Transcript How Atoms Differ

Atomic Number (Z)
Identity of each element is determined
by the number of protons in the nucleus.
 The number of protons in the nucleus is
the atomic number.
 In a normal atom, the number of
electrons equals the number of protons.

Atomic Mass (A)

…is equal to the number of protons and
the number of neutrons in the nucleus.
Isotopes
1.
2.

Atoms of the same element having
different atomic masses.
Atoms of the same element having
different numbers of neutrons.
Hydrogen has three isotopes:
Protium – 0 neutrons
Deuterium – 1 neutron
Tritium – 2 neutrons
Isotopes - Naming



It is often important to identify one
isotope from another. This is done in
two ways.
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
Isotopes - Naming


It is often important to identify one
isotope from another. This is done in
two ways.
Nuclide or nuclear symbol.
Atomic mass (A)
Atomic number (Z)
4
2
He
Some Important Isotopes
Carbon – 14
 Carbon - 12

Average Atomic Mass
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
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
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
