Download Atomic Mass

Lesson 12
What is the difference
between atomic mass and
atomic number?
Atoms of different elements
have different numbers of
protons and electrons.
When scientists talk about
different kinds of matter, they
often refer to the matter by its
atomic number.
The atomic number is the
number of protons in the atom.
What is the atomic number of
this atom?
Yellow = neutron
Blue =
electron
Red = proton
4 protons = Atomic number of 4
Scientists also describe atoms
by their atomic mass.
Scientists do not measure the
mass of atoms in grams or
ounces.
They use Atomic Mass Units
or a.m.u.
Each proton has a mass of
1 a.m.u.
Each neutron has a mass of
1 a.m.u.
So………………..
The Atomic Mass of an atom
is the number of protons plus
the number of neutrons in the
atom.
What is the atomic mass of
this?
Blue =
neutrons
Yellow =
electrons
Red = protons
Atomic mass = protons +
neutrons.
3+2=5
Atomic mass = 5
Can you figure out why
scientists don’t count electrons
in the atomic mass?
Think back to what you know
about electrons.
Anyone????
Electrons are very, very light.
They have almost no mass.
Therefore, scientists don’t
count them in the atomic mass.
Sometimes, two atoms of the
same kind of matter do not
have the same atomic mass.
How in the world is that
possible???
They have a different number
of neutrons.
Atoms of the same type of
matter always have the same
number of protons.
When the number of neutrons
is different, it makes the
atomic mass different.
The atoms still have the same
atomic number. (the number
of protons is the same)
Isotopes are atoms of the
same kind of matter that have
different number of neutrons.