Download Atoms

The History of Atoms
Not the history of the atom itself, but
the history of the idea of the atom.
1. Democritus 460-370 B.C
Democritus (Greek
Philosopher 384-322
B.C.
 Matter is made of
Atomos
(He thought it
looked like this!)

Some weaknesses of Democritus's
idea

No empirical (experimental) evidence,
just a well thought out idea.

“What holds the Atomos together?” was
one important question that Democritus
could not answer.
2. Aristotle 384-322 B.C.
Completely rejected
Democritus’s idea
Aristotle was so well
regarded and influential that
his rejection of the Atomos
caused the idea of Atoms to
be dismissed for nearly 2000
years .
3. John Dalton 1766-1844
He revisited and revised
Democritus’s ideas.
He made careful
observations on numerous
chemical reactions that
helped him refine his
theories.
Dalton’s Atomic Theory

All atoms of an element are identical
having the same size, mass and chemical
properties. Atoms of an elements are
different from all other atoms of different
elements.
Dalton’s Atomic Theory

Atoms cannot be created, destroyed or
divided in to smaller particle.
Dalton’s Atomic Theory
Atoms cannot be created, destroyed or
divided in to smaller particle.
 Different atoms combine in simple whole
number ratios to form compounds.
 Atoms are neither created nor destroyed
in chemical reactions.

CO2
Law of Conservation of Mass
The total mass of substances present at
the end of a chemical process is the same
as the mass of substances present before
the process took place.
Is Dalton’s Theory Completely
Accurate?

It was a major step in the right direction.

No, atoms of the same element can have
slightly different masses (Isotopes).

No, atoms are divisible into neutrons
protons and electrons.
4. J. J. Thomson
Around 1900
 “Plum
Pudding Model”
5. Rutherford’s Discovery of the
Nucleus
Ernest Rutherford
shot  particles at a
thin sheet of gold foil
and observed the
pattern of scatter of
the particles.
Protons were discovered by Rutherford in
1919.
6. Niels Bohr “Planetary Atomic Model”
1913
Electrons only occupy well-defined orbits with fixed
energy levels
 Discovered that electrons may absorb energy by
jumping to higher energy levels and fall back to the
original level or orbit. This release of energy is called a
photon.

7. Modern View of the Atom
 Tiny
nucleus surrounded by electron
“cloud”
 Nucleus accounts for all of the mass
Subatomic Particles
Protons (p+) - positive charge – inside the
nucleus – weight 1 amu (atomic mass
unit)
 Neutron (n0) – neutral charge – inside the
nucleus – weight 1amu
 Electron (e-) - Negative Charge – in
Electron Cloud – weight 1/1840 amu

Using the Periodic Table to
Determine the Subatomic Particles
Equals the Number of
Protons (Also the
Number of Electrons
in a neutral atom)
Equals the Number of
Protons + Neutrons
(since the nucleus is
where all the weight
is located)