1. Chapter 3.3
Counting Atoms

2. Objectives Explain what isotopes are
Define atomic number and mass number, and describe how they apply to isotopes.
Given the identity of a nuclide, determine its number of protons, neutrons, and electrons.
Define mole in terms of Avogadro’s number, and define molar mass.
Solve problems involving mass in grams, amount in moles, and number of atoms of an element

3. Atomic Number Symbol – Z Number of protons in nucleus of atom
Determines the identity of the atom!!!!!
C - 6
He - ___
F - ___
Pb - ___
Use periodic table to find these!
Since atoms are neutral atomic number is also number of electrons
So C also has 6 electrons
He - ____ electrons 2 9 82 2

4. 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 n n
Protium %
Deuterium
0.015 %
Tritium
Trace
Three isotopes of Hydrogen

5. 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
What is the mass number deuterium and tritium?
Find on periodic table

6. 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
235U Z 92
Number of protons Neutrons Electrons 92
143 92
Nuclide – general term for any isotope of any element

7. 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
# protons
# electrons
#neutrons
Atomic number from periodic table 17 17
# protons = # electrons 20
Mass # = # protons + # electrons

8. Practice Pg. 80
How many protons, electrons, and neutrons make up an atom of bromine-80?
Write the nuclear symbol for carbon-13.
Write the hyphen notation for the isotope with 15 electrons, and 15 neutrons.
Answer : 35 protons, 35 electrons, 45 neutrons
Answer : 13 C 6
Answer : phosphorus – 30

9. 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 amu
Magnesium – 24 , Slightly less than twice that of carbon -12
Precise value is amu

10. 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 g = 275 g (TOTAL MASS)
275 g ÷ 100 = g average marble mass

11. Calculating average atomic massOr
(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 % with mass of amu
Copper – % with mass of amu
( x amu) + ( x amu)
=63.55
Match with atomic mass on periodic table
Round to two decimal places in calculations
Visual Concept – Click Here

12. 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.
63.55 g of copper or 1 mol of copper
Visual Concept – Click Here

13. 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

14. 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 x 1023 atoms oo g
One mole Li x 1023 atoms g
One mole Cu x 1023 atoms o g
One mole Fe x 1023 atoms g
Visual Concept – Click Here

15. Gram/Mole Conversions
Mass
In grams
Moles
In mol
Atoms
In atoms
Avogadro’s Number
Molar mass
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:
Step 2:
Step 3:
Step 4:
Write the given value
Use molar mass from periodic table
Cancel out units
Calculate
Always round to 2 places after the decimal

16. Sample Problem C Sample Problem D
A chemist produced 11.9 g of Aluminum, Al. How many moles of aluminum were produced?
1 mol Al
11.9 g Al x =
0.441 mol Al
26.98 g Al
Sample Problem D
How many moles of silver, Ag, are in 3.01 x 1023 atoms of silver?
1 mol Ag
3.01 x 1023 atoms Ag x =
0.500 mol Ag
6.02 x 1023 atoms Ag
Avogadro’s number

17. Sample Problem E
What is the mass in grams of 1.20 x 108 atoms of copper, Cu?
63.55 g Cu
1 mol Cu x x
1.20 x 108atoms Cu
6.02 x 1023 atoms Cu
1 mol Cu =
1.27 x g Cu
Avogadro’s #
****Requires 2 conversions
Molar mass

18. The Ten Commolements
(found at
Thou shalt not use 6.02 x 1023 in vain.
Thou shalt not use the term mole if thou has no true knowledge of the term mole.
Thou shalt not kill a mole.
Thou shalt not covet your neighbor's mole.
Thou shall always remember to celebrate Mole Day.
Thou shalt not disparage Mole Day.
Thou shalt not use a mole out of season.
Thou shalt always honor the one who introduced thou to Mole Day.
You shalt always keep sacred 10/23.
Thou shalt always remember these commolments or thou will never properly celebrate Mole Day.