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Unit E
Chapter 1 “Atoms, Elements and Compounds”
Lesson 1
ATOMS
Matter is Made of Atoms
Democritus
460BCE –
370BCE
 If you were to cut something
in half, how many times could
you cut it?
 Around 400BCE, the Greek
philosopher Democritus said
you would get a piece that
could not be divided any
further.
 He named these tiny pieces
“atoms.”
 An atom is the smallest unit
of a pure substance that still
has the properties of that
substance.
Known as the
“Father of
Modern
Science”
He was also
known as the
“laughing
philosopher” for
his cheerfulness.
Later writers
thought it was
his amusement
with human
follies.
The Atomic Theory
 While scientists and
philosophers believed atoms
were real, it wasn’t until the
1800’s that scientists could use
experiments to study them.
 An English chemist named John
Dalton was the first to use
experiments to support the
“atomic theory.”
 The atomic theory is the theory
that matter is made of atoms.
 Dalton thought atoms were the
smallest pieces of matter and
could not be divided further. He
was proven wrong near the end
of the century.
John Dalton, not known as the
“laughing chemist.”
The Atomic Theory
 In 1897, J.J. Thomson
showed that atoms are
made of even smaller
particles called
“subatomic particles.”
 He made a model of an
atom to explain his
findings, it was the first
to show subatomic
particles.
Thomson’s atomic model
The Subatomic Particles
 The nucleus is the center
of an atom. It has a
positive electrical charge
and is made of at least
one proton.
 A proton is a subatomic
particle with a positive
charge.
 A neutron is a subatomic
particle with a neutral
charge. It has roughly
the same mass as a
proton.
The Subatomic Particles
 An electron is a negatively
charged subatomic
particle. They are much
smaller than protons.
 Protons and neutrons have
nearly 2000 times the mass
of an electron.
 Electrons are in constant
motion around the
nucleus, acting like a cloud.
 The flow of electrons from
one atom to another is
electricity.
Electrons may be small but they can
pack a wallop. This is a small example of
the power of an atom.
The Nucleus
 In 1911, Ernest Rutherford
proved the existence of the
nucleus.
 He shot charged particles
at gold foil, most traveled
straight through but a few
bounced back.
 He inferred that atoms are
mostly empty space. The
diameter of the nucleus is
1/10,000 the diameter of
the whole atom.
Rutherford with his gold foil experiment
Isotopes and Ions
 The atomic number is the
number of protons in the
nucleus.
 Isotopes are atoms that
have the same amount of
protons but different
amounts of neutrons.
 Scientists measure the
mass of an atom in atomic
mass units or amu for
short.
 To find the atomic mass,
add the number of protons
and neutrons.
These are all carbon atoms, but with
different amounts of neutrons.
Isotopes and Ions
 Ions are atoms that
have gained or lost
electrons, causing the
atom to receive an
electric charge.
 Electrons are
constantly being taken
and lost between
atoms.
Ion cannons, found in Star Wars…..not in
real life…..not yet anyway……..darn it.
Charges in Atoms
 Because opposite charges
attract, the positive charge of
protons holds the negative
electrons around the nucleus.
 But if similar charges repel,
why don’t the positive
protons push apart from each
other?
 Overpowering this repulsion
is a strong nuclear force.
 This nuclear force holds the
nucleus of an atom together.
This is the first underwater nuclear
test. This nuclear force holds the
atoms together. When this force is
released, the chain reaction is terrible.
Charges in Atoms
 To find the charge of an
 If an atom has 5 protons
atom, you must compare
the amount of positive
and negative charges
(protons are positive,
electrons are negative)
 If an atom has 5 protons
and 4 electrons, it has a
charge of +1 because it
has one more positive
charge than negative
charge.
but 7 electrons, it has a
charge of -2 because it
has 2 more negative than
positive charges.
 Positive and negative
charges neutralize each
other. If an atom has the
same amount of protons
and electrons, the atom
has no charge, or is
neutral.