2. Thales of Miletus - discovered that a piece of
amber, after rubbing it with fur, attracts bits of
hair and feathers and other light objects. He
suggested that this mysterious force came from
the amber. Thales, however, did not connect this
force with any atomic particle.
Democritus – (460 – 370 B.C.) a Greek
philosopher proposed that all matter is
composed of tiny indivisible particles.
- He called this particles atomos, which meant
indivisible or indestructible.
History of the Atom
3. Plato (427 – 347 B.C.) and Aristotle (384 – 322 B.C.)
- They were of the opinion that all materials
could be broken down into smaller parts
and smaller parts without end.
Atomism – was the notion that all matter was composed of very
tiny, individual, finite, and indivisible particles.
John Dalton (1766 – 1844) – an English
schoolmaster concluded that the properties of
matter could be explained in terms of atoms.
4. Matter consists of tiny particles called atoms.
Atoms are indestructible. In chemical reactions, the
atoms rearrange but no new atoms are created and
atoms themselves do not break apart.
The atoms of one particular element are all identical
in mass and other properties. The atoms of different
elements differ in mass and other properties.
When atoms of different elements combine into a
compound, the constituent atoms are always present
in the same fixed numerical ratio.
DALTON’S ATOMIC THEORY
6. 1. Electrons – negatively charged particles
Discovered by Sir joseph John Thomson
2. Protons – positively charged particles
Discovered by Eugen Goldstein
3. Neutrons – neutral charged
Discovered by James Chadwick
SUBATOMIC PARTICLES
7. Nucleus – central part of the atom
- It is where the neutrons and protons are found
- Discovered by Ernest Rutherford
8. A Summary of the Subatomic Particles of an Atom
Mass Number – the whole number which is closest to
the atomic mass
Mass number = number of protons + number of
neutrons
Atomic number (Z) = no of proton = no. of electron
Name Symbol Electrical Charge Mass (amu) Mass (g)
Electron e- -1 0.000549 9.1093 x 10-28
Proton p+ +1 1.00728 or 1 1.673 x 10-24
Neutron no 0 1.00867 or 1 1.675 x 10-24
9. Eelement Atomic number No. of Protons No. of Electrons No. of neutrons Atomic Mass
(amu)
Mass number
Carbon 6 6 6 6 12.011 12
Sodium 11 11 11 12 22.991 23
Sulfur 16 16 16 16 32.06 32
Calcium 20 20 40.08
Silver 47 107.88 108
Gold 118 197.00 197
Lead 82 125 207.21
Subatomic Particles of Some
Elements
10. Isotopes – are atoms of the same element having the
same atomic number but different atomic masses
Protium Deuterium Tritium
Isotopes can be classified as natural (found in nature)
or man made (artificial or synthetic)
Isotopes can be classified as stable or unstable
