Download Chapter 2 Atoms and Elements

Atoms and Elements
The Atom
Atomic Number and Mass Number
Isotopes
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The Atom
An atom is the smallest particle
that retains the characteristics
of an element
Atomic Theory
 Atoms are building blocks of elements
 Similar atoms in each element
 Different from atoms of other elements
 Two or more different atoms bond in simple ratios
to form compounds
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The Parts of the Atom
Atoms are composed of
three subatomic
particles:
• Protons have a positive
charge (+)
• Electrons carry a
negative charge (-)
• Neutrons are electrically
neutral.
Subatomic Particles
Particle
Symbol
Charge
Relative
Mass
0
Electron
e-
1-
Proton
p+
+
1
Neutron
n
0
1
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Location of Subatomic Particles
10-13 cm
electrons
nucleus
protons
neutrons
10-8 cm
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The Structure of the Atom
The protons and
neutrons, each
with a mass of
about 1 amu are
found in the tiny,
dense nucleus.
The electrons are
located outside the
nucleus.
Models of the Atom
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Atomic Number
Counts the number
of
protons
in an atom
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Atomic Number on the Periodic
Table
Atomic Number
Symbol
11
Na
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All atoms of an element have the
same number of protons
11 protons
Sodium
11
Na
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Periodic Table
 Represents physical and chemical behavior of
elements
 Arranges elements by increasing atomic
number
 Repeats similar properties in columns known
as chemical families or groups
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Periodic Table
1
2
3
4 5
6 7
8
11
Na
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Learning Check
State the number of protons for atoms of each of
the following:
A. Nitrogen
1) 5 protons
2) 7 protons 3) 14 protons
B. Sulfur
1) 32 protons
2) 16 protons
3) 6 protons
C. Barium
1) 137 protons 2) 81 protons
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3) 56 protons
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Solution
State the number of protons for atoms of each of the
following:
A. Nitrogen
2) 7 protons
B. Sulfur
2) 16 protons
C. Barium
3) 56 protons
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Number of Electrons
 An atom is neutral
 The net charge is zero
 Number of protons = Number of electrons
 Atomic number = Number of electrons
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Mass Number
Counts the number
of
protons and neutrons
in an atom
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Atomic Symbols
 Show the mass number and atomic number
 Give the symbol of the element
mass number
23 Na
atomic number
sodium-23
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Subatomic Particles in Some Atoms
16
O
31
P
65
8
15
30
15 p+
16 n
15 e-
30 p+
35 n
30 e-
8 p+
8n
8 e-
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Zn
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Isotopes
 Atoms with the same number of protons, but
different numbers of neutrons.
 Atoms of the same element (same atomic
number) with different mass numbers
Isotopes of chlorine
35Cl
37Cl
17
17
chlorine - 35
chlorine - 37
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Learning Check
Naturally occurring carbon consists of three isotopes,
12C, 13C, and 14C. State the number of protons,
neutrons, and electrons in each of these carbon atoms.
12C
6
13C
14C
6
6
#p _______
_______
_______
#n _______
_______
_______
#e _______
_______
_______
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Solution
12C
6
13C
14C
6
6
#p
6
6
6
#n
6
7
8
#e
6
6
6
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Learning Check
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
2) 35
3) 65
B. Number of neutrons in the zinc atom
1) 30
2) 35
3) 65
C. What is the mass number of a zinc isotope
with 37 neutrons?
1) 37
2) 65
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3) 67
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Solution
An atom of zinc has a mass number of 65.
A. Number of protons in the zinc atom
1) 30
B. Number of neutrons in the zinc atom
2) 35
C. What is the mass number of a zinc isotope
with 37 neutrons?
3) 67
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Learning Check
Write the atomic symbols for atoms with the
following:
A. 8 p+, 8 n, 8 e-
___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
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Solution
16O
8
A. 8 p+, 8 n, 8 e-
B. 17p+, 20n, 17e-
37Cl
17
C. 47p+, 60 n, 47 e-
107Ag
47
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Learning Check AT 5
An atom has 14 protons and 20 neutrons.
A. Its atomic number is
1) 14
2) 16
3) 34
B. Its mass number is
1) 14
2) 16
3) 34
C. The element is
1) Si
3) Se
2) Ca
D. Another isotope of this element is
1) 34X
2) 34X
3) 36X
16
14
14
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Solution AT 5
An atom has 14 protons and 20 neutrons.
