Transcript Chapter 7: Chemical Formulas and Bonding

Some Questions to Consider
• Why are so few elements (such as Au, S,
Ar, N, O, Ag) found free in Nature as
atoms?
• Why do atoms of different elements
combine (react) to form compounds?
• What is happening in this process?
• How can we explain the tremendous
number of compounds that are known
today?
• Many of the answers will be found in
Chapter 6 (“Chemical Formulas and
Bonding”), and they are the result of a ‘tug
of war’ between elements. (Example)
Chapter 7:
Chemical Formulas
and Bonding
PPowell 05
7-1 Objectives
• Describe the distinguishing characteristics
of an ionic bond.
• Describe some properties of ionic
compounds.
• Explain the “octet rule.”
• Draw Lewis dot diagrams to show the
valence electrons of an atom.
• Distinguish among anions, cations, and
polyatomic ions.
• Name binary ionic compounds.
• Write the empirical formula for binary ionic
compounds.
Ions: A Review
• Cation: a positively charged ion.
– Metals tend to form cations. (How?)
• Anion: a negatively charged ion.
– Nonmetals tend to form anions. (How?)
• How do you think cations and anions
will interact?
OBJ: Describe the distinguishing characteristics of an ionic bond.
Ionic Compounds
• Ionic bond: results from the attraction
between a positive ion and a
negative ion.
– Most metals will form ionic bonds with
most nonmetals.
• Ionic compound: consists of cations
and anions.
– Electrically neutral; the total charges of
the cations and anions must balance.
OBJ: Describe the distinguishing characteristics of an ionic bond.
Video Clip:
Ionic Bonds In Action
• Reaction of magnesium and oxygen
releases light energy.
• Product (MgO) is more stable than the
reactants alone.
OBJ: Describe the distinguishing characteristics of an ionic bond.
Why is magnesium oxide more
stable than elemental magnesium
and oxygen?
• Compare electron configurations of the
atoms to their ions:
 Mg: [Ne]3s2
 Mg  Mg2+ + 2e• Mg2+ = [Ne]2+
 O:[He]2s22p4
 O + 2e-  O2• O2-: [He]2s22p6, or [Ne]2-
• Noble Gas electron configurations are
more stable!
7-1A: Describe the distinguishing characteristics of an ionic bond.
The Octet Rule
• Atoms tend to gain, lose, or share
electrons in order to get a full set of
valence electrons.
• Most atoms have eight valence electrons
in a full set.
– Exceptions: H, He (Why?)
• The representative elements act like
“noble gas impersonators.”
OBJ: Explain the octet rule.
The Octet Rule
• Metals will lose e- to “impersonate”
a noble gas.
– Metals form cations.
• Nonmetals will gain e- to
“impersonate” a noble gas.
– Nonmetals form anions.
OBJ: Explain the octet rule.
The Role of Valence Electrons
• Note that the valence electrons were
involved in these changes, NOT the core
electrons.
– Why? (Which orbitals & electrons are
encountered first when two atoms interact?)
• Chemists focus on the valence electrons
(s and p outer electrons) to understand
the chemistry of atoms.
• To aid us, we use shorthand diagrams,
called Lewis Dot Diagrams, where dots
represent the valence electrons around an
atom.
LEWIS
DOT
DIAGRAMS
Lewis Dot diagrams
• A way of keeping track of
valence electrons.
• How to write them:
– Write the symbol.
– Put one dot for each
valence electron.
– Don’t pair electrons until
you have to.
X
OBJ: Draw Lewis dot diagrams to show the valence electrons of an atom.
Lewis Dot Structure:
Nitrogen
• Nitrogen has 5 valence electrons.
• Write the symbol.
• Draw one dot per side of the symbol:
top, right, bottom, left.
• Don’t pair electrons until you have to.
N
OBJ: Draw Lewis dot diagrams to show the valence electrons of an atom.
Dot Diagram:
Ionic Bond Formation
Na
Cl  Na
Cl  Na+ Cl
This is sodium chloride. In
solid form, it would have
the lattice structure.
Is the octet rule still satisfied for Na+?
OBJ: Draw Lewis dot diagrams to show the valence electrons of an atom.
-
With a partner, decide what IONS
those elements are likely to form,
based on their Lewis dot structure.
• Na  Na+ + 1e- gives Na+ = [Ne]1+
• Mg 
• N+
Mg2+ +
3e-

N3-
2e-
gives
gives
N3-
Mg2+
=
=
[Ne]2+
[Ne]3-
• F + e-  F- gives F- = [Ne]1-
F
N
+
Na
Mg
3-
-
• Most ions form in order to get the electron
configuration of the nearest noble gas.
OBJ:Explain the octet rule.
2+
Lewis Dot Diagram Practice:
Element
Electron Configuration
Li
[He]2s1
Be
[He]2s2
B
[He]2s22p1
C
[He]2s22p2
N
[He]2s22p3
O
[He]2s22p4
F
[He]2s22p5
Ne
[He]2s22p6
Al
[Ne]3s23p1
P
[Ne]3s23p3
Lewis Dot
Diagram
Important Note
• When an anion is formed, the
electron(s) come(s) from some
other atom.
• When a cation is formed, the original
atom has donated its electron(s) to
some other atom.
• In other words, the previous slide
only showed you only half the story.
OBJ: Describe the distinguishing characteristics of an ionic bond.
What about Argon?
• Ar is a noble gas.
• Ar has an octet.
• Ar is INERT, or unreactive.
Ar
OBJ:Explain the octet rule.
Types of Ions
• Monatomic ion: has one atom.
– May be cations or anions.
• Polyatomic ion: has >1 atom.
– May be cations or anions.
OBJ: Distinguish among anions, cations, and polyatomic ions.
Monatomic Cations
• See p. 231 Figure 7-10
Monatomic Anions
• See p. 232 Figure 7-11
Polyatomic Ions
• Can you find them in Figures 7-10
and 7-11?
OBJ: Distinguish among anions, cations, and polyatomic ions.
Structure of Ionic
Compounds
• The ions within an ionic compound
arrange to maximize their electrical
attraction.
• This results in a repetitive pattern called
a “lattice network.”
Cl -
Na+
7-1B: Describe some
properties of ionic
compounds.
Properties of Ionic Compounds
• High bond strength means high
melting points.
• Often dissolve in water (soluble).
– Ions in solution move freely and allow
the solution to conduct electricity.
+
+
-
+
+
• Ionic compounds are not good
conductors as a solid, but they are
- when melted.
+ • Ionic compounds are brittle.
+
+ - +
7-1B: Describe some
properties of ionic
compounds.
Binary Ionic Compounds
• Have ions of two different elements.
• Do not always have 1:1 ratio of the
two ions, because charges have to
balance (electrical neutrality).
• Ratio of ions in a compound is given
by the empirical formula:
CaF2
1 Ca2+ for every 2 FOBJ: Name binary ionic compounds.
Naming Binary Ionic
Compounds
CaF2
• Name the cation, then the anion.
• Change the ending of the anion to
“-ide.” Calcium Fluoride
Try naming these:
a) Potassium Chloride
b) Iron Oxide
a) KCl
b) FeO
c) Zinc Fluoride
c) ZnF2
d) Ammonium Chloride
d) NH4Cl
OBJ: Name binary ionic compounds.
Writing Formulas for Binary
Ionic Compounds
Given: Potassium Nitride
1) Write the element symbols and the
charges for both the cation and the anion.
1+
3-
K3 P1
2) Criss-cross the two charges.
3) Reduce (simplify) the formula if necessary.
OBJ: Write the formula for binary ionic compounds.
7-1 Practice Problems
• Do problems 1-5 with a partner.
• Do problems 6-10 on your own.
OBJ: Write the formula for binary ionic compounds.
Using ion cards, write the correct formulas for the
following:
F-
NH4+
K+
Li+
Na+
Ag+
Mg2+
Ca2+
Fe2+
Al3+
Cl-
Br-
SO42-
CO32-
NO3-
PO43-
CrO42-
CH3COO-
Can you now meet the
objectives for 7-1?
• Describe the distinguishing characteristics
of an ionic bond.
• Describe some properties of ionic
compounds.
• Explain the “octet rule.”
• Draw Lewis dot diagrams to show the
valence electrons of an atom.
• Distinguish among anions, cations, and
polyatomic ions.
• Name binary ionic comounds.
• Write the formula for binary ionic
compounds.