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Drill – What do you know?
1. Write your definition of an ATOM.
2. What is inside an atom?
3. What determines the identity of an atom?
The Atom
What is an atom?
• We know that an element is a substance
that cannot be separated into simpler
substances…
• An atom is the smallest unit
of an element that maintains
the chemical properties of
that element.
Gold = Element
Atoms of Gold
How little is an atom???
• If you expand a hydrogen atom to the size
of an apple, the apple would expand to the
size of the Earth.
is to
as
is to
How little is an atom???
• A human hair is about 1 million
carbon atoms wide.
• A speck of dust contains about 3 trillion
atoms.
Atoms
of Gold
Gold = Element
Each
Individual
Gold Atom
• The nucleus is a very small region in
the center of the atom that is made up
of protons and neutrons
• Surrounding the nucleus are
electrons
Inside the atom
• Subatomic Particles:
Particle
Charge
Mass
(kg)
Relative
Mass (amu)
Proton
Positive
1.7x10-27
1
Neutron
Neutral
1.7x10-27
1
Electron
Negative
9.1x10-31
0
Protons:
An atom’s identity is determined by the
number of protons. If you change the number
of protons, you change the element.
Neutrons:
Neutrons maintain stability. If you change the
number of neutrons, you have an ISOTOPE.
Electrons:
Electrons determine an atom’s behavior. If
you change the number of electrons, then
you have an ION.
What the Periodic Table tells us!
Atomic Number (Z):
3
Li
Lithium
6.941
The number of protons
An atom, which is neutral, will have
an equal number of protons and
electrons…so that the positive and
negative cancel out
(not true for ions)
What the Periodic Table tells us!
3
Li
Lithium
6.941
Average Atomic Mass:
The weighted average
of the atomic masses of
the naturally occurring
isotopes of the element
The Average Atomic Mass rounded
to a whole number is the MASS
NUMBER
Because protons and neutrons are much larger than electrons,
they are the only 2 particles that impact the mass of the atom.
Therefore:
Mass Number = # of protons + # of neutrons
# of neutrons = Mass Number - # of protons
What the Periodic Table tells us!
Average Atomic Mass:
The weighted average of the atomic masses of the naturally occurring
isotopes of the element
Isotopes are atoms of the same element
that have different masses (ie same
protons, different neutrons)
Isotopes are written using the name of the
element followed by a hyphen and then the
mass number. Ex: Hydrogen-1, Hydrogen-2, &
Hydrogen-3 are isotopes
What is a weighted average?
• Mrs. Tang’s English Class:
– 20% Homework
– 45% Test
– 35% Essay
If Sue got a 94% on Homework, a 82% on
Tests, and a 89% on Essays, what is her
English grade?
What is a weighted average?
• Sue’s English Grade:
Homework: (.20)(.94) = 0.19
Test: (.45)(.82) = 0.37
Essay: (.35)(.89) = 0.31
Add all three up and you get:
0.19 + 0.37 + 0.31 = 0.87 = 87%
Now let’s do an actual example:
• Copper consists of 69.15% copper-63 and
30.85% copper-65. Calculate the Average
Atomic Mass of copper?
Electrons
• If electrons are added or removed, we
ION This is indicated with a + or –
have an ____.
charge.
NEGATIVELY
• Since electrons are ____________
charged,
if we have extra electrons the atom will be
NEGATIVE
_____________.
If the atom has lost an
POSITIVELY
electron, the atom will be _____________
charged.
Ca2+
Cl-
O-2