This document outlines the development of atomic theory from Democritus' idea of indivisible atoms to modern atomic structure. It describes key contributors such as Dalton who proposed atoms as fundamental units of matter, Thomson who discovered the electron, and Rutherford whose gold foil experiment showed atoms have a small, dense nucleus. Later scientists such as Bohr, de Broglie, Schrödinger, Chadwick, and Curie further refined atomic models through discoveries like isotopes, wave-particle duality, and subatomic particles like neutrons. Their work disproved early ideas of atoms as indivisible and showed matter is made of even smaller constituents that can change form through radioactivity.

1. ATOMIC THEORY

2. Democritus
“Could matter be divided into smaller and
smaller pieces forever?”
His theory: Matter could not be divided into
smaller and smaller pieces forever, eventually
the smallest possible piece would be
obtained.This piece would be indivisible. He
named the smallest piece “atomos” meaning
“not to be cut”.

3. Democritus
 Greek philosopher
 No experiments to support idea
 No protons, electrons, or neutrons
 Small, solid particles that were made of the
same material but were different shapes and
sizes

4. Democritus
 Atoms are infinite in number, always moving
and capable of joining together to form
different objects.
 The characteristics of an object are
determined by the shape of its atoms.

5. Democritus (400 BC)
 Hard substances have rough atoms that stick
together
 Smooth atoms are in liquids and they slide
over each other
 Sleep is caused by atoms escaping the brain

6. Dalton
 1803 proposed that elements consist of
individual particles called atoms
 Four parts of the theory
 He had the first atomic theory that had
evidence to support it
 Called elements “pure”
 Atom looked like a marble

7. Dalton
 All matter is made of atoms
 Atoms are indivisible and indestructible
 All atoms of a given element are identical in
Mass and properties
 Compounds are formed by a combination of
two or more different kinds of atoms
 A chemical reaction is a rearrangement of
atoms
 Atoms can be neither created nor destroyed

8. Dalton
 The essence of Dalton’s theory remains valid.
 His ideas were basis for chemistry

9. Dalton
 Developed the Law of Multiple Proportions
 When elements form multiple compounds, the
proportions can be expressed as a whole number
ratio.

10. Mendeleev
 He noticed a pattern in various elements
according to their masses.
 He later arranged the elements into a table—
the periodic table.

11. Sir William Crookes
 First person to confirm the existence of
cathode rays by displaying them with his tube
invention.
 He was able to show that cathode rays are
negatively charged by studying the direction
in which cathode rays are deflected by a
magnetic field.

12. Sir William Crookes
 Cathode rays—streams of electrons observed
in vacuum tubes.
 Crookes tube—an early experimental
electrical discharge tube in which cathode
rays, streams of electrons, were discovered
 A cathode ray tube is a vacuum tube that
produces images when its phosphorescent
surface is struck by electron beams.

13. Becquerel
 Contributed to the knowledge of radiation
and radioactive decay
 His photographic plate didn’t need a source
of energy to make an image—discovered
radioactivity! (He was using sunlight and
artificial light previously)

14. JJ Thomson
 Plum Pudding Model (Chocolate Chip Cookie)
 Totally changed the view of an atom by
discovering the electron
 Thomson’s atomic theory suggested that the
atom is divisible because it is made of smaller
pieces—electrons and protons
 An atom consists of a sphere of positive
charge with negatively charged electrons
embedded in it

15. Thomson
 The positive and negative charges in an atom
are equal in magnitude due to which an atom
is electrically neutral.
 It has no overall negative or positive charge.

16. Thomson
 Discovered the electron when he was
experimenting with gas tubes
 He noticed a movement and called the
movement cathode rays
 The rays travel from negative to positive
 He concluded that atoms do contain subatomic
particles—they are divisible
 This contradicted Dalton’s theory and was not
widely accepted by fellow physicists and
chemists

17. Thomson continued….
 Cathode rays originate at the cathode
(negative) and move to the anode (positive)
 The rays bent toward the positive pole,
indicating the negative charge

18. Thomson continued….
 Thomson proved that atoms of any element
can be made to emit tiny negative particles—
therefore, all atoms must contain these
negative particles

19. Marie Curie
 Discovered polonium and radium which were
radioactive
 Radium, a new element, spontaneously
disintegrated into other elements. This
proved that the atoms of one element at least
were not indivisible.

20. Rutherford
 Learned physics inThomson’s lab
 Conducted gold foil experiment
 He took a thin sheet of gold foil and used
special equipment to shoot alpha particles
(positive) at the gold foil

21. Rutherford continued….
 Most particles passed straight through the
foil like the foil wasn’t even there…
 Some particles went straight back or were
deflected as if they had hit something…

22. Rutherford continued…
The experiment shows:
 Atoms are made of a small, positive nucleus;
the positive nucleus repels (pushes away)
positive alpha particles
 Atoms are mostly empty space
 Planetary Model

23.  He named protons—a mass much larger than
the electron.
 Through his experiment, he realized that an
atom is mostly empty space with a nucleus
taking up most of the mass of the atom.
Electrons orbit the nucleus at a great
distance, relatively speaking.

24. Niels Bohr (early 1900s)
 Danish physicist
 Refined Rutherford’s model
 He proposed a model of the atom that is
similar to the model of the solar system.
 The electrons go around the nucleus like
planets orbit around the sun—fixed orbits.

25. Bohr continued…
 All electrons have energy levels—a certain
distance from the nucleus
 Each energy level can hold a certain number
of electrons

26. Louis De Broglie
 His ideas were a basis for developing the
Wave MechanicsTheory
 His theory helped explain how atoms,
molecules, and protons behave
 He inspired Schrodinger in the formulation of
wave mechanics

27. De Broglie
 QuantumWave Mechanical Model
 Electrons could act as both particles and waves
 Waves around a nucleus

28. Schrödinger
 Orbitals can be described as electron density
clouds
 Densest area—greatest probability of finding
atom
 Least dense—lowest probability of finding
electron

29. James Chadwick
 Discovered the neutron
 He spoke about isotopes—Isotopes of the
same element have the same number of
protons and electrons but differ in the
number of neutrons found in their nucleus.

30. Isotopes
 Different forms of an element that have the
same number of protons but a different
number of neutrons.
 Mass of individual isotopes remain mostly
unchanged but differ in stability…as
such…some isotopes may emit neutrons,
protons, and electrons.To attain a more
stable atomic configuration. (potential
energy) through radioactive decay.

31. Radioactivity
 Radioactivity is the spontaneous
disintegration of an unstable nucleus, by the
emission of a small particle (alpha or beta) or
by the change of the energy level of the
nucleus by the emission of a gamma ray

32.  Elements may change into another element
naturally due to an unstable nucleus
 The ability to emit radiation does not depend
on the arrangement of the atoms in a
molecule; it must be linked to the interior of
the atom itself.

33. Advancement of atomic theory
 If atoms emitted alpha and beta particles,
then they could not be indivisible and
unchangeable.
 Atoms are made up of smaller particles, and
these can be rearranged.