Introduction to Chemical Bonding

SECTION 6.1
Introduction to
Chemical Bonding
Teacher Notes and Answers
SECTION 1 Introduction to Chemical
Bonding
1.Atoms form chemical bonds to form more stable
arrangements of matter.
2.nonpolar-covalent bond
Practice
A.3.0 − 1.0 = 2.0; ionic; chlorine
B.3.5 − 3.0 = 0.5; polar-covalent; oxygen
C.3.0 − 2.8 = 0.2; nonpolar-covalent; chlorine
Review
1.Ionic bonding involves the electrical attraction
Covalent bonding involves the sharing of
electron pairs between two atoms.
2.A large difference in electronegativity between
two atoms in a bond will result in ionic bonding.
A small difference in electronegativity between
two atoms will result in covalent bonding.
3a.ionic
3b.polar-covalent
3c.nonpolar-covalent
4.I and Br, Cu and S, Li and F
5a.Cu and Cl
5b.Cu and Cl
6a.ionic
6b.the ​Br​−​ ion
between large numbers of anions and cations.
Chemical Bonding
1
SECTION 6.1
Introduction to
Chemical Bonding
Nature favors arrangements in which potential energy is
minimized. For example, a boulder is less likely to balance
at the top of a hill than it is to roll to the bottom of a valley.
The boulder at the top of the hill is not stable. Atoms are
usually not stable when isolated. They usually combine to
form more stable arrangements of matter.
Key Terms
chemical bond
ionic bonding
covalent bonding
nonpolar-covalent bond
polar
polar-covalent bond
Atoms become compounds by gaining, losing, or
sharing electrons.
A chemical bond is a mutual electrical attraction between
the nuclei and valence electrons of different atoms that
binds the atoms together. When atoms form a chemical
bond, their valence electrons are redistributed to make the
atoms more stable. The way the electrons are redistributed
determines the type of bond.
READING CHECK
1. Why do atoms form
chemical bonds?
Chemical bonding that results from the electrical
­attraction between positive ions and negative ions is called
ionic bonding. In a purely ionic bond, the metal atom gives
up its electron or electrons to the nonmetal atom. In
­covalent bonding, a bond forms from the sharing of electron
pairs between two atoms. In a purely covalent bond, the
shared electrons are “owned” equally by the bonded atoms.
IONIC BONDING
Many atoms
Anion B
Electrons transferred
from atoms A to
atoms B
+
+
+
Atoms A
Atoms B
+
-
-
Cation A
+
+
-
+
+
COVALENT BONDING
+
Atom C
Atom D
Two atoms
2
C HA P T E R 6
Electron pair shared
between atom C and
atom D
Atom C
Atom D
In ionic bonding, many atoms
transfer electrons. The atoms are then
held together by the attraction of
opposite charges. In covalent bonding,
atoms share electrons and form
independent molecules.
The second diagram at the right shows that there are two
types of covalent bonds. Bonding between two atoms of the
same element is completely covalent, because the two atoms
have the same electronegativity. For example, hydrogen is
usually found in pairs of atoms that are bonded together
covalently. The hydrogen-hydrogen bond is an example of a
nonpolar-covalent bond. Any bond formed between atoms
that have an electronegativity difference of 0.3 or less is
considered nonpolar-covalent.
A nonpolar bond has an even distribution of charge.
A bond that is polar has an uneven distribution of charge.
The electrons are more strongly attracted to the more
electronegative atom in a polar bond. A covalent bond in
which the atoms have an uneven attraction for the electrons
is called a polar-covalent bond. Bonds between atoms with a
difference in electronegativities from 0.3 to 1.7 are considered
polar-covalent bonds.
The hydrogen-chlorine bond in the second diagram at the
right is an example of a polar-covalent bond. Because
hydrogen has an electronegativity of 2.1 and chlorine has an
electronegativity of 3.0, the difference is 0.9. The electrons in
this bond tend to be closer to the chlorine atom than the
hydrogen atom. As a result, the chlorine end of the bond has a
partial negative charge, indicated by the symbol δ−. The
­hydrogen end of the bond has a partial positive charge,
­indicated by the symbol δ+.
