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Table 2-1
Concept 2.2: An element’s properties
depend on the structure of its atoms
• Each element consists of unique atoms
• An atom is the smallest unit of matter that still
retains the properties of an element.
• Atoms of the same element share similar
properties.
• Atoms have symbols which are the same as
that of the element made up of those atoms.
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Subatomic Particles
• Atoms are composed of subatomic particles
• Relevant subatomic particles include:
– Neutrons (no electrical charge, mass of one atomic
mass unit, or amu)
– Protons (positive charge, mass of 1 amu)
– Electrons (negative charge, mass of 1/2000 amu)
____________________________________
Dalton is a unit used to express mass at the atomic level.
1 Dalton = 1.67 x 10-24 gram.
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Subatomic Particles
• Neutrons and protons form the atomic
nucleus at the center of the atom.
• Electrons form a cloud around the nucleus
Fig. 2-5
Cloud model
Shell model
Cloud of negative
charge (2 electrons)
Electrons
Nucleus
(a)
(b)
Subatomic Particles
Nucleus
Protons (+ charge)
determine element
Neutrons (no charge)
determine isotope
Electrons (– charge)
form negative cloud
and determine
chemical behavior
Atom
An atom’s nucleus contains __________.
• protons
• neutrons
• electrons
• protons and neutrons
An Element’s Atomic Number
• Atoms of the various elements differ in number
of subatomic particles
• An atom before interacting with other atoms is
electrically neutral, i.e. in this atom:
the # of protons = the # of electrons
• An element’s atomic number is the number of
protons in its nucleus. Therefore,
Atomic number = # of protons = # of electrons
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Atomic Mass number
• An element’s mass number is the sum of protons
plus neutrons in the nucleus
• Mass number = # of protons + # of neutrons
• Atomic mass, the atom’s total mass, can be
approximated by the mass number
The element lithium has 3 protons and 4 neutrons
in its nucleus. Its mass number is __________.
• 1
• 3
• 7
• 12
Bohr Model
What is the atomic # and Mass # of each of these atoms in the Bohr models
Isotopes
• All atoms of an element have the same
number of protons but may differ in number
of neutrons
• Isotopes are two or more atoms of an
element that differ in number of neutrons.
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Electron Distribution and Chemical Properties
• The distribution of electrons in the electron
shells follow usually the formula:
X = 2n2
Where:
X is the maximum number of electrons in an
electron shell and
n is the number of that electron shell in
terms of its distance from the nucleus.
Hydrogen
1H
Atomic mass
First
shell
2
He
4.00
Atomic number
Helium
2He
Element symbol
Electrondistribution
diagram
Lithium
3Li
Beryllium
4Be
Boron
5B
Carbon
6C
Nitrogen
7N
Oxygen
8O
Fluorine
9F
Neon
10Ne
Silicon
14Si
Phosphorus
15P
Sulfur
16S
Chlorine
17Cl
Argon
18Ar
Second
shell
Sodium Magnesium Aluminum
12Mg
11Na
13Al
Third
shell
The Octet Rule
• Valence electrons are those in the outermost
shell, or valence shell
• The chemical behavior of an atom is mostly
determined by the valence electrons
• Elements with a full valence shell are
chemically inert
• The Octet Rule: Rule that a valence shell is
complete when it contains eight electrons
except for H and He
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Chemically Inert Elements
• Stable and unreactive
• Inert elements have their outermost energy
level fully occupied by electrons
Chemically Reactive Elements
• Reactive
elements do
not have their
outermost
energy level
fully occupied
by electrons
Concept 2.3: The formation and function of
molecules depend on chemical bonding between
atoms
• Atoms with incomplete valence
shells can share or transfer
valence electrons with certain
other atoms
• These interactions usually result in
atoms staying close together, held
by attractions called chemical
bonds.
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Losing, Gaining, or Sharing Electrons
• Atoms having 1, 2, or 3 electrons in
their valence shell tend to lose
electrons.
• Atoms having 4 or 5 electrons in their
valence shell tend to share electrons.
• Atoms having 6 or 7 electrons in their
valence shell tend to gain or share
electrons depending on their partner
atom.
The most important determinant of an atom’s
bonding behavior is __________.
• the number of protons in the nucleus
• the total number of electrons
• the number of valence shell electrons
• the number of neutrons in the nucleus
Types of Chemical Bonds
1- Covalent bonds (the strongest).
