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14.1 Structure of the Atom
In order to understand atoms, we
need to understand the idea of
electric charge.
We know of two
different kinds of
electric charge and we
call them positive and
negative.
14.1 Electric charge in matter
We say an object is electrically neutral
when its total electric charge is zero.
14.1 An early model
In 1897 English physicist J.
J. Thomson discovered
that electricity passing
through a gas caused the
gas to give off particles
that were too small to be
atoms.
These negative particles
were eventually called
“electrons.”
14.1 The nuclear model
In 1911, Ernest
Rutherford, Hans
Geiger, and Ernest
Marsden did a clever
experiment to test
Thomson’s model.
We now know that
every atom has a tiny
nucleus, which
contains more than
99% of the atom’s
mass.
14.1 The Nucleus
The mass of the nucleus
determines the mass of
an atom because protons
and neutrons are much
larger and more massive
than electrons.
In fact, a proton is 1,836
times heavier than an
electron.
14.1 The Nucleus
Protons have a
positive(+) charge that is
equal and opposite to an
electron
Neutrons have no
charge(neutral), but have
the same mass as a
proton
14.1 The Nucleus
The atomic number of an
element gives the
number of protons in the
nucleus
The mass number of an
element gives the
number of protons and
neutrons in the nucleus
14.1 The Electron Cloud
This is where the
electron is found
Electrons have a
negative (-) charge
Electrons have a very
small mass compared to
protons
14.1 The Electron Cloud
Since atoms are
electrically neutral, the
atomic number of an
element also gives the
number of electrons in
the electron cloud
14.1 Force inside atoms
The Electromagnetic
Force:
 Electrons are bound
to the nucleus by the
attractive force
between electrons (-)
and protons (+).
14.1 Force inside atoms
What holds the
nucleus together?
There is another
force that is even
stronger than the
electric force.
We call it the strong
nuclear force.
14.1 Force inside atoms
The Strong Nuclear
Force is the glue that
holds the nucleus
together
14.1 Force inside atoms
Other Forces:
 Gravity
Too weak to really
matter
 Weak Force
Force inside of a
proton or neutron
14.1 How atoms of various
elements are different
No two elements have
the same nucleus
Every element has its
own Atomic Number!!
If you change the atomic
number you change the
element.
14.1 How atoms of various
elements are different
Isotopes are atoms of the
same element that have
different numbers of
neutrons.
How are these carbon
isotopes different?
The mass number of an
isotope tells you the
number of protons plus
the number of neutrons.
14.1 Radioactivity
Almost all elements have
one or more isotopes
that are stable.
“Stable” means the
nucleus stays together.
Carbon-14 is radioactive
because it has an
unstable nucleus.
14.1 Average Atomic Mass
Mass #  total
particles in the
nucleus (protons &
neutrons)
Atomic Mass  a
weighted average of
all stable isotopes
14.1 Average Atomic Mass
Mass # can be found
in most Periodic
Tables by rounding
the Average Atomic
Mass
14.1 Average Atomic Mass
Isotope Notation
helps to avoid
having to round
 Carbon—12
Can also be written
in this form
Solving Problems
How many neutrons are present in an
aluminum atom that has an atomic
number of 13 and a mass number of 27?
Solving Problems
1. Looking for:
 …number of neutrons in aluminum-27
2. Given
 … atomic no. = 13; mass no. = 27
3. Relationships:
 Periodic table says atomic no. = proton no.
 protons + neutrons = mass no.
4. Solution
 neutrons = mass no. – protons
 neutrons = 27 – 13 = 14