• Are atoms of the same element that have
different masses. Due to a different number
of neutrons in the nucleus.
• Is a general term for any isotope of any
• Represented in mathematical equations by
the letter Z in the book examples.
• Remember. The number of protons is
represented by the atomic number, and will
never change. It is the “fingerprint” of the
• Is the total number of protons and neutrons
in the nucleus of an isotope.
• The neutron and the proton mass values are
each approx 1.
• The periodic table reports the average
atomic mass of the many nuclides of a
Atomic mass unit
• The mass of atoms is extremely small.
• Therefore, we use relative atomic masses
because it is easier to do calculations.
• Arbitrarily chose a standard and assigned a
relative mass value.
• Selected the carbon –12 nuclide.
Atomic mass unit or amu
• Carbon -12 has been assigned the relative
value of 12 amu.
• Or 1 amu is exactly 1/12th the mass of a
carbon-12 atom ( nuclide)
• Mass of any nuclide is compared to the
carbon –12 nuclide
• An atom that is 4x the mass of the carbon12 atom will be 4 x 12 amu = 48 amu.
• An atom that is 2/3 the mass of the carbon12 atom will have a mass of 2/3 x 12 amu =
Finding average atomic mass
• Multiply the abundance factor, (%), by the
mass number of the isotope
• Add up all the isotopes
• Divide by 100
• Neon has two isotopes Ne-20 has a mass of
19.992 amu and occurs in nature 90% of the
time. Ne-22 has a mass number of 21.991
amu and occurs 10% of the time.
• ((90 x 19.992) + (10 X 21.991) ) / 100
= 20.192 amu
How many in a dozen?
• How many in 2 dozen
• If you have 36 shoes, how many dozen?
• The chemist dozen
• Instead of 12 units of something it is
6.02 x10 23 units of something.
• 6.02 x 1023 is called Avogadro’s number
– Or the number of particles in exactly one mole
of a substance.
• The mass of one mole of a substance.
• Units are g/mol
• Numerically equal to the atomic mass listed
on the periodic table.
• The molar mass of an element contains one
mole, or 6.02 x1023 atoms of that element.
• He has a molar mass of 4.00 g/mol
• How many grams of He are there in 5
• 4.00 g/mol x 5 mol = 20 grams
• Note: the mole unit crosses out.
• How many grams will there be in 4 moles
• How many grams are there in 4.5 moles of
• Page 83 sample problem 3-2
Now the other way
• Given the grams calculate the moles.
• You have 72 grams of copper, how many
moles do you have?
• 72 grams / 63.55 g/mol = 1.13 moles of Cu
• Page 83 sample problem 3-3 1,2
How many moles is a given # of
• If you have 2.0 x 10 23 atoms of Ca, how
many moles do you have.
• 2.0 x 1023 atom Ca x 1 mol Ca/6.02x 1023
atom of Ca = 0.33 mol Ca
• Page 84 sample problem 3-4 1,2,3
Given # atoms find the mass
• Find the mass of 5 x 10 16 atoms of copper.
• 5 x 10 16 atoms Cu x 1mole/6.02x10 23atoms
multiplied by 63.55 g/mol = 1.31 x 10-9
• Page 85 practice problem 3-5 1,2,3