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The Modern Model
The modern atom is composed of two
regions:
1. Electron cloud- Most of the volume of an
atom and is the region where the electron
can be found (extranuclear)
2. The nucleus – with protons and neutrons
so that it has a Positive charge and almost
all the mass
Major Subatomic Particles
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 is 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
Nuclear Symbols
• 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
Locations on periodic table…
• Atomic Number = 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 = number of protons + neutrons
• Electrons have a negligible contribution to overall mass
• In a neutral atom there is the same number of
electrons (e-) and protons (atomic number)
• 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?
Isotopes
• 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
2
3
1
H
Protium
1
H
Deuterium
1
H
Tritium
Naming Isotopes
• When naming, write the mass number
after the name of the element
•
•
•
•
Examples:
carbon- 12
carbon -14
uranium-235
Average Atomic Mass
• Did you ever notice that the mass number
on the periodic table is not a whole
number?
• There are the decimal numbers on the
periodic table.
**This is because it is an average mass for
all the isotopes of each atom
How heavy is an atom of oxygen?
•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
Calculating Averages
You have five rocks, four with a mass of 50 g, and
one with a mass of 60 g. What is the average mass
of the rocks?
Total mass =
4 x 50 g + 1 x 60 g = 260 g
Average mass = (4 x 50 g + 1 x 60 g)/5 = (260 g)/5
= 52 g
Calculating Averages when given %:
80% of the rocks were 50 grams
20% of the rocks were 60 grams
What is the average mass of the rocks?
Average mass = 0.8 x 50 + 0.2 x 60
= 40 + 12
= 52 g
Average = (% as decimal) x (mass1) +
(% as decimal) x (mass2) +
(% as decimal) x (mass3) + …
EXAMPLES:
1. Calculate the av 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
2. 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 the decimal
numbers on the periodic table
To do :
• Page 45
– Questions 1, 2, 3, 4
• Page 46
– Questions 2, 3, 4, 5a