Chapter 3
Chapter Three
Atoms and the Periodic
Table
Dalton’s Atomic Theory (1808)
1. Elements are composed of extremely small
particles called atoms. All atoms of a given
element are identical, having the same size,and
chemical properties. The atoms of one element
are different from the atoms of all other elements.
2. Compounds are composed of atoms of more
than one element. The relative number of atoms
of each element in a given compound is always
the same.
3. Chemical reactions only involve the rearrangement
of atoms. Atoms are not created or destroyed in
chemical reactions.
2 2.1
What
are atoms composed of
?
3
The Structure of Atoms

Cathode-Ray Tube (Thomson, 1856–1940):

Cathode rays
consist of tiny
negatively
charged particles,
now called
electrons.
4
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.
5
6
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.
7
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.
8
The Structure of Atoms

Structure of the Atom:
9
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
10
Atomic Numbers and Protons for
Lithium and Carbon Atoms
11
Subatomic Particles
 Chemists are interested
in three particles in
atoms: protons,
electrons, and neutrons.
 Protons have a positive
(+) charge; electrons
have a negative (-)
charge.
 Like charges repel and
unlike charges attract.
12
Summary of Subatomic Particles
13
Structure of the Atom
 An atom consists of a
nucleus and a cloud of
electrons.
 The nucleus contains
all of the protons and
neutrons.
 The rest of the atom,
which is mostly empty
space, is occupied by
the electrons.
14
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
15
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
16
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.
17
Mass Number
 The mass number gives the total number of protons
and neutrons in the nucleus.
Mass number = # protons + # neutrons
18
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
19
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 e20
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
______
______
______
21
Solution
12C
6
13C
6
14C
6
Protons
6
6
6
Neutrons
6
7
8
Electrons
6
6
6
22
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-
___________
23
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
24
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
25
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
26
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
27
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
28
2.5 Isotopes and Atomic Mass
24Mg
12
25Mg
12
26Mg
12
29
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.
All elements have two or more isotopes.
30
Isotopes of Magnesium
In naturally occurring
atoms of magnesium,
there are three isotopes.

24Mg
makes up 78.9% of
magnesium atoms.
 25
Mg makes up 10.0%
and 26Mg makes up
11.1%.
24Mg
12
25Mg
12
26Mg
12
31
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
__________
32
Solution
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
33
3. 6 Electronic Structure of Atoms



Quantum mechanical model of atomic
structure:
Electrons are not perfectly free to move
about in an atom.
Each electron is restricted to moving about
only in a certain region of space within the
atom, depending on the amount of
energy the electron has.
34
Probability distribution ( 2) for an
electron in a 1s orbital.
A boundary surface encloses the region
where the probability of finding an electron
is high—on the order of 90-95%
1s
2s
Boundary surfaces of a 1s orbital
and a 2s orbital.
Different orbitals have different shapes.
Orbitals in s subshells are spherical (a), while
orbitals in p subshells are roughly dumbbell
shaped (b).
37
s and p Orbitals
 In an atom, all the orbitals are centered
around the nucleus. For example, the
illustration of the combination of s and p
orbitals is
38
Subshells and Orbitals
Each subshell has a specific number of orbitals.
 Each s subshell contains one s orbital.
 Each p subshell contains three p orbitals.
 Each d subshell contains five d orbitals.
 Each f subshell contains seven f orbitals.
39
d-Orbitals
40
f-Orbitals

41




The first shell has only an s subshell
The second shell has an s and a p
subshell
The third shell has an s, a p, and a d
subshell.
The fourth shell has an s, a p, a d, and
an f subshell.
42



The number of subshells in a given shell is equal to
the shell number. For example, shell number 3 has
3 subshells.
Within each subshell, electrons are further grouped
into orbitals, regions of space within an atom where
the specific electrons are more likely to be found.
There are different number of orbitals within the
diff
erent kinds of subshells.
43
Energy of Electrons




Different electrons have different amounts
of energy and thus occupy different regions
within the atom.
The energies of electrons are quantized, or
restricted to having only certain values.
The electrons in an atom are grouped
around the nucleus into shells.
Within the shells, electrons are further
grouped into sub shells of four different
types, identified as s, p, d, and f in order of
increasing energy.
44
Electrons fill orbitals in ascending order, from the lowest-energy
orbitals upward. Generally, this means that lower-numbered
shells fill before higher-numbered shells. However, some overlap
in energy levels occurs starting in the third energy level.

