Download Measuring the Atom

Measuring the Atom
The Atom
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All atoms are made up of subatomic particles
There are many subatomic particles, but we
will limit our discussion to protons, neutrons,
and electrons
Protons and neutrons are found in the
nucleus and are therefore called nucleons.
The electrons are found outside of the
nucleus (more on that in a month or so)
Isotopes
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Not all the atoms of an element are equal
They may differ in the number of neutrons in
the nucleus.
Example
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Carbon-12 – 6p+, 6n0, 6e- (6+6 = 12)
Carbon-14 – 6p+, 8n0, 6e- (6+8 = 14)
Weighing an Atom
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A proton weighs 1.673 x 10-24 g
A neutron weighs 1.675 x 10–24 g
An electron weighs 1.1 x 10-29 g
All of these masses are very small so
chemists needed a standard. They chose
Carbon-12. Why?
Atomic Mass Unit
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Chemists decided that Carbon-12 would
weigh 12 atomic mass units (u)
Making each neutron and proton about 1u
Average Atomic Mass
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If we look on the periodic table we will see
that atomic mass of Carbon is 12.01u
But didn’t we just say that Carbon was the
standard at 12u????
The standard was Carbon-12 not Carbon
(which is made up of all the isotopes: 12, 13,
and 14)
Calculating Average Atomic Mass
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Carbon is made up of 3 isotopes.
Carbon-12 is 98.9%
Carbon-13 is 1.1%
Carbon-14 is 0.0000000001%
To find the atomic mass we must take a weighted
average
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(12u x 98.9% + ~13u x 1.1% + ~14u x 1 x10-10) / 100%
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= 12.01u
Homework
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Text pg 82 #19, 25