1. The Atom
- Are the building blocks of matter that
make up everything that we encounter
every day.
- The basic unit of matter
- Are the smallest individual part of an element
and it consists of subatomic particles such as
protons, electrons and neutrons.

2. TIMELINE OF ATOMIC
NATURE
He proposed that all substances are composed
of four elements such as air, earth, fire and
water in different proportion.
fire
air
water
earth
Aristotle
322 – 384 BC

3. Empedocles divided matter into
four elements such as air, earth, fire
and water.
Approximately
450 BC

4. • Democritus and Leucippus –
proposed that everything is
composed of small bit of matter
that is indivisible and call it atom
(atomos – “indivisible”)
500 BC

5. • Lavoisier clarified the concept of an
element as a simple substance that
could not be broken down by any
known method of chemical analysis,
and he devised a theory of the
formation of chemical compounds from
elements.
1777

6. • Coulomb explained that like fluids
repel and unlike attract. This was
important in the development of the
theory of absolute measurement, and
had a great impact on electrical
potential.
1780

7. (The Solid Sphere Model)
– Billiard Ball Model - small solid
sphere
– Developed notion of
conservation of mass and that
atoms combine in specific
ratios
Dalton’s Atomic Theory1803

8. The postulates of this
theory are the ff;
1. All matter is composed of extremely
small, indivisible, indestructible
particles called atoms.
2. All atoms of each element are exactly
alike but they differ from atoms of
the other elements. The atoms of
different elements have different
mass and properties.

9. The postulates of this
theory are the ff;
3. When atoms of different elements
combine to form a compound, the
constituent atoms are always present in
the same fixed numerical ratio.

10. The postulates of this
theory are the ff;
4. A chemical reaction involves a
rearrangement of atoms. No atom is
created or destroyed.

11. Sir William Crookes
- constructed the forerunner of the
modern television picture tube in the
1870s to investigate the properties of
cathode rays.
- Crookes tube – a vacuum discharge
tube
1870

12. Wilhelm Roentgen
- discovered X-ray by observing the
fluorescence they produced.
1896

13. • J.J. Thomson
– Plum Pudding Model - positive and
negative particles dispersed
throughout the atom
– Used Cathode Ray tubes to
discover the electron - first
subatomic particle discovered!
1898

14. Cathode Ray Tube
• A tube that contains a stream of
electrons going from a negative disk
(cathode) to a positive disk.
• Deflected stream showed that
electrons are negative.

15. Rutherford
– Nuclear Model of an atom
– Discovered dense positively charged
nucleus of the atom while working with
alpha particles
• Gold Foil Experiment

16. • Neils Bohr -
– Planetary Model of an Atom
– Electrons travel in specific,
circular orbits
Neils Bohr and Arnold Sommerfeld
- Arnold Sommerfeld improved
Bohr’s model and introduced the
concept of elliptical orbit.