Fig 3.5 order of orbital
energy levels
45
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p,
7s, 5f, 6d, 7p
46
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p,
7s, 5f, 6d, 7p
Some Electron Configurations
First Period
H
1s1
He
1s2
Second Period
Li
1s2
C
1s2
Third Period
Na
1s2
Al
1s2
S
1s2
2s1
2s2
2p2
2s2
2s2
2s2
2p6
2p6
2p6
3s1
3s2
3s2
3p1
3p4
47
Electronic configuration of a few elements are
shown below:

48
49
Electron Configurations for
Elements
50
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p,
7s, 5f, 6d, 7p
Noble Gas Notation

A noble gas notation is used to abbreviate the
completed subshells as [noble gas].
For example, the complete configuration of
bromine is
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5

Ar configuration
The noble gas notation for bromine is
[Ar] 4s2 3d10 4p5

51
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d,
6p, 7s, 5f, 6d, 7p
Learning Check
Write the complete electronic configuration and
noble gas notation for each of the following
elements:
A. Cl
B. Sr
C. I
52
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p,
7s, 5f, 6d, 7p
Solution
A. Cl
1s2 2s2 2p6 3s2 3p5
[Ne] 3s23p5
B. Sr
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2
[Kr] 5s2
C. I
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5
[Kr] 5s2 4d10 5p5
53
Group Numbers and Subshells

The Group Number is the sum of the s and p
electrons in the outermost shell.
54
Fig 3.2 The periodic table

55
The table has 113 boxes, each of which tells
the symbol, atomic number, and atomic weight
of an element.
Last update: 117 elements known to date

56
Main Groups: The two large groups on the far left
and the six on the far right are called the main groups.
Lanthanides: 14 elements following lanthanum in
group 6.
Actinides: 14 elements following actinium in group
7.
Transition Metal Groups: Elements in the groups
numbered 3 through 12 (Sc-Zn).
Inner Transition Metal Groups:
The 14 groups
shown separately at the bottom of the table and are not
numbered.

57
3.8 Electron Configuration and the
Periodic table
The periodic table can be divided into four
regions or blocks, of elements according to the
shells and subshells as shown in Fig 3.6.
Valence Shell : Outermost shell of an atom.
Valence electrons: An electron in an outermost
shell of an atom. These electrons are loosely
held, they are most important in determining an
element’s properties.

58
Group Numbers and Subshells

The Group Number is the sum of the s and p
electrons in the outermost shell.
59
4f
5f
60
ns2np6
ns2np5
ns2np4
ns2np3
ns2np2
ns2np1
d10
d5
d1
ns2
ns1
Electron Configuration and the Periodic Table
Electron Configuration of Atoms
06
61
62
Using periodic table write Noble gas notation
for the following elements:
a)S
b)Fe
[Ne]3s23p4
[Ar] 4s23d6
c)Se
[Ar] 4s23d104p4
d)Gd
[Xe]6s2 5d1 4f7
63
64

Fig 3.6 electron configurations and the periodic
table
65
Fig 3.2 The periodic table

66
Chapter Summary




All matter is composed of atoms.
An atom is the smallest and simplest unit into
which an element can be divided while
maintaining the properties of the element.
Atoms are made up of subatomic particles called
protons, neutrons and electrons. Protons have
positive, electrons have negative, and neutrons have
zero electrical charges.
Elements differ from each other according to the
number of protons their atoms contain (Atomic
number, Z)
67
Chapter Summary Contd.




Mass number (A): Total number of protons and
neutrons in an atom.
Isotopes: Atoms with identical number of protons
but different numbers of neutrons.
Periodic table: Tabular arrangement of elements
according to their valence shells.
The electrons surrounding an atom are grouped into
shells. Within each shell, electrons are grouped
into subshells, and within subshells into orbitals
– regions of space where electrons are more
likely to be found.
68
Chapter Summary Contd.
s orbitals are spherical and p orbitals are bumbbell
shaped.

Each orbital and each shell can hold a specific
number of electrons.
- The first shell can hold only two electrons. 2
electrons in an s orbital (1S2).
- The second shell can hold 8 electrons.
2 electrons in an s orbital and 6 electrons in 3 p
orbitals. (2S2 2p6).
- The third shell can hold 18 electrons. 2 electrons
in an s orbital, 6 electrons in 3 p orbitals, and 10
electrons in 5 d orbitals (3S2 3p6 3d10); and so on.

69
End of Chapter 3

70