Download average atomic mass

Bellwork Friday, 10/9
Find your notes on isotopes.
At the top of pg. 2: Fill in the following…
Finish this and we’ll check it.
How many protons, neutrons, and electrons?
Draw good nuclide symbols.
6 protons, 8 neutrons, 6 electrons
6 protons, 5 neutrons, 6 electrons
Let’s check…
The only stable isotope of naturally occurring
fluorine has a mass number of 19.
How many p+, n0 and e- are in an atom of F?
Write the isotope symbol.
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19
F
9
4
Word Wall
Isotope
Atomic number
Mass number
Nuclide
Chapter 3
Calculating average atomic mass
In chemistry there are many different
concepts of mass…
We need to talk about one of them.
Atomic mass
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Please don’t panic…
I like to rewrite my notes from my slides. I think it helps
you later when you study. (Y’all are all studying every
night, right??)
Also… there are different terms for things
and it depends on the textbook, the
professor…
and it gets confusing when there are truly
different types of mass…
I’ll do my best to keep it clear for you.
But promise not to panic on me.
Promise?!
Isotopes have mass numbers
Isotopes are atoms of the same element (the same
number of p+ and e-) but with different masses due to
having different numbers of n0.
Right?
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Question of the day:
What’s the difference between atomic mass (on
the periodic table) and mass number?
Today we get to work it out!
We know that the mass of an atom is
very, very, very, very small.
The mass of an atom is incredibly small and it
doesn’t make sense to use units like grams or
even picograms to measure it…
It’s easier to measure it in atomic mass units
(amu)
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Scientists like a standard…
The chosen isotope is carbon-12.
It was assigned a mass of exactly 12 atomic mass
units. (This is handy because it has 6 p+ and 6 n0.)
Sooooo…
An amu is defined as one-twelfth the mass of a
carbon-12 atom.
(our textbook uses just plain “u”)
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Scientists use a reference isotope for these units.
This means amu is a relative measurement. (relative
to carbon-12).
Average atomic mass
This is the number you see on the periodic table.
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Average atomic mass.
The average atomic mass of an element is the
weighted average of the masses of its isotopes on
this scale…
Remember relative abundance?
Think about “weighted” like your grades: 40%
formative and 60% summative.
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The average atomic mass of an element depends
of both the mass and relative abundance of each
of the element’s isotopes.
Most elements occur in nature
as a specific mixture of
isotopes. They don’t occur
equally… some are more
abundant than others.
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The average atomic mass is the weighted
average of all of the naturally occurring
isotopes of the element.
Percent
Atomic Mass,
Isotope Abundance
amu
12C
98.90%
12.00000
13C
1.10%
13.00335
• Elements rarely occur as only one isotope.
• They exist as mixtures of different isotopes of
various masses.
• Using average atomic mass means the less
common isotopes are accounted for.
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Don’t confuse mass number with average
atomic mass.
Mass number is the mass of one particular atom
(isotope).
Average atomic mass is the average mass of a
group of atoms of the same element that takes
into consideration all the isotopes.
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Go back and answer the
question at the
beginning of your notes.
Make sure you both
understand this concept.
How do we calculate
the average atomic mass?
The average atomic mass of an element
depends of both the mass and relative
abundance (percent abundance) of each of the
element’s isotopes.
Let’s look at a problem.
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Copper consists of 69.15% copper-63, which has an
atomic mass of 62.929 601 amu, and 30.85% copper65, which has an atomic mass of 64.927 794 amu.
Calculate the average atomic mass for Cu.
Breaking down the calculation:
Step 1: multiply the atomic mass of each
isotope by its relative abundance.
Step 2: add the results.
0.6915 x 62.929 amu + 0.3085 x 64.927 794 amu
= 43.52 amu + 20.03 amu
= 63.55 amu
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Here’s another one!
Oxygen has three naturally occurring isotopes in
the following proportions:
O-16 99.762 % (15.99491 amu);
O-17 0.038000% (16.99913 amu);
O-18 0.20000% (17.99916 amu).
What is the average atomic mass of oxygen?
Did you get it?
1) .99762 x 15.99491 =
2) .00038000 x 16.99914 =
3) .0020000 x 17.999 =
15.957
0.0064597
.035998
15.999 amu
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More problems for practice!
Textbook, pg. 117
Look at the sample problem then work
problems #1 and 2.
Bellwork 10/13
Calculate the atomic mass of bromine. The two
isotopes of bromine have atomic masses and
relative abundance of 78.92 amu (50.69%) and
80.92 amu (49.31%).
Bullseye comparison
Isotope
Atom
Atomic mass
Electron
Mass number
Nucleus
Atomic number
Periodic table
Proton
neutron
Activity Time!
The atomic mass of candium