Counting Atoms - Shanksville
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Transcript Counting Atoms - Shanksville
Chapter 3.3
COUNTING ATOMS
Objectives
1. Explain what isotopes are
2. Define atomic number and mass number,
and describe how they apply to isotopes.
3. Given the identity of a nuclide, determine its
number of protons, neutrons, and electrons.
4. Define mole in terms of Avogadro’s number,
and define molar mass.
5. Solve problems involving mass in grams,
amount in moles, and number of atoms of
an element
Atomic Number
Symbol – Z
Number of protons in nucleus of atom
Determines the identity of the atom!!!!!
C - 6
2
He - ___
9
F - ___
82
Pb - ___
Use periodic table to find these!
Since atoms are neutral atomic number is also
number of electrons
So C also has 6 electrons
2 electrons
He - ____
Isotopes
All Hydrogen atoms contain 1 proton
Many naturally occurring elements can
contain different numbers of neutrons.
Isotope – at0ms of the same element that
have different masses.
n
Protium
99.985 %
Deuterium
0.015 %
n
n
Tritium
Trace
Three isotopes of Hydrogen
Mass Number
Mass number – total number of
protons and neutrons in the
nucleus of an isotope
Symbol – A
Protium has 1 proton and 0
neutrons
Mass number = #p + #n
=1 + 0
= 1
What is the mass number deuterium and
tritium?
Find on periodic table
Designating Isotopes
Hyphen Notation
Name of atom – mass number
Uranium - 235
Nuclear symbol
A
X
A : mass number, Z : atomic number, X : symbol of element
Z
235
U
92
Number of protons
92
Neutrons
Electrons
143
92
Nuclide – general term for any isotope of any
element
Sample Problem A
How many protons, electrons, and neutrons
are there in an atom of chlorine-37?
atomic number = number of protons = number of
electrons
mass number = number of neutrons + number of
protons
Atomic number from
# protons
# electrons
#neutrons
17
periodic table
17
# protons = # electrons
20
Mass # = # protons + # electrons
Practice Pg. 80
1. How many protons, electrons, and neutrons
make up an atom of bromine-80?
Answer : 35 protons, 35 electrons, 45 neutrons
2. Write the nuclear symbol for carbon-13.
Answer :
13
6
C
3. Write the hyphen notation for the isotope
with 15 electrons, and 15 neutrons.
Answer : phosphorus – 30
Relative Atomic Mass
Oxygen – 16 , mass of 2.657 x 10-23 g
Easier to use relative scale
Need arbitrarily chosen standard
All others are compared to carbon - 12
One atomic mass unit (1 amu) – exactly 1/12 the
mass of a carbon – 12 atom.
All others are compared to carbon -12
Hydrogen – 1 , about 1/12 the mass of carbon -12
Precise value is 1.007825 amu
Magnesium – 24 , Slightly less than twice that of
carbon -12
• Precise value is 23.985042 amu
Average Atomic Mass of Elements
Most elements occur naturally as mixtures of
isotopes
Average atomic mass : weighted average of
the atomic masses of the naturally occurring
isotopes of an element
Ex. of weighted average
Box contains two types of marbles, 25% have
mass of 2.00g, and 75% have a mass of 3.00 g.
25 marbles x 2.00 g = 50 g
75 marbles x 3.oo g = 225 g
so 50 g + 225 g = 275 g (TOTAL MASS)
275 g ÷ 100 =
2.75 g average marble mass
Calculating average atomic mass
Or
(2.00 g x 0.25) + (3.00 g x .75) = 2.75 g
Calculate the average atomic mass of copper
Use table 3-4 pg. 80 in text
Copper - 63
Copper – 65
69.17% with mass of 62.929599 amu
30.83% with mass of 64.927793 amu
(0.6917 x 62.929599 amu) + (0.3083 x 64.927793 amu)
=63.55
Match with atomic mass on periodic table
Round to two decimal places in calculations
Visual Concept – Click Here
Relating Mass to Numbers of Atoms
The MOLE
SI base unit for amount of a substance
Abbreviated as mol
Defined as : amount of substance that contains as
many particles as there are atoms in exactly 12 g
of carbon – 12
Counting unit just like a dozen
We don’t order 12 or 24 eggs, we order 1 or 2 dozen
In same way, a chemist may want 1 mol of carbon or
2 mol of iron.
Visual Concept – Click Here
63.55 g of copper
or 1 mol of copper
Avogadro’s Number
The number of particles in a mole –
AVOGADRO’S NUMBER
6.022 x 1023 particles in exactly one mole of a
pure substance
How big is this number?
If every person on earth (5 billion people)
counted 1 atom per second, it would take 4
million years to count all the atoms.
Visual Concept – Click Here
Molar Mass
Molar mass – mass of one mole of a pure
substance
Units : g/mol
Numerically equal to atomic mass of element
Mass of 6.02 x 1023 atoms of element
One mole He 6.02 x 1023 atoms
One mole Li
6.02 x 1023 atoms
One mole Cu 6.02 x 1023 atoms
One mole Fe 6.02 x 1023 atoms
Visual Concept – Click Here
4.oo g
6.94 g
63.5o g
55.78 g
Gram/Mole Conversions
Mass
In grams
Molar
mass
Moles
In mol
Avogadro’s
Number
Atoms
In atoms
Sample Problem B
What is the mass in grams of 3.50 mol of the element
copper, Cu?
63.55 g Cu
3.30 mol Cu x
= 222 g Cu
1 mol Cu
Step 1:
Write the
given value
Step 2:
Step 3:
Use molar mass
Cancel out units
from periodic
table
Always round to 2 places
after the decimal
Step 4:
Calculate
Sample Problem C
A chemist produced 11.9 g of Aluminum, Al. How many
moles of aluminum were produced?
11.9 g Al x
1 mol Al
26.98 g Al
= 0.441 mol Al
Sample Problem D
How many moles of silver, Ag, are in 3.01 x 1023 atoms of
silver?
3.01 x 1023 atoms Ag x
1 mol Ag
6.02 x 1023 atoms Ag
Avogadro’s number
= 0.500 mol Ag
Sample Problem E
What is the mass in grams of 1.20 x 108 atoms of copper,
Cu?
1.20 x 108atoms Cu x
1 mol Cu
x
6.02 x 1023 atoms Cu
63.55 g Cu
1 mol Cu
= 1.27 x 10-14 g Cu
Avogadro’s #
****Requires 2 conversions
Molar mass
The Ten Commolements
(found at http://members.tripod.com/~smalls42/commolements.html)
1. Thou shalt not use 6.02 x 1023 in vain.
2. Thou shalt not use the term mole if thou has no true knowledge of
the term mole.
3. Thou shalt not kill a mole.
4. Thou shalt not covet your neighbor's mole.
5. Thou shall always remember to celebrate Mole Day.
6. Thou shalt not disparage Mole Day.
7. Thou shalt not use a mole out of season.
8. Thou shalt always honor the one who introduced thou to Mole Day.
9. You shalt always keep sacred 10/23.
10. Thou shalt always remember these commolments or thou will never
properly celebrate Mole Day.