Transcript electrons.
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