Download Chemistry I Honors

Chemistry I Honors
Atomic Theory
The Atomic Mass Unit
The Size of an Atom
• Atoms are small.
• The diameter of a typical atom is between 3 to
5 Angstroms across.
• To help with that unit:
1 Angstrom is 0.00000001 centimeters
That would be one one-hundred millionth of a
centimeter.
(really small)
The Mass of Atoms
• This is even more ridiculous…
• The mass of a “typical” atom is around
1 x 10-23 grams.
• That would be:
0.00000000000000000000001 grams
(Just for understanding – a dollar bill has a mass
of 1 gram)
(really light too)
A Better Way to Describe the Mass
• Since the actual mass
(in grams) of atoms is
such an obnoxious
number, Chemistry
decided to invent a new
measurement to
describe the mass of
individual atoms.
• Step 1: The actual mass
of a carbon-12 atom
was determined. (it
gave them that
ridiculous 10-23 value)
• Step 2: They divided
that actual mass by
“12” (because it was a
carbon-12 isotope).
Continuing…
• Your math skills tell you
that taking a 10-23
number and dividing by
12 will give you an even
more ridiculous
number. (It does – the
result is something like
10-24 )
• So how does this help??
• This is the new part –
Chemistry simply (??)
defined the result of the
division as
1 Atomic Mass Unit
(1 amu).
• This now gives us the
ability to describe the
masses of atoms in
terms of amu’s instead
of grams.
Picture it this way…
• The entire “pie” represents the carbon-12 atom (note the 12 “slices” – each
representing either a proton or an electron).
• Each “slice” has a mass of 1 amu.
• Since the entire “pie” (the entire atom) has 12 slices (protons and neutrons), the
mass of the entire carbon-12 atom is simply 12 amu.
What does that do for us?
• We can conclude that
each proton and each
neutron has a mass of 1
amu. (actually, the
neutrons are just a bit
heavier than the
protons – but the
difference is not
significant.
• Since all of the mass of
an atom is centered in
the nucleus (the
electrons are regarded
as having a mass that is
essentially zero), the
mass number of an
isotope is the mass of
that atom in amu’s.