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Introduction to
Atoms
Eighth Grade --- Unit 2
EQ: How is the periodic table organized?
Atomic Models
O If you could look into an
atom, what would you
see?
O Figuring out what is
inside atoms has not
been easy.
O Because atoms are so
small, it’s been a difficult
task has been tricky and
our ideas about the
structure and shape of
atoms have changed
many times.
Atomic Models
O The ancient Greeks were the
first to think about what
made up matter.
O The Greek philosopher
Democritus first proposed the
idea that matter is made of
small pieces that could not be
cut into smaller pieces
around the year 430 BC
O He described these small
pieces as atoms, which
means “uncuttable” in Greek.
O Because the Greeks did not
do experiments they did not
prove the existence of atoms
Atomic Theory
O It wasn’t until the late
1600’s (2000 years later!)
that the idea of atoms
were considered again.
O Since then scientists such
as John Dalton, JJ
Thompson, Ernest
Rutherford, Niels Bohr,
and James Chadwick have
researched and learned
about Democritus’s
atoms.
O We are adding to our
knowledge every day
Atoms
O Atoms are the
foundation of
chemistry. They are the
basis for everything in
the Universe.
O As you know, matter is
composed of atoms.
O Solids are made of
densely packed atoms
while gases have
atoms that are spread
out.
The Modern Atomic Model
O The modern atomic
model describes an
atom as consisting of
a nucleus that
contains protons and
neutrons,
surrounded by a
cloudlike region of
electrons
Structure of An Atom
O Electrons are the
smallest of the three
particles that make up
atoms.
O Electrons are found in
shells or orbitals that
surround the nucleus of
an atom.
O Protons and neutrons are
found in the nucleus.
O They group together in
the center of the atom.
Structure of An Atom
O There are almost 120
known elements in the
periodic table.
O The atoms of different
elements have different
numbers of electrons,
protons, and neutrons.
O Every element is unique
and has an atomic
number.
O That number tells you the
number of protons in every
atom of the element
Structure of An Atom
O The center of the atom
is called the nucleus.
O Electrons are found in
areas called shells. A
shell is sometimes
called an energy level.
O Shells are areas that
surround the center of
an atom.
O Each of those shells
has a name (K, L, M...).
Charges of Atoms
O You can see that each part
of the atom is labeled with
a "+", "-", or a "0."
O Those symbols refer to the
charge of the particle.
O Have you ever heard about
getting a shock from a
socket, static electricity, or
lightning?
O Those are all related to
electric charges. Charges
are also found in tiny
particles of matter.
Charges of Atoms
O The electron always has a "-",
O
O
O
O
or negative, charge.
The proton always has a "+",
or positive, charge.
If the charge of an entire
atom is "0", or neutral, there
are equal numbers of positive
and negative charges.
Neutral atoms have equal
numbers of electrons and
protons.
The third particle is the
neutron. It has a neutral
charge, also known as a
charge of zero.
Charges of Atoms
O Since the number of
protons in an atom
does not change, fewer
or extra electrons can
create a special atom
called an ion.
O Cations have fewer
electrons and have a
positive charge.
O Anions have extra
electrons that create a
negative charge.
Electrons
O Electrons are the
negatively charged
particles of atom.
O Together, all of the
electrons of an atom
create a negative charge
that balances the positive
charge of the protons in
the atomic nucleus.
O Electrons are extremely
small compared to all of
the other parts of the
atom.
Electrons
O Electrons are found
in clouds that
surround the nucleus
of an atom.
O Those clouds are
specific distances
away from the
nucleus and are
generally organized
into shells.
Electrons
O Each electron shell can hold
a certain number of
electrons
O Electron shells are filled
from the inside out
O Noble gases have full outer
electron shells
O All other elements have
partially filled outer electron
shells
Protons
O A proton is a tiny particle,
smaller than an atom.
O Protons are too small to see,
even with an electron
microscope, but we know they
must be there because that's
the only way we can explain
how atoms behave.
O To give you an idea how small
a proton is, if an atom was
the size of a football stadium,
then a proton would still be
smaller than a marble.
O They are made of sub-atomic
particles called quarks
Protons
O Protons make the
nucleus, or center, of
an atom and have a
positive charge.
Neutrons
O Neutrons are the particles in
O
O
O
O
O
an atom that have a neutral
charge.
They aren't positive like
protons.
They aren't negative like
electrons.
But don't start thinking that
they aren't important.
Every piece of an atom has
huge importance to the way
the atom acts and behaves.
Neutrons are no exception.
Neutrons
O Neutrons are part of
the nucleus, or
center, of an atom
and have no charge
(neutral)
Particle Masses
O Although electrons may
balance out protons
charge for charge
(negative to positive),
they can’t compare when
it comes to mass.
O It takes 2,000 electrons
to equal the mass of just
one proton.
O On the other hand,
protons and neutrons are
about equal in mass
Atomic Mass and Number
EQ: How is the periodic table organized?
Size and Scale of Atoms
O While atoms are
composed of
electrons, protons
and neutrons, they
are mostly empty
space.
Size and Scale of Atoms
O To give you an idea of
the scale of an atom,
picture an empty
baseball stadium, like
Turner Field
O If the nucleus (the size
of a pencil eraser) is
on the pitcher’s
mound, where would
the electrons be?
Size and Scale of Atoms
O In the top row of seats!
Size and Scale of Atoms
O Atoms are amazingly
small.
O The tiniest speck of
dust may contain 10
billion atoms.
O Today’s powerful w
microscopes can give a
glimpse of atoms, but
do not yet show the
structure of atoms and
how they work
Atomic Number
O Every atom of an
element has the same
number of protons.
O For example, the
nucleus of every
hydrogen atom has 1
proton
O Every nitrogen atom
has 7 protons
O Every potassium atom
has 19 protons
Atomic Number
O This unique number of
O
O
O
O
O
protons in the nucleus of
an atom is the element’s
atomic number.
The atomic number
identifies the element.
Carbon’s atomic number is
6
Oxygen’s atomic number is
8
Iron’s atomic number is
26
Atomic number = number
of protons in the nucleus
of an atom
Isotopes and Mass Number
O Carbon-12, 13,14
O Atoms with the same
number of protons,
but a different
number of neutrons
are called isotopes.
Isotopes and Mass Number
O Although all atoms of
an element have the
same number of
protons, their
number of neutrons
may vary.
Isotopes and Mass Number
O An isotope is identified by
its mass number.
O The mass number is the
sum of protons and
neutrons in the nucleus
of an atom
O Although the atoms that
make up an element
may have different mass
numbers, the atoms all
react the same way
chemically.
Electrons
O Electrons are always
moving. They spin very
quickly around the
nucleus of an atom.
O As the electrons zip
around, they can move
in any direction, as
long as they stay in
their shell.