Chapter 9
Chemical Bonding
ALABAMA 8TH
5
GiADE SCIENCE STANDARDS
COVERED IN THIS CHAPTER INCLUDE:
Differentiate between ionic and covalent bonds.
.
Illustrating the transfer or sharing of electrons using electron dot
diagrams
BONDING OF ATOMS
Recall from Chapter 5 that an element is a substance composed of identical atoms.
When atoms oftwo or more elements combine chemically, they form a compound. This
new compound has completely different properties than the individual elements from
which it is made. (Recall the example of hydrogen and oxygen combining to form
water.)
r.
:
A compound is made up of identical molecules. One molecule of water consists of two
atoms of hydrogen (H) and one atom of oxygen (0). The chemical formula that
describes each molecule of water is H20. A molecule is the product of two or more
atoms joined by chemical bonds. The chemical bond is the force that holds atoms
together. Recall, atoms of different elements can combine chemically to form molecules
by sharing or by transferring valence electrons. Valence electrons are either lost, gained
or shared when bonds are formed. Chemical bonding can occur in two main ways: ionic
bonding and covalent bonding.
0
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IONIC BONDS
.
I
I
An atom with a charge is called an ion, and it is formed by the transfer of electrons.
When one atom “takes” electrons from another atom, both are left with a charge. The
atom that took electrons has a negative charge. (Recall that electrons have a negative
charge). The atom that gave electrons has a positive charge. The bond formed by this
transfer is called an ionic bond.
Ionic bonds are very strong. Ionic compounds have high melting points and high boiling
points. These compounds tend to have ordered crystal structures and are usually solids
at room temperature. Ionic compounds will usually dissolve in water, and they have the
ability to conduct electricity in a watery solution or a molten state.
75
Chemical Bonding
Sodium chloride (table salt) is an example of a
compound with an ionic bond. In salt, one atom
of sodium reacts with one atom of chlorine. The
sodium gives up one electron. This forms
positive ions with 1 charge. The chlorine atom
gains one electron given up by the sodium atom.
This forms a negative chloride ion with a 1
charge. Figure 9. 1 illustrates this electron
transfer. Note that the orbital shape (circular)
has been simplified for clarity.
COVALENT BONDS
Covalent bonds are formed when two or more
Sodium Chloride
elements share valence electrons. They share
Figure 9.1 Ionic Bonding ofTable Salt
electrons in such a way that their valence
electron orbital is filled. The sharing arrangement creates a more stable structure in the bound
elements than was present before the bonding occurred. In general, there are two rules about
elements that form covalent bonds:
1 Elements with similar electronegativities form covalent bonds.
.
2. Nonmetals form covalent bonds.
Covalent bonding results in compounds that have low melting points and low boiling points. In
general, they do not conduct electricity well. Let’s look at an example of a covalent bond.
Carbon and hydrogen have very similar electronegativities and commonly form covalent
bonds. The covalent bond between one atom of carbon and four hydrogen atoms is methane
( CM4). Each of the four hydrogen atoms shares one electron with a single carbon atom.
Likewise, the carbon atom shares its four valence electrons. One is shared with each ofthe four
hydrogen atoms. This arrangement gives the carbon atom a full valence shell containing eight
electrons. Hydrogen’s valence orbital can only contain two electrons. This means that each
hydrogen atom has a full valence shell as well. The valence orbitals ofcarbon and hydrogen are
drawn in Figure 9.2. The overlap of orbitals represents shared electrons.
Ii
I;
,
‘%
“%
C’
I:
/ 1
_•_•••%
,
(H
%
H
Figure 9.2 Methaie Molecule (CR4)
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0
0
76
Chapter 9
ELECTRON DOT DIAGRAMS
An electron dot diagram is the symbol of an element surrounded by dots. The dots represent
the number of electrons in the elements outer energy level. Electron dot diagrams are used to
show how atoms bond with each other.
The dots around the symbol are placed in a particular order: in pairs
..
on all four sides ofthe element symbol. Figure 9.3 shows the electron
dot diagram for nitrogen. Nitrogen contains five electrons in its outer
.
