2. Defining the Atom
• An Atom is the smallest particle of an
element that retains its identity in a
chemical reaction.
• The Greek Philosopher
Democritus, was among the
First to suggest the existence
Of atoms.
3. •Democritus believed that
atoms were indivisible and
indestructible.
•Although Democritus’s ideas
agreed with later scientific
theory, they did not explain
chemical behavior.
•They also lacked experimental
support.
4. Electrons
• In 1897, English
physicist J.J.
Thomson discovered
the Electron.
• Electrons are
negatively charged
subatomic particles.
• Thomson called these
particles corpuscles;
later they were
named electrons.
5.
6. »You need to understand….
•Atoms have NO
CHARGE!!
»They are electrically neutral.
7. • This model of the atom turned out to be short
lived, however, due to the groundbreaking
work of Ernest Rutherford (1871-1937), a
former student of Thomson.
9. Rutherford Atomic Model
• He proposed that the atom is mostly empty
space, thus explaining the lack of
deflection of most of the alpha particles.
• He concluded that all the positive charge
and almost all the mass are concentrated in
a small region.
• He called this region the Nucleus.
• The nucleus is the tiny central core of an atom and is
composed of protons and neutrons.
10. Key Concept
• In the nuclear atom, the protons and
neutrons are located in the nucleus. The
electrons are distributed around the
nucleus and occupy almost all the volume
of the atom.
11. Review Cont….
• There are 3 subatomic particles
1. Proton
– Identifies the atom
– Positive charge
2. Electron
– Determines chemical reactions
– Negative Charge
3. Neutron
– Determines Isotope
– Neutral Charge
