Download Isotopes and Atomic Mass Unit

Isotopes and Atomic Mass Unit
Isotopes
• All atoms of a certain element will
always contain the same number of
protons, however the number of
neutrons can vary.
• Isotopes are atoms that have the same
number of protons, but different
numbers of neutrons.
235
92
U
238
92
U
2.3
Do You Understand Isotopes?
How many protons, neutrons, and electrons are in?
14
6C
6 protons, 8 (14 - 6) neutrons, 6 electrons
How many protons, neutrons, and electrons are
11
in? 6 C
6 protons, 5 (11 - 6) neutrons, 6 electrons
2.3
Isotopes
• Carbon-12 has 6 protons and 6
neutrons. Carbon-14 has 6 protons and
8 neutrons.
• The atomic mass is an average of the
isotopes so is not always a whole
number.
• Isotopes are useful in determining age
of fossils, diets of animals, and origins
of wood.
Atomic Mass Unit
• Scientists have chosen the most common
isotope of carbon (carbon-12) as a
reference.
Atomic Mass Unit
• An atom of carbon-12 is assigned a mass
of 12 atomic mass units (amu).
• The masses of all other atoms are
compared with this mass.
• Let’s calculate atomic mass using carbon12
1. An isotope of neon has a mass which is 3/2 the mass of an atom of
carbon-12. What is the atomic mass of this neon isotope?
2. An atom of sodium has a mass which is 1.92 times the mass of a carbon12 atom. What is the atomic mass of sodium?
3. An atom of argon has a mass which is 10/3 the mass of an atom of
carbon-12. What is the atomic mass of argon?
4. What is the atomic mass of an atom that is 7/3 the mass of a carbon-12
atom? What element is this?
5. A copper atom has a mass which is 5.29 times the mass of a carbon-12
atom. What is the atomic mass of copper?
6. An atom of helium has 1/3 the mass of an atom of carbon-12. What is
the atomic mass of helium?
7. What is the mass of an atom which has a mass 7/6 times as much as
that of a carbon-12 atom?
8. What is the mass of an atom that has 4/13 the mass of a chromium
atom? The mass of a chromium atom is 13/3 times the mass of an atom
of carbon-12.
9. What is the mass of an atom which has a mass 4/5 the mass of a
calcium atom? The mass of a calcium atom is 10/3 the mass of an atom
of carbon-12.
• One atomic mass unit is 1/12 the mass
of a carbon-12 atom.
• The mass of an atom expressed in atomic
mass units is called its atomic mass.
Average Atomic Mass
• The atomic mass of an element
found in the periodic table is
actually calculated from the masses
of the element’s isotopes.
• It is a weighted average of the
masses of the isotopes – you must
take into consideration the percent
abundance of each element.
• Example:
• There are two isotopes for lithium:
• Lithium-6 makes up 7.4% of all lithium
atoms
• Lithium-7 makes up the other 92.6%.
• Calculate the average atomic mass.
• SOLUTION:
(6amu x 0.074) + (7amu x
0.926) = 0.44 + 6.482
= 6.922 = 6.9amu
Now it’s
your
turn……
• Natural carbon consists of:
– 98.89% carbon-12 (mass 12.000 0 u) and
– 1.11% carbon-13 (actual mass 13.003 0u).
• What is the average atomic mass of
natural carbon?