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Atoms and Molecules
Parts of the Atom
What is an atom?
Democritus, a Greek philosopher theorized
that at some point a substance can no
longer be divided. He called this atomos,
greek for indivisible.
Atom = the smallest part, indivisible.
What is Atomic Theory?
Atomic theory states that atoms are the
building blocks of all matter.
• Matter is any material substance that
occupies space and has weight.
– Matter makes up ALL the observable
universe
Parts of the atom
Atoms have a nucleus and an outer cloud that are
made up of smaller parts called subatomic
particles.
Protons
In the Nucleus
Neutrons
In the electron cloud
Electrons
Subatomic Particles
• Proton = the
positive “+” charged
particle in the
nucleus of an atom.
– Protons have mass.
• Neutron = the
neutral (no charge)
particle in the
nucleus of an atom.
– Neutrons have mass
• Electron = the
negative “-” charged
particle orbiting
around the nucleus
of an atom.
– Electrons have very
little mass.
What do atoms look like?
What is matter?
Subatomic Particle Comparison
Relative
Mass
Electrical
Charge
Location in
the atom
Proton
1 amu
1+
Part of the
nucleus
Neutron
1 amu
None
Part of the
nucleus
Electron 0.0005 amu
1-
Moves
around the
nucleus in
the electron
cloud
Models: Why use them?
• Models are used to help us see, and
understand, something that can not be
observed directly, or easily.
• Models help us understand how
something is built or how something
works.
What is a model?
What is a model?
• A model is a description or physical
representation of something.
• We must make mental models for
things that we can not see.
Scientific Models
Early Models of the Atom
John Dalton
Ernest Rutherford
J. J. Thomson
Neils Bohr
Early Models of the Atom
John Dalton
J. J. Thomson
Ernest Rutherford
Neils Bohr
Evolution of Atom Models
The Electron Cloud Model
Atomic Number
• Atomic number is the number of protons in
the atom of an element.
• The atomic number of an element is found
using the Periodic Table of the Elements.
Atomic Mass
• Atomic mass is the mass of the atom,
or how much the atom weighs.
– Different atoms have different masses
because of the different number of protons
and neutrons.
• Atomic mass is equal to the number of
protons plus the number of neutrons
Atomic mass = protons + neutrons
Atom Terms
Atoms Form Elements
• An element is the simplest form of a
substance.
• If we break an element down any
smaller we get protons, neutrons and
electrons in known proportions. If we
do this we lose the characteristics of
the substance.
Atoms of Elements Combine to
Form Molecules
• Atoms will combine with other atoms and
form molecules.
• Examples are:
Oxygen + Oxygen = O2 (oxygen molecule)
Hydrogen + Oxygen = H2O (water molecule)
Carbon + Hydrogen + Oxygen = C6H12O6 (sugar
molecule)
What are Molecules?
• A molecule is two or
more atoms joined
together.
– Example is water,
H2O
What do molecules look like?
Salt Molecule
What Are Subatomic Forces?
• Subatomic forces are forces within the
atom that are responsible for holding the
atom together.
• Subatomic forces explain the behavior of
the subatomic particles.
• The four subatomic forces are:
– Electromagnetic force
– Strong force
– Weak force
– Gravity
What is Electromagnetic
Force?
• Electromagnetic force is a force of
attraction.
– It can either attract or repel the particles on
which it acts.
– If the particles have opposite charges, such
as a proton (+) and an electron (-), the
electromagnetic force is a force of attraction.
– If the particles have the same charges, such
as two protons (+) (+), the electromagnetic
force is a force of repulsion.
What is Strong Force?
• Strong force is the force that “glues” the
protons together to form the nucleus of an
atom.
• The strong force opposes the
electromagnetic force.
What is Weak Force?
• The weak force is the key to the power of
the sun, it is responsible for a process
called radioactive decay.
– During radioactive decay a neutron in the
nucleus changes into a proton and an
electron.
What is Gravity?
• Gravity is the force of attraction exerted
between all objects in nature.
• Gravity is the weakest force known in
nature.
Atomic Structure Review
• Parts of the atom:
• Subatomic particles
– Proton: “+” charge
found in the nucleus
– Neutron: no charge
found in the nucleus
– Electron: “-” charge
found outside
nucleus in electron
cloud
• Atomic number = #
of protons
• Atomic mass =
protons + neutrons
• Models of the atom:
–
–
–
–
–
Dalton
Thomson
Rutherford
Bohr
Electron Cloud