© 2003 John Wiley and Sons Publishers
Chapter 2: Atoms and Elements
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The Building Blocks of Chemistry
© 2003 John Wiley and Sons Publishers
Courtesy Ken Karp
Atoms and Paper Clips
Compounds are result of
combination of elements
(in a fix ratio)
What elements composed of ?
2.1 Origins of Atomic
Theory
Atomistic PhilosophersLeucippus & Democritus (ca., 400 BC)
Atomos - indivisible (Greek)
Democritus
Matter is composed of Atoms
http://dbhs.wvusd.k12.ca.us/AtomicStructure/AtomicStructure.html
© 2003 John Wiley and Sons Publishers
Courtesy Ken Karp
Piles of paper clips, individual paper clips, and fragments of a paper clip.
© 2003 John Wiley and Sons Publishers
Figure 2.1: In a parallel to the demonstration with the paper clips, repeatedly
subdividing a piece of gold produces smaller and smaller groups of atoms.
What
are atoms composed of
?
The Structure of Atoms
• Cathode-Ray Tube (Thomson, 1856–1940):
• Cathode rays
consist of tiny
negatively
charged particles,
now called
electrons.
The Structure of Atoms
• Deflection of electron depends on three factors:
– Strength of electric or magnetic field
– Size of negative charge on electron
– Mass of the electron
• Thomson calculated the electron’s charge to mass
ratio as 1.758820 x 108 Coulombs per gram.
The Structure of Atoms
• Oil Drop Experiment (Millikan, 1868–1953): Applied a
voltage to oppose the downward fall of charged drops and
suspend them.
• Voltage on plates place
1.602176 x 10-19 C of
charge on each oil drop.
• Millikan calculated the
electron’s mass as
9.109382 x 10-28 grams.
The Structure of Atoms
• Discovery of Nucleus (Rutherford, 1871 –
1937):
• Rutherford irradiated
gold foil with a beam
of alpha () particles
to search for positive
charged particles.
The Structure of Atoms
Discovery of Nucleus (Rutherford, 1871–1937):
Rutherford irradiated
gold foil with a beam
of alpha () particles
to search for positive
charged particles.
Atom must be mostly
empty space except
for a central positive
mass concentration.
© 2003 John Wiley and Sons Publishers
Figure 2.10: Relative distances and masses in the hydrogen atom.
The Structure of Atoms
• Structure of the Atom:
© 2003 John Wiley and Sons Publishers
Figure 2.9: The structure of the hydrogen atom.
© 2003 John Wiley and Sons Publishers
Figure 2.7: The nucleus and electron shells of a lithium atom, a typical small atom.
Atomic Number
 The atomic number is equal to the number of
protons in an atom.
 On the periodic chart, the atomic number
appears above the symbol of an element.
Atomic Number
Symbol
11
Na
Fig 3.2 The periodic table

18
Atomic Numbers and Protons for
Lithium and Carbon Atoms
Learning Check
State the number of protons in each.
A. Nitrogen
1) 5 protons
2) 7 protons
3) 14 protons
B. Sulfur
1) 32 protons
2) 16 protons
C. Barium
1) 137 protons 2) 81 protons
3) 6 protons
3) 56 protons
Solution
State the number of protons in each.
A. Nitrogen
2) Atomic number 7; 7 protons
B. Sulfur
2) Atomic number 16; 16 protons
C. Barium
3) Atomic number 56; 56 protons
Electrons in An Atom
 An atom is electrically neutral; the net charge is
zero.
 In an atom, the number of protons is equal to the
number of electrons.
# protons = # electrons
 Therefore, the atomic number is also equal to the
number of electrons in a neutral atom.
What Else is There in an Atom?
• Mp = 1.67 X 10 –24 g
• Me = 0.0009 X 10 –24 g,
• M(atom) > Mp+Me
• Mn = 1.67X10 –24 g
1amu = 1.661X10 –24 g
Neutrons?
Mass Number
 The mass number gives the total number of protons
and neutrons in the nucleus.
Mass number = # protons + # neutrons
Atomic Symbol
 The atomic symbol represents a particular atom of
an element.
 The atomic symbol shows the mass number in the
upper left corner and the atomic number in the
lower left corner.