.
energy level. To draw the dots, place one dot on the top ofthe element
to
the
clockwise,
a
dot
adding
around
way
symbol. Then work your
right, bottom and left side ofthe symbol. Add the fifth dot to the top
.
to make a pair. If there were more electrons in the outer shell, you
Figure 9.3 Electron
would continue to add dots in this same manner until you complete Dot Diagram for Nitrogen
the level.
Activity
Lets practice making some electron dot diagrams ofelements. Remember, we are using dots
to represent valance electrons. Draw dot diagrams around the following elemental symbols:
3.
2.
1.
P
Ne
Mg
Na
F.
Co
In
6.
5.
4.
S
In
CO
Co
0
-J
a.
D
9.
8.
7.
H
F
I-
0
z
0
C-
E
0
0
.
0
0
a,
E
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.
C0
0
77
Ar
Chemical Bonding
Let’s return to our example of a covalent bond, methane (CH4). Figure 9.2 showed us how the
valence electrons are shared between the atom of carbon and the four atoms of hydrogen in a
molecule of methane. Figure 9.4 below shows how the electrons are shared using an electron
dot diagram. Carbon has four electrons in its outer shell, so its dot diagram is a “C” with one
dot on each side of its symbol. An atom of hydrogen has only one electron in its outer shell.
Carbon needs eight electrons total to fill its outer shell, so it shares an electron with four
different hydrogen atoms. Hydrogen atoms need two electrons to become stable. The dot
diagram shows that the bonding between carbon and hydrogen atoms creates full valence shells
for the carbon atom and each hydrogen atom. The white circles in the figure represent the
shared electrons. In a normal diagram, the dots are all the same color. Here, they are shown
differently for your learning benefit.
H
HCH
H
Figure 9.4 Electron Dot Diagram for Methane
Electron dot diagrams also illustrate the transfer of electrons during ionic bonding. Let’s return
to our earlier example of our ionic bond, sodium chloride. Figure 9. 1 illustrated the transfer of
one electron from a sodium atom. The chlorine atom only needs one electron to become stable,
so it gained this electron from the transfer. This gave both the sodium and the chlorine a more
stable electron configuration. The sodium atom was left with a positive charge (+f) while the
chlorine atom now has a negative charge (l). This electron transfer is illustrated in Figure 9.5
using an electron dot diagram.
N.C1:—Na:C1:
Figure 9.5 Electron Dot Diagram for Sodium Chionde
0
z
0
0
E
0
0
0
0
0)
2
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.
0.
0
0
78
Chapter 9
Activity
For each compound in the table, determine the type ofbond that formed to create the
compound. Write “I” for ionic or “C” for covalent under the second column. Draw the
appropriate electron dot diagram to represent the bonding of electrons in the space
provided. (Hint: Refer to the Periodic Table on page 54 to determine the number of
electrons in the outer shell of each atom).
Compound
Dot Structure
Type of Bond
(IorC)
Co2
H20
Co
C2H6
c.1
Co
Co
C?
LU
0
-J
0
I-
CaCl2
0
0
0
E
0
0
0
0
Cu
C)
C)
E
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0)
0
0
0
79
Chemical Bonding
CHAPTER 9 REvIEw
1.
Which ofthe following characteristics does a covalent bond have?
A
B
C
B
2.
high melting and high boiling points
atoms share electrons to bond
conducts electricity
all ofthe above
Ionic and covalent bonding take place between
A
B
C
B
neutrons.
protons.
valence electrons.
polar molecules.
3.
Which ofthe following molecules is most likely to have a covalent bond?
A
C
MgO
°2
B
NaCl
D
Fe203
4.
One atom of lithium and one atom of chlorine undergo ionic bonding. One atom of
lithium has one electron in its outer shell and chlorine has 7. Which ofthe following
shows the correct charges for the new compound lithium chloride (LiC1)?
A
B
C
B
Li Cl
Lf1 C1
Li2 Cl2—
Li2 C12
5. Two atoms of sodium (Na) and one atom of oxygen undergo ionic bonding. One atom
ofNa has 1 electron in its outer shell, and one atom of 0 has 6 electrons. Which of
the following electron dot structures shows the correct transfer of electrons?
A
C
B
:NaONa:
NaO1a
B
N\
••:Na
•Q•%A
-Na
Na
80
..
.0: