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Counting Atoms
Big Ideas: All atoms of an element must
have the same number of protons but not
neutrons, atomic mass is a relative measure,
average atomic mass is a weight value
Counting atoms
All atoms contain the same particles, but
atoms of different elements have different
numbers of protons.
Atomic number of an element is the number
of protons in each atom of that element.
Elements are identified using the atomic
number.
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What does a box on the periodic table tell us?
atomic number
symbol
mass number
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Isotopes
All H atoms have the same number of protons, but
can have different numbers of neutrons.
Example: H as we know it is made up of three types
of H atoms. Protium, deuterium and tritium. Each
type has the same number of protons but different
numbers of neutrons. Lets take a look at each
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Isotopes - are atoms of the same element
that have different masses.
Atomic
number
Number of Mass
neutron
number
1
o
1+0=1
Deuterium 1
1
1+1=2
Tritium
2
1+2=3
Protium
1
Counting atoms
In the last slide we looked at three types of H
atoms. Each had a different number of
neutrons and a different mass number.
Mass number is the total number protons and
neutrons that make up the nucleus of an
isotope.
There are two ways to ID an isotope of an
atom.
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hyphen notation: Hydrogen-1 (protium)
compared to Hydrogen-2 (deuterium)
nuclear symbol: 11H (protium) compared to
2 H (deuterium)
1
3 thing to remember:
Atomic number equals the number of protons
In neutral atoms the number of protons equals the
number of electrons
Mass number (p + n) minus the atomic number (p)
equals the number of neutrons.
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Nuclide is a general term for a specific
isotope of an element.
Lets work out a problem:
How many protons, electrons, and neutrons make up
an atom of bromine-80? (hint: 80 is the mass
number)
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Atomic mass is a relative measure
In order to set up a relative atomic scale, scientists
arbitrarily chose one atom as the standard and
assigned a mass value.
All other atoms masses are expressed in relation to
this standard. Explain using example on pg. 82 in
TE.
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The atom chosen was carbon-12, which has a mass of
exactly 12 unified atomic mass units or 12u.
Hydrogen-1 is about 1/12 that of carbon-12. It has a
mass of about 1u.
Magnesium-24 is 24/12 that of carbon-12. It has a mass
of about 2u.
The masses of subatomic particles (p, e, & n) can also be
expressed on the atomic mass scale.
Counting Atoms
Average atomic mass is a weighted value
Most elements occur in nature as a mixture of
isotopes.
We determine the average atomic mass of a sample
of an element's isotopes by determining the
percentage of each of the isotopes and then giving
the proper weight to each value.
Average atomic mass is the weighted average of the
atomic masses of the naturally occurring isotopes of
an element.
Counting Atoms
Average atomic mass is a weighted value
Example: Your grade is a weighted average. Test
weight - 30%, Lab weight 30%, Quiz weight - 30 %
and Classwork - 10%. If you Test weight is 80, Lab
weight is 90, Quiz weight is 85 and Classwork
weight is 70, what is your average? workout
w/student on board or docu camera.
Counting atoms
Formative Assessment
1.Determine the number of protons, electrons,
and neutrons in each of the following:
a.sodium-23
b.calcium-40
c. 6429Cu
2.To two decimal places, what is the relative
atomic mass of the element potassium?
Formative Assessment
3. Calculate the average atomic mass of argon
to two decimal places, give the following
relative atomic masses and abundances of
each of the isotopes: argon-36 (35.97u;
0.377%), argon-38 (37.96u; 0.063%), and
argon-40 (39.96u; 99.600%).