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Calculating Atomic
Mass
Name
Symbol
Charge Relative Mass Actual Mass (g)
(amu)
Electron
e-
-1
1/1840
9.11x10-28
Proton
p+
+1
1
1.67x10-24
Neutron
no
0
1
1.67x10-24
• Atoms are measured in picometers, 10-12 meters
• Hydrogen atom, 32 pm radius
• Nucleus tiny compared to atom
• If the atom were a stadium, the nucleus would be a marble
• Radius of the nucleus is on the order of 10-15 m
• Density within the atom is near 1014 g/cm3
• Atomic Number (Z) = number of protons (p+) in the nucleus
•
Determines the type of atom
• Li atoms always have 3 protons in the nucleus, Hg always 80
• Mass Number (A) = number of protons + neutrons [Sum of p+
and nº]
•
Electrons have a negligible contribution to overall mass
• In a neutral atom there is the same number of electrons (e-)
and protons (atomic number)
• Every element is given a corresponding symbol which is
composed of 1 or 2 letters (first letter upper case, second lower),
as well as the mass number and atomic number
mass number
A
elemental symbol
atomic number
Z
E
• Find the
number of protons
• number of neutrons
• number of electrons
• atomic number
• mass number
•
19
9
F
80
35
Br
184
74
W
• If an element has an atomic number of 34 and a mass number
of 78 what is the:
number of protons in the atom?
• number of neutrons in the atom?
• number of electrons in the atom?
• complete symbol of the atom?
•
• If an element has 91 protons and 140 neutrons what is the:
• atomic number?
• mass number?
• number of electrons?
• complete symbol?
• Atoms of the same element can have different numbers of
neutrons and therefore have different mass numbers
• The atoms of the same element that differ in the number of
neutrons are called isotopes of that element
1
1
H
Hydrogen-1
2
1
H
Hydrogen-2
3
1
H
Hydrogen-3
• When naming, write the mass number after the name of the
element
•There are different kinds of oxygen atoms (different isotopes)
• 16O, 17O, 18O
• We are more concerned with average atomic masses, rather
than exact ones
•
Based on abundance of each isotope found in nature
• We can’t use grams as the unit of measure because the
numbers would be too small
Instead we use Atomic Mass Units (amu)
• Standard amu is 1/12 the mass of a carbon-12 atom
• Each isotope has its own atomic mass
•
Average =
(% as decimal) x (mass1) +
(% as decimal) x (mass2) +
(% as decimal) x (mass3) + …
Try Again…THINK!!
Your test and quiz mark is 30%. 10% is made up of quiz,
20% is made up of tests
If you made 85%, 78% and 79% on your quizzes
If you made 87% on your only test…
What would be your test quiz mark for your report card?
Calculating Averages
• Calculate the atomic mass of copper if copper has two
isotopes


69.1% has a mass of 62.93 amu
The rest (30.9%) has a mass of 64.93 amu
• Magnesium has three isotopes




78.99% magnesium 24 with a mass of 23.9850 amu
10.00% magnesium 25 with a mass of 24.9858 amu
The rest magnesium 26 with a mass of 25.9826 amu
What is the atomic mass of magnesium?
Average Atomic Masses
• If not told otherwise, the mass of the isotope is the mass
number in amu
• The average atomic masses are not whole numbers because
they are an average mass value
• Remember, the atomic masses are the decimal numbers on
the periodic table
Isotope Worksheet
 Question 2a:
 10B = 18.8 %, 11B = 81.2 %
 Avg = (10)(0.188) + (11)(0.812)
= 10.812 amu
 Skip 3f and 3g, unless you really want a
challenge 
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