Atomic mass

Report
Notes #5
I. Parts of an atom
A.
B.
C.
Protons – positive charge
Neutrons – no charge
Electrons – negative charge
1. in constant motion around
nucleus of atom; travel in
orbitals, or shells
II. Determining # of each subatomic
particle
Atomic number - # of protons in the
nucleus of an atom
 Atomic mass - # of protons and neutrons
in the nucleus
21
 Example for Scandium atom

Sc
45.0
atomic # = # of p+
= ___
# p+ = # e= ___
atomic mass – atomic # = n0 = ___
II. Determining # of sub. Part.
• Atomic number - # of protons in the
nucleus of an atom
• Atomic mass - # of protons and
neutrons in the nucleus
21
• Example for Sc atom
Sc
45.0
atomic # = # of p+
= 21
# p+ = # e= 21
atomic mass – atomic # = n0 = 24
III. Atomic Number
Identity of each element is determined
by the number of protons in the
nucleus.
 In a neutral atom, the number of
electrons equals the number of protons.
 Practice problems pg. 99

IV. Isotopes
A. Atoms of the same element having
different atomic masses.
B. Atoms of the same element having
different numbers of neutrons.
 Hydrogen has three isotopes:
Protium – 0 neutrons
Deuterium – 1 neutron
Tritium – 2 neutrons
IV. Isotopes
C. Naming
It is often important to identify one
isotope from another. This is done in
two ways.
1. Hyphen notation. This gives the
element name (which you can use to
determine the atomic number) followed
by the atomic mass which you can use
to determine the number of neutrons.
 Hydrogen – 2

2. Nuclide or nuclear symbol notation
Atomic mass
Atomic number
4
2
He
Practice problems find p+, e-, n0
1. Symbol notation
107
Ag
2. Hyphen notation
Potassium – 39
Potassium – 40
Potassium – 41
Solve practice problems pg. 99,101 #’s1114. For #14 add on hyphen notation
and symbol notation
V. Average Atomic Mass
3.An average sample of hydrogen in
nature contains 99.985% protium,
0.015% deuterium and a miniscule
amount of tritium.
 Protium has an atomic mass of
1.007825 amu.
 Deuterium has an atomic mass of
2.014102 amu.
Average Atomic Mass
 Multiply
the masses of each
isotope by its percentage. These
are weighted values.
 Add all of these values together.
(Do NOT divide by the number of
isotopes. This is unnecessary.)
Average Atomic Mass
1.007825 * .99985 = 1.0076738 amu protium
.00015 * 2.014102 = 0.0003021 amu deuterium
1.0076738 + 0.0003021 = 1.00798 amu
 1.00798
amu is a weighted average
because most of the hydrogen is
protium.
Average Atomic Mass
1.00798 amu is a weighted average because
most of the hydrogen is protium.
 Multiply the masses of each isotope by its
percentage. These are weighted values.
 Add all of these values together. (Do NOT
divide by the number of isotopes. This is
unnecessary.)

Average Atomic Mass

The normal way to determine the
average mass of something would be to
add the two masses and simply divide
by two.
Average Atomic Mass

The normal way to determine the
average mass of something would be to
add the two masses and simply divide
by two.
1.007825 + 2.014102 = 3.021927
3.071927 / 2 = 1.5109635 amu
Sample Problem
4. What is the atomic mass of silicon if 92.21%
of its atoms have mass 27.977u, 4.70%
have mass 28.976 u and 3.09% have mass
29.974?
 28.1 u
 .9221 x 27.997 = 25.82 u
 .0470 x 28.976 = 1.36 u
 .0309 x 29.974 = 0.926 u
Sample #2
5. Neon has two isotopes. Neon-20 has a mass
of 19.992 u and neon-22 has a mass of
21.991 u. In any sample of 100 neon atoms,
90 will be neon-20 and 10 will be neon-22.
Calculate the average atomic mass of neon.
 20.192 u
 (.90 x 19.992) + (.10 x 21.991)
 17.9928 + 2.1991

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