A. It has atomic number
1) 14
B. It has a mass number of
3) 34
C. The element is
1) Si
D. Another isotope of this element would be
3) 36X
14
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Masses of Atoms
 A scale designed for atoms gives their small atomic
masses in atomic mass units (amu)
 An atom of 12C was assigned an exact mass of 12.00
amu
 Relative masses of all other atoms was determined
by comparing each to the mass of 12C
 An atom twice as heavy has a mass of 24.00 amu. An
atom half as heavy is 6.00 amu.
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Atomic Mass on the Periodic
Table
Atomic Number
Symbol
Atomic Mass
11
Na
22.99
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Atomic Mass
 Listed on the periodic table
 Gives the mass of “average” atom of
each element compared to 12C
Na
22.99
 Average atom based on all the isotopes
and their abundance %
 Atomic mass is not a whole number
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Atomic Mass
Atomic mass is the weighted average mass
of all the atomic masses of all the isotopes of
that atom.
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Weighted Average
• You have two types of coins: pennies and
quarters.
The pennies have a mass of 5g.
The quarters have a mass of 15g.
57% of the coins are pennies, 43% of the coins
are quarters.
What is the average mass of your coins?
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Weighted Average – Atomic Mass
• You have two isotopes of Boron: 10B and 11B.
The 10B has a mass of 10amu.
The 11B has a mass of 22 amu.
57% of the atoms are 10B, 43% of the atoms
are 11B.
What is the average mass of your atoms?
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Example of an Average Atomic
Mass
Cl-35 is about 75.5 % and Cl-37 about
24.5% of natural chlorine.
35 x 75.5 = 26.4
100
35.5
37 x 24.5 =
9.07
100
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Problem AT6
Using the periodic table, specify the atomic mass
of each element (round to the tenths place):
A. calcium
________
B. aluminum
________
C. lead
________
D. barium
________
E. iron
________
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Solution AT6
Using the periodic table, specify the atomic mass
of each element (round to the tenths place):
A. calcium
_40.1 amu _
B. aluminum
_27.0 amu _
C. lead
_207.2 amu_
D. barium
_137.3 amu_
E. iron
_55.8 amu__
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Calculating Atomic Mass
Add :
Percent (%) abundance of each isotope
Multiplied by the
Mass of each isotope of that element
Weighted average = mass isotope1(%) + mass isotope2(%) + …
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Atomic Mass of Magnesium
Isotopes
24Mg
=
Mass of Isotope Abundance
24.0 amu
78.70%
25Mg
=
25.0 amu
10.13%
26Mg
=
26.0 amu
11.17%
Atomic mass (average mass) Mg = 24.3 amu
Mg
24.3
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Learning Check AT7
Gallium is a metallic element found in small
lasers used in compact disc players. In a sample
of gallium, there is 60.2% of gallium-69 (68.9
amu) atoms and 39.8% of gallium-71 (70.9 amu)
atoms. What is the atomic mass of gallium?
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Solution AT7
Ga-69
68.9 amu x 60.2
=
41.5 amu for
69Ga
=
28.2 amu for
71Ga
=
69.7 amu
100
Ga-71 (%/100)
70.9 amu x 39.8
100
Atomic mass Ga
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Finding An Isotopic Mass
A sample of boron consists of 10B (mass 10.0
amu) and 11B (mass 11.0 amu). If the average
atomic mass of B is 10.8 amu, what is the %
abundance of each boron isotope?
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Assign X and Y values:
X = % 10B
Y = % 11B
Determine Y in terms of X
X + Y = 100
Y = 100 - X
Solve for X:
X (10.0) + (100 - X )(11.0)
100
100
= 10.8
Multiply through by 100
10.0 X + 1100 - 11.0X = 1080
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Collect X terms
10.0 X - 11.0 X
=
1080 - 1100
- 1.0 X
= -20
-1.0 X
-1.0
= -20
- 1.0
X
=
20 % 10B
Y = 100 - X
%
11B
= 100 - 20%
= 80% 11B
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Learning Check AT8
Copper has two isotopes 63Cu (62.9 amu)
and 65Cu (64.9 amu). What is the %
abundance of each isotope? (Hint: Check
periodic table for atomic mass)
1) 30%
2) 70%
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3) 100%
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Solution AT8
2) 70%
Solution
X
+ Y
Y
= 100
= 100 - X
Solve for X:
X (62.9) + (100 - X )(64.9) = 63.5
100
100
Multiply through by 100
62.9X + 6490 - 64.9X = 6350
-2.0 X = -140
X = 70%
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