 For example, an atom of sodium with atomic
number 11 and a mass number 23 has the following
atomic symbol:
23 Na
mass number
atomic number 11
Information from Atomic Symbols
 When we know the atomic symbol of an atom,
we can determine the number of protons (p+),
neutrons, (n), and electrons (e-).
16
31
O
65
P
Zn
8
15
30
8 p+
8n
8 e-
15 p+
16 n
15 e-
30 p+
35 n
30 e-
Learning Check
Naturally occurring carbon consists of three
isotopes, 12C, 13C, and 14C. State the number of
protons, neutrons, and electrons in each of the
following.
12C
13C
14C
6
6
6
protons
______
______
______
neutrons
______
______
______
electrons
______
______
______
Solution
12C
6
13C
6
14C
6
Protons
6
6
6
Neutrons
6
7
8
Electrons
6
6
6
Learning Check
Write the atomic symbols for atoms with the
following subatomic particles:
A. 8 p+, 8 n, 8 e___________
B. 17p+, 20n, 17e-
___________
C. 47p+, 60 n, 47 e-
___________
Solution
Write the atomic symbols for atoms with the
following subatomic particles:
16O
8
A. 8 p+, 8 n, 8 eB. 17p+, 20n, 17e-
37Cl
17
C. 47p+, 60 n, 47 e-
107Ag
47
Learning Check
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30
2) 35
3) 65
B. How many neutrons are in the zinc atom?
1) 30
2) 35
3) 65
C. What is the mass number of a zinc atom
that has 37 neutrons?
1) 37
2) 65
3) 67
Solution
An atom of zinc has a mass number of 65.
A. How many protons are in this zinc atom?
1) 30
B. How many neutrons are in the zinc atom?
2) 35
C. What is the mass number of a zinc atom
that has 37 neutrons?
3) 67
Learning Check
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
2) Ca
3) Se
Solution
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
Atoms and Elements
Isotopes and Atomic Mass
24Mg
12
25Mg
12
26Mg
12
Isotopes
 Isotopes are atoms of the same element that have
different mass numbers.
 Isotopes have the same number of protons, but
different numbers of neutrons.
 Most elements have two or more isotopes.
Atomic Mass
 Atomic mass is the weighted average mass of all the
isotopes of that element.
 The atomic mass of each element is compared to
12C, whith a mass of 12 amu.
 The atomic mass of each element is listed below the
symbol of the element on the periodic table.
Atomic Mass of Magnesium
 The individual atomic
masses of all the
isotopes contribute to
the atomic mass of
Mg.
 As a weighted
average, the atomic
mass is not a whole
number.
© 2003 John Wiley and Sons Publishers
Figure 2.11: Relative abundance of deuterium.
Learning Check
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
__________
Solution
Using the periodic table, specify the atomic
mass of each element (round to the tenths
place):
A. calcium
40.1
B. aluminum
27.0
C. lead
207.2
D. barium
137.3
E. iron
55.9
Periodic Table
The periodic
table
arranges
elements
according to
similar
properties.
Groups on the Periodic Table
• On the periodic table, each vertical column
is called a group of elements.
• A group contains elements with similar
chemical and physical properties.
• Each group is identified by a group number
at the top of the column.
• The representative elements have group
numbers of 1A – 8A. The transition
elements use the letter “B.”
Groups and Group Numbers
Numbering Groups
• One numbering system for groups assigns
the letter A to the representative elements
and the letter B to the transition elements.
• Another system assigns the numbers 1-18
to the columns across the periodic table.
Group Numbers
Group numbers
Representative Elements
• Several groups of representative elements are
classified by name.
Group 1A
Group 2A
Group 7A
Group 8A
Alkali Metals
Alkaline Earth Metals
Halogens
Noble Gases
Names of Some Representative
Groups
Representative Elements
• Group 1A, the
alkali metals,
includes
sodium, lithium,
and potassium.
• Group 7A, the
halogens,
includes chlorine,
bromine,
and iodine.
Periods on the Periodic Table
• On the periodic table, each horizontal row
is called a period.
• A period is identified by a number such as
1, 2, 3, or 4.
• Periods 2-7 include representative elements
and transition elements.