Difference in electronegativities
The first diagram at the right summarizes how the
­difference in electronegativity affects the percentage
of the bond that is ionic in nature. If the difference in
­electronegativity of two atoms is 1.7 or greater, the bond
is at least 50% ionic in nature, and the bond is considered
an ionic bond.
3.3
100%
Ionic
1.7
50%
Polar-covalent
0.3
0
Nonpolar-covalent
Percentage ionic character
Ionic or Covalent?
Bonding between atoms of different elements is usually not
purely ionic or purely covalent. It usually falls somewhere
between the two extremes. The difference between the
electronegativities of the two atoms determines the type of
bond they form.
5%
0%
The electronegativity difference
between two atoms determines the
type of bond that forms.
Hydrogen nuclei
(a) Nonpolar-covalent bond
Hydrogen
nucleus
δ+
Chlorine
nucleus
δ-
(b) Polar-covalent bond
The difference in electron density
in (a) a nonpolar hydrogen-hydrogen
bond and (b) a polar hydrogenchlorine bond.
READING CHECK
2. A bond between two atoms in
which the shared electron or
electrons are equally likely to be
found near each atom is called a
.
Chemical Bonding
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SAMPLE PROBLEM
Use the electronegativity differences given in the chapter “The Periodic
Law” and the first diagram on the previous page to classify bonding
between sulfur, S, and the following elements: hydrogen, H; cesium, Cs; and
chlorine, Cl. In each pair, which atom will be more negative?
SOLUTION
1 ANALYZE
Determine what information is given and unknown.
iven: electronegativity values for S, H, Cs, and Cl
G
Unknown: bond types for S with H, Cs, and Cl
2 PLAN
Determine the information necessary to answer the questions.
Look up the electronegativity values for each element. Find
the difference between the electronegativities. Use the
diagram on the previous page to determine the type of bond.
3 SOLVE
Determine the types of bonds from the given information.
The electronegativities are 2.5 for S, 2.1 for H, 0.7 for Cs,
and 3.0 for Cl. In each pair, the atom with the greater
electronegativity will be the more negative atom.
Electronegativity
difference
Bond type
More-negative
atom
S and H
2.5 − 2.1 = 0.4
polar-covalent
S
S and Cs
2.5 − 0.7 = 1.8
ionic
S
S and Cl
3.0 − 2.5 = 0.5
polar-covalent
Cl
Bonded atoms
4 CHECK
YOUR
WORK
PRACTICE
Determine if the answers make sense.
Cs is a metal, so it is likely to form ionic compounds with a
nonmetal. H and Cl are more likely to form covalent bonds.
Complete the following table.
Bonded atoms
A. Cl and Ca
B. Cl and O
C. Cl and Br
4
C HA P T E R 6
Electronegativity
difference
Bond type
More-negative
atom
SECTION 6.1 REVIEW
VOCABULARY
1. What is the main distinction between ionic and covalent bonding?
REVIEW
2. How is electronegativity used in determining the ionic or covalent character
of the bonding between two elements?
3. What type of bonding would be expected between the following pairs
of atoms?
a. Li and F b. Cu and S c. I and Br 4. List the three pairs of atoms from Question 3 in order of increasing ionic
character of the bonding between them.
Critical Thinking
5. INTERPRETING CONCEPTS Compare the following two pairs of atoms:
Cu and Cl; I and Cl.
a. Which pair would have a bond with a greater ionic character?
b. In which pair would Cl have the greater negative charge?
6. INFERRING RELATIONSHIPS The isolated K atom is larger than the isolated
Br atom.
a. What type of bond is expected between K and Br?
b. Which ion in the compound KBr is larger?
Chemical Bonding
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