2- Ionic Bonds (strong, but fragile).
http://www.youtube.com/watch?v=QqjcCvzW
www
3- Hydrogen Bonds (weak).
4- Van der Waals Interactions (weak).
5- Hydrophobic Interactions (weak).
Covalent Bonds
• A covalent bond is the sharing of a
pair of valence electrons by two atoms
• In a covalent bond, the shared
electrons count as part of each atom’s
valence shell
• A molecule consists of two or more
atoms held together by covalent bonds
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Covalent Bonds
• The atoms of four elements of the human body
form covalent bonds most commonly:
– oxygen (O)
– carbon (C)
– hydrogen (H)
– nitrogen (N)
Covalent Bonds
• Covalent bonds can form between atoms
of the same element or atoms of different
elements
• Every atom has a characteristic total
number of covalent bonds that it can
form, this number is called an atom’s
valence.
– The valence of H=1, O=2, N=3, and C=4
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
An Atom’s Valence
Hydrogen
(valence = 1)
Oxygen
(valence = 2)
Nitrogen
(valence = 3)
Carbon
(valence = 4)
H
O
N
C
Electrons Are Shared in Covalent Bonds
Single, double, and triple covalent bonds
• Single covalent bond
– One pair of electrons shared
• e.g., between two hydrogen atoms
• Double covalent bond
– Two pairs of electrons shared
• e.g., between two oxygen atoms
• Triple covalent bond
– Three pairs of electrons shared
• e.g., between two nitrogen atoms
Nonpolar and polar covalent bonds
• Atoms in a covalent bond may share electrons equally or
unequally
• How they share is determined by electronegativity
• Electronegativity:
• Is the relative attraction of particular atom for the
electrons of a covalent bond.
• High electronegativity = electrons spend more time
orbiting the nucleus
• Is determined by the number of protons in the
nucleus and the proximity of valence electrons to the
nucleus
Fig. 2-9
Hydrogen
1H
Atomic mass
First
shell
2
He
4.00
Atomic number
Helium
2He
Element symbol
Electrondistribution
diagram
Lithium
3Li
Beryllium
4Be
Boron
5B
Carbon
6C
Nitrogen
7N
Oxygen
8O
Fluorine
9F
Neon
10Ne
Silicon
14Si
Phosphorus
15P
Sulfur
16S
Chlorine
17Cl
Argon
18Ar
Second
shell
Sodium Magnesium Aluminum
12Mg
11Na
13Al
Third
shell
Copyright © The McGraw–Hill Companies, Inc. Permission required for reproduction or display.
IIA
IIIA
Increasing electronegativity
1
IVA
VA
VIA
VIIA
2
H
He
1.008
3
4
Li
Be
6.941
9.012
11
12
Na
Mg
22.99
24.31
19
VIIIA
5
1
Atomic number
H
Element symbol
B
20
K
Ca
39.10
40.08
22
23
24
25
Fe
13
14
Al
Si
26.98
26
21
C
12.01
10.81
Atomic mass number
1.008
6
27
30
4.003
8
7
O
N
9
F
10
Ne
14.01
15.99
15
16
17
P
S
Cl
30.97
2.07
35.45
35
36
Br
Kr
79.90
83.80
53
54
19.00
28.09
20.18
18
Ar
39.95
28
29
31
32
33
34
Co
Ni
Cu
58.93
58.69
63.55
Zn
Ga
Ge
As
Se
65.38
69.72
46
47
48
49
50
51
Sc
Ti
V
Cr
Mn
44.96
47.87
50.94
52.00
54.94
41
42
43
44
45
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
98.00
101.1
102.9
106.4
107.9
112.4
114.8
118.7
121.8
127.6
126.9
131.3
37
38
39
40
Rb
Sr
Y
Zr
Nb
Mo
85.47
87.62
88.91
91.22
92.91
95.94
55.85
72.64
74.92
78.96
52
55
56
57
72
73
74
75
76
77
78
79
80
81
82
83
84
Cs
Ba
La
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
132.9
137.3
138.9
178.5
180.9
183.8
186.2
190.2
192.2
195.1
197.0
200.6
204.4
207.2
209.0
209.0
210.0
222.0
87
88
89
104
105
106
107
108
109
110
111
112
112
114
115
116
117
118
Fr
Ra
Ac
Rf
Db
Sg
Bh
Hs
Mt
Ds
Rg Uub Uut
223.0
226.0
227.0
267.0
268.0
271.0
272.0
270.0
276.0
281.0
60
61
62
63
64
274
85
86
Rn
Uuq Uup Uuh Uus Uuo
277
277
289.0
288.0
293.0
292.0
294.0
65
66
67
68
69
70
71
58
59
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
140.1
140.9
144.2
145.0
150.4
152.0
157.3
158.9
162.5
164.9
167.3
168.9
173.0
175.0
90
91
92
103
Th
Pa
232.0
231.0
U
238.0
93
94
Np
Pu
237.0
244.0
95
96
Am
Cm
243.0
247.0
97
Bk
247.0
98
99