Location of Periods
Period 4
Learning Check
Identify the element described by the following:
A. Element in Group 7A, Period 4
1) Br
2) Cl
3) Mn
B. Element in Group 2A, Period 3
1) beryllium 2) magnesium
3) boron
Solution
A. Group 7A, Period 4
1) Br
B. Group 2A, Period 3
2) magnesium
© 2003 John Wiley and Sons Publishers
Figure 2.12: Names, symbols, electron structures, and compositions of nuclei of the first
10 elements.
Which particle has a mass approximately
equal to the mass of a proton?
a.
b.
c.
d.
e.
Atom
Electron
Neutron
Nucleus
quark
Which particle has a mass
approximately equal to the mass of
a proton?
a.
b.
c.
d.
e.
Atom
Electron
Neutron
Nucleus
quark
Protons possess a ________ charge, and
electrons possess a ________ charge.
a.
b.
c.
d.
e.
negative, negative
negative, positive
positive, negative
positive, zero
zero, positive
Protons possess a ________
charge, and electrons possess a
________ charge.
a.
b.
c.
d.
e.
negative, negative
negative, positive
positive, negative
positive, zero
zero, positive
In a neutral atom the number of ________ is
equal to the number of ________.
a.
b.
c.
d.
e.
protons, electrons
protons, neutrons
neutrons, electrons
protons + electrons, neutrons
none of the above
In a neutral atom the number of
________ is equal to the number of
________.
a.
b.
c.
d.
e.
protons, electrons
protons, neutrons
neutrons, electrons
protons + electrons, neutrons
none of the above
An atom with a mass number of 58 and with
32 neutrons will have ________ protons.
a.
b.
c.
d.
e.
16
26
32
58
90
An atom with a mass number of 58
and with 32 neutrons will have
________ protons.
a.
b.
c.
d.
e.
16
26
32
58
90
The symbol of the element with 23 protons
is
a.
b.
c.
d.
e.
Mg
Na
V
B
None of the above.
The symbol of the element with 23
protons is
a.
b.
c.
d.
e.
Mg
Na
V
B
None of the above.
An atom with Z = 26 and A = 58 contains
________ protons, ________ electrons, and
________ neutrons.
a.
b.
c.
d.
e.
26; 26; 58
58; 26; 26
26; 26; 32
32; 26; 32
26; 32; 84
An atom with Z = 26 and A = 58
contains ________ protons,
________ electrons, and ________
neutrons.
a.
b.
c.
d.
e.
26; 26; 58
58; 26; 26
26; 26; 32
32; 26; 32
26; 32; 84
An atom containing 29 protons, 29
electrons, and 34 neutrons has a mass
number of
a.
b.
c.
d.
e.
5
29
34
58
63
An atom containing 29 protons, 29
electrons, and 34 neutrons has a
mass number of
a.
b.
c.
d.
e.
5
29
34
58
63
The value of Z for an atom containing 29
protons, 29 electrons, and 34 neutrons, is
a.
b.
c.
d.
e.
5
29
34
58
63
The value of Z for an atom
containing 29 protons, 29
electrons, and 34 neutrons, is
a.
b.
c.
d.
e.
5
29
34
58
63
An atom containing 47 protons, 47
electrons, and 60 neutrons has a mass
number of
a.
b.
c.
d.
e.
13.
47.
60.
107.
154.
An atom containing 47 protons, 47
electrons, and 60 neutrons has a
mass number of
a.
b.
c.
d.
e.
13.
47.
60.
107.
154.
The atomic number of an atom containing
47 protons, 47 electrons, and 60 neutrons is
a.
b.
c.
d.
e.
13
47
60
107
154
The atomic number of an atom
containing 47 protons, 47
electrons, and 60 neutrons is
a.
b.
c.
d.
e.
13
47
60
107
154
© 2003 John Wiley and Sons Publishers
Courtesy Andy Washnik
Figure 2.15: Two paper clips of the same kind but different sizes can be used as models
for atomic isotopes.
© 2003 John Wiley and Sons Publishers
Courtesy Andy Washnik
Figure 2.14: The many varieties of paper clips reflect the varieties of elements and their
atoms.
© 2003 John Wiley and Sons Publishers
Figure 2.6: Weight depends on the size of the gravitational force.
© 2003 John Wiley and Sons Publishers
Courtesy NASA
The astronauts have mass but no weight.