provides
the centripetal force to keep the electrons in orbit.
Limitations included the fact that Rutherford’s model was later
adjusted by NielsBohr because in Rutherford’s model, the electrons
should spiral into the nucleus in a few microseconds due to a
constant acceleration, which would emit electromagnetic waves that
would take energy from the orbiting electrons.
15. Niel’s Bohr’s Life And Contribution
Corrected the critical ﬂaw in Rutherford’s model.
Focuses on the quantization of energy of electrons
- Basic principles of Bohr’s model:
1. Electrons can orbit the nucleus only at certain speciﬁc distances from
the nucleus. These distances are particular multiples of the radius of the
smallest permitted orbit meaning the orbits in an atom are quantized.
2. The electron's distance from the nucleus determines both the kinetic
and electric potential energy of an electron in orbit. So forth the energy in an
electron is also quantized and each orbit corresponds to a speciﬁc energy
level for the electron.
3. Only by emitting or absorbing photons of equal energy to the difference
between energy levels can an electron move from one energy level to
another. When an electron continues to orbit at particular energy level, no
energy is radiated. Also, since the size and shape of the orbit remains the
same and at a ﬁxed energy level, these orbits are often referred to as
16. Niel’s Bohr’s Life And Contribution
Limitations included him not explaining as to why energy is
quantized, why orbiting electrons do not radiate electromagnetic
energy, why a magnetic ﬁeld splits the main spectral lines into
multiple closely spaced lines, and the fact that it is not accurate for
electrons to have two or more electrons.
Electrons travel around the nucleus in definite paths and fixed
Electrons can jump from one level to a path in another level.
1913 - Danish physicist; discovered energy levels
17. Quantum Model (De Broglie)
In 1924, Louis de Broglie developed his theory that particles have wave
properties. He concluded this through diffraction experiments. So
forth, the principles of interference and standing waves apply for electrons
orbiting a nucleus.
For most sizes of orbit, successive cycles of the electron wave will be out
of phase, and destructive interference will reduce the amplitude of the
wave. For constructive interference to occur, the circumference of the
orbit must be equal to a whole number of wavelengths.
The wave nature of matter provides a natural explanation for quantized
In 1926, Erwin Schrödinger derived an equation for determining how
electron waves behave in the electric ﬁeld surrounding a nucleus. The
solutions to his equation are functions that deﬁne the amplitude of the
electron wave in the space around a nucleus.
In quantum model, electrons behave as waves, which do not have a
18. ATOMIC SPECTRA
The atomic spectra is a range of characteristic frequencies of
electromagnetic radiation that are readily absorbed and emitted by an
An electron can jump from one ﬁxed orbital to another. If theorbital it
jumps to has a higher energy, the electron must absorb photon of a
certain frequency. If it’s a lower energy, the electron must give off a
photon of a certain frequency.
The frequency depends on the difference in energy between the orbitals.
This relates to Bohr’s model because Bohr’s model describe show in order
for electrons to move from one orbital to another, the electron must
release or absorb a photon of appropriate energy.
19. Models Presented
Solid Sphere Model or
Bowling Ball Model Plum Pudding Model or Nuclear Model
Proposed by John Dalton Raisin Bun Model Proposed by Ernest
Proposed by J.J. Thomson Rutherford
Bohr Model or
Planetary Model Electron Cloud Model
Proposed by Niels Proposed by Erwin