Transcript The Atom

ISOTOPES
Chemistry
I. Why Atoms Differ?
A. Atoms of different elements have
their own unique structures
1. Because they have different
structures they have different
properties
2. An atom of hydrogen in a water
molecule is very different from an
atom of sodium in salt.
B. How we know atoms are different
1. Each element has a different number
of protons
2. The number of protons is called the
atomic number
3. Since all atoms are neutral – the
atomic number also means
the number of electrons
C. The subatomic particle that is different is
the neutrons
1. Since each element has a
specific number of protons
(which is the same as the
atomic number) then any
change in atomic mass is caused by
the change in the number of
neutrons
• Neutrons exist to stabilize the
nucleus – without them, the
nucleus would consist of nothing
but positively-charged protons in
close proximity to one another.
• Because there are different ways
of stabilizing the protons, there
are different isotopes.
4. Each element has a different
atomic number
5. Ex. Hydrogen has 1 proton and 1
electron = so the atomic
number is 1
6. Ex. Uranium has 92 protons = so
has an atomic number of 92
7. The atomic number for an element
NEVER changes
V. Isotopes
A. Isotopes are atoms of the same
element with different atomic mass
numbers.
Nuclear symbol:
Mass #
Atomic #
12
6
Hyphen notation: carbon -12
C
Isotopes
© Addison-Wesley Publishing Company, Inc.
B. Ex. of isotope
• Chlorine-37
–atomic #:
17
–mass #:
37
–# of protons:
17
–# of electrons:
17
–# of neutrons:
20
–Name:
37
17
Chlorine-37
Cl
III. Isotopes
A. Atoms of an element will always have
the same atomic number
1. But they do not always have the
same atomic mass number
2. They always have the same atomic
number because they always have the
same number of protons
3. But, atoms don’t always have the
same number of neutrons
B. Atoms that have the same
number of protons but a
different number of neutrons
are called isotopes
1. Different number of neutrons
give elements different
mass numbers
Ex. Hydrogen has a mass number of 1 on
average
But hydrogen also has two isotopes….
– So it also has a mass number of 2
– And a mass number of 3
1H
1
2H
1
3H
1
3. Although isotopes have different number
of neutrons these atoms still have very
similar properties of the element since
number of protons and electrons are the
same
4. But because the isotopes are slightly
different that changes their properties
slightly
5. Note – the actual mass of an atom as
measured by mass spectrometer is still
incredibly small
6. More useful to compare relative masses of
atoms using a reference isotope as standard
(carbon-12)
7. An atomic mass unit (amu) = 1/12 that of
carbon-12
8. The atomic mass unit listed in the periodic
table is usually the weighted average mass
of all isotopes of an element based on
percent / relative abundance (or on some
that are very unstable it is the most
common isotope)
Abundance
• A factor that is most common, occurs
naturally, most plentifully
Percent Abundance
• Percent abundance is parts per
hundred
• The individual percent abundances
add up to 100%
Relative Abundance
• Relative abundance is parts per one, or
the decimal form of the percent
• The individual relative abundances add
up to 1
• Relative abundance tells you the
decimal fraction of particles
Percent Abundance & Relative
Abundance
Weighted average best represents atomic
mass because it considers differences
in mass and abundance among the
particles (isotopes)
Useful Isotopes
Carbon - 14 for dating artifacts
Cobalt – 60 for cancer radiation
treatment
Iodine – 131 for treating thyroid
disorders
Americium – 241 for smoke alarms
IV. Finding the Number of Neutrons
A. To find the number of neutrons an atom has
1. First you have to know the number of protons –
that’s the atomic number
2. Then you have to know the atomic mass
number
3. When you subtract the atomic number from the
mass number rounded to nearest whole number
4. You get the number of neutrons
Mass Number = Atomic No. + No. of Neutrons
Mass Number = No. of Protons + No. of Neutrons
5. Ex. Chlorine 37
has an atomic number of 17
has a mass number of 37
so 37-17 = 20
this chlorine atom has 20 neutrons
Ex. Chlorine 35
has an atomic number of 17
has an atomic mass number of 35
35 – 17 = 18
this atom of chlorine has 18 neutrons
Review Key Points
• Define isotopes
• Describe characteristics of isotopes
• Give 3 examples of the uses of isotopes
Independent Practice
• Mass of Candium Lab – link
• Practice Sets –
– Isotopes 1
– Isotopes 2
link
link