100
101
102
Cf
Es
Fm
Md
No
Lr
251.0
252.0
257.0
258.0
259.0
262.0
Increasing electronegaativity
IA
Nonpolar and polar covalent bonds (continued)
• Atoms in a covalent bond may share electrons equally or
unequally
• Two atoms of the same element have the same
electronegativities and share electrons equally,
• This results in forming a nonpolar covalent bond
• Atoms with different electronegativity share electrons
unequally
• This results in a polar covalent bond. Exception is
the bond between carbon and hydrogen, considered
nonpolar
Nonpolar Covalent Bond
• In a nonpolar covalent bond, the atoms
share the electron equally
Polar Covalent Bond
• In a polar covalent bond, one atom is more
electronegative, and the atoms do not share
the electron equally
Polar Covalent Bond
• More electronegative atom
develops a partial negative
charge
• Less electronegative atom
develops a partial positive
charge
– In a bond between oxygen
(more electronegative) and
hydrogen (less
electronegative), oxygen is
slightly negative, hydrogen
slightly positive
–
–
O
+
H
H
H2O
+
Covalent Bonds
What type of bond is formed between two
oxygen atoms?
Covalent Bonds
Why are some covalent bonds nonpolar and
others polar?
Different atoms have different electronegativities and
share electrons unequally, causing polar bonds.
Bonds between atoms of the same type are nonpolar.
(An exception is the carbon-hydrogen bond, which is
considered nonpolar, though the atoms are of different
types.)
Covalent bonds occur when __________.
• electrons are shared between atoms
• electrons are transferred between atoms
• electrons are lost
• opposite charges attract atoms together
Ionic Bonds
• Atoms sometimes strip electrons from their
bonding partners
• An example is the transfer of an electron
from sodium to chlorine
• After the transfer of an electron, both atoms
have charges
• A charged atom (or molecule) is called an
ion
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Ionic Bonds
• A cation is a
positively
charged ion
• An anion is a
negatively
charged ion
• An ionic bond is
an attraction
between an
anion and a
cation
When atoms gain electrons __________.
• the atoms become electrically neutral
• the atoms become positively charged
• their atomic mass significantly increases
• the atoms become negatively charged
Fig. 2-14-2
Ionic Bonds
Na
Cl
Na
Cl
Na
Sodium atom
Cl
Chlorine atom
Na+
Sodium ion
(a cation)
Cl–
Chloride ion
(an anion)
Sodium chloride (NaCl)
Figure 2.6 Ionic Attraction between Sodium
and Chlorine
Ionic bonds connect atoms together by __________.
• overlap of valence shells
• charge attractions
• overlap of the nucleus
• Ionic bonds do not connect atoms together.
Ionic Compounds
• Compounds formed by ionic bonds are called
ionic compounds, or salts
• Salts, such as sodium chloride (table salt), are
often found in nature as crystals
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Comparison of Ionic, Polar Covalent, and Nonpolar Covalent Bonds
Figure 2.9
Hydrogen Bonds
• A hydrogen bond
• forms between polar
molecules
• attraction between a
partially positive
hydrogen atom and a
partially negative atom
• individually weak,
collectively strong
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings
Hydrogen Bond in Water
Figure 2.4 Hydrogen Bonds Can Form
between or within Molecules
Copyright © The McGraw–Hill Companies, Inc. Permission required for reproduction or display.
Hydrogen atom
C
H +
O
C
O
Hydrogen bond
O
C

C
Oxygen
atom
C
O
H +
C
O
O
O
Glucose molecule
Water molecule
Fig. 2-16

+
Water (H2O)
+
Hydrogen bond

Ammonia (NH3)
+
+
+
Hydrogen bonds are similar to ionic bonds because
__________.
• both hydrogen bonds and ionic bonds are
due to opposite charge attractions
• they both occur between like charged atoms
• they both form molecules
• they are both very strong bonds