Counting Atoms
3.3
Counting Atoms
• Very difficult to count
• Atomic Number – # of p+ of each
atom of that element
• Whole numbers
• Elements arranged by atomic #
• Identifies the element
Isotopes
• Isotope – an atom of the same
element with different # of neutrons
• Elements are mix of isotopes
• Hydrogen has 3
• Mass Number – total # of protons
and neutrons that make up the
nucleus of an isotope
Designating Isotopes
•
•
•
•
•
•
Hyphen notation = Hydrogen – 3
238
Nuclear symbol- 92 U
92 is atomic # (# of protons)
# of n0 = mass # - atomic #
How many p+,n0,e- r in Cl-37, Br-80?
What’s the notation for an isotope
with 15 e- and 15 n0?
Relative Atomic Mass
• Atomic Mass Standard is C-12
• Atomic Mass Unit – (amu) exactly
1/12 the mass of a carbon-12 atom
• Masses are relative to C-12 atoms.
• All masses are averages of amu’s
b/c of isotopes
Average Atomic Mass
• AAM – the weighted average of the
atomic masses of the naturally
occurring isotopes of an element
• The mass of Cu-63 is 62.94 amu
and the mass of Cu-65 is 64.93
amu. The abundance of Cu-63 is
69.17% and Cu-65 is 30.83%.
Calculate the relative atomic mass of
copper.
R.A.M. Problem
• Chlorine exists as Cl-35 which has
a mass of 34.969 amu and makes
up 75.8 % of Cl atoms. The rest
of naturally occurring chlorine is
Cl-37 with a mass of 36.996 amu.
What is the average mass of
chlorine?
R.A.M. Problem
• Uranium-234 makes up 0.005% of
uranium atoms and has a mass of
234.041 amu. Uranium-235 makes
up 0.720% and has a mass of
235.044 amu. Uranium-238 has a
mass of 238.051 amu and makes up
99.275 %. What is the average
atomic mass of Uranium?
R.A.M. Problem
• Carbon-12 makes up 98.90% of
existing carbon. Carbon-13,
with a mass of 13.003 amu,
makes up 1.10%. Traces of
carbon-14 also exist. What is
the average atomic mass of
carbon?
Relating Mass # to Atoms
• Mole – Amount of a substance that
contains as many particles as there
are atoms in exactly 12 g of C-12.
• Way of counting (like 1 dozen = 12)
• Avogadro’s number – the # of
particles in 1 mole = 6.022 x 1023
Molar Mass
• Molar Mass – mass of 1 mole of a
substance
• Expressed as g/mol
• For an element it’s = to atomic
mass
• What’s the molar mass of 1 mole
of Li, Hg, C, He, and Cl
Conversions with Molar Mass
• Convert …
2 mole of He to grams of He
3.5 mole of Cu to grams of Cu
100 grams of C to mole of C
250 grams of Br to mole of Br
Conversions with Avogadro’s #
• How many moles are in 3.01 x 1022
atoms of Ag?
• How many moles are in 4.97 x 1020
atoms of Cu?
• How many atoms are in 15 moles of
C?
• How many atoms are in 4 moles of S
Practice
• Complete the section review on
page 87 and do numbers 2-7.