Chemistry 161
Lecture 8: Trends in Compound Formation, Naming Compounds & Writing
Formulas Sections 2.7, 2.8 in Gilbert, Kirss, Foster, and Davies 2nd edition
10/13/08, 10/14/08
Objectives
1. Know compounds come in 2 flavors – ionic and covalent. Define
covalent and ionic bonding. Define molecule as a covalently bonded
species.
2. Use electronegativities to rationalize the type of bonding found
between B,C,N,O with H, as well as the type of bonding found between
the halogens and the alkali metals, alkaline earth metals
3. Define the “Rule of 8” and utilize it to predict the bonding of period 2
elements with H
4. Write molecular and structural formulas for the row 2 elements
bonded to hydrogen; predict the degree and direction of polarity
5. Apply the formal system for binary molecular compounds
6. Appreciate why pure haloacids and aqueous solutions of haloacids are
named differently, and be able to name the common strong acids
7. Be able to rank polyatomic anions in terms of oxygen content based on
the perXate, Xate, Xite, hypoXite, and how this relates to the
corresponding acid
8. Describe the difference between molecular and ionic compounds, and
appreciate the how position in the periodic table influences the
formation of molecular and ionic species
o In particular, rationalize the bonding of the halogens and group
16 non-metals with group 1 and 2 metals
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o Develop the concept that compounds can be a combination of
ionic and covalent bonding – there are combinations of nonmetals that are particularly stable and ionizable. These are the
polyatomic ions
o Rationalize why ionic compounds formed from group 1 or 2
metals and group 16 or 17 non-metals have higher melting points
but better solubility in water than molecular compounds
9. For binary compounds – which may include transition metals of variable
oxidation state - given the charge on one element, deduce the charge
on the other
o Use the noble gas configuration of the group 1,2, 16, and 17 ions
to establish their charge, and use this to predict the charge on
the transition metal or polyatomic anion
10. Describe the name and charge of the major polyatomic anions
“derived” from major acids, and appreciate what confers their
stability (i.e. why they are major ions)
11. Describe the ammonium ion as the major polyatomic cation
12. Name simple (binary) ionic compounds
13. Name ionic compounds containing a polyatomic anion bonded to group 1
or group 2 metals
14. Name ionic compounds containing a transition metal bonded to a group
16 or 17 non-metal
o Use the numbering system for indicating the oxidation state of
the transition metal
o Define what is meant by the –ous and –ic endings
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2.7: Trends in Compound Formation
Ionic vs. Covalent Compounds
 Ionic Compounds:
 Covalent Compounds:
In Simple Ionic Bonding…
 Alkali metals: +1 Always
 Alkaline Earth metals: +2 Always
 Halogens: -1 Always
 Group 16 Non-metals: -2 Always
Question: Why do ionic compounds formed from group 1 or 2 metals and
group 16 or 17 non-metals have higher melting points but better solubility in
water than molecular compounds?
Question: How many electrons are in Na+? How many electrons are in O-2?
Deducing the charge on a transition metal
Question: What is the charge on iron in FeCl3? How about FeO?
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Question: What is the charge on Mn in potassium permanganate, KMnO4?
Question: Gold is a very stable metal (one of the few that exists as an
elemental form in its native state). However, gold can be solubilized with
aqua regia (a 3:1 mixture of 12M HCl and 16M HNO3), the end result being
AuCl4-. What is the oxidation state of solubilized gold cation?
On Electronegativites and Bonding Type
 The (loose) rule of thumb: Electronegativity differences greater than
1.9 result in an ionic bond
 Rule of 8 Defined:
The Bonding of H with the Period 2 Elements
Li
Be
B
C
N
O
F
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Properties of period 2 elements bonded to hydrogen
The curious behavior of LiH and and HF

LiH mp = 680 oC; highly basic

HF bp = 19.5 oC; HF is a weak acid in aqueous solution1
Of greater interest is the comparative behavior of CH4, NH3, and H2O:
Question: Aqueous HF is a weak electrolyte. What is occurring in aqueous
solution to this molecular species?
 Electrolyte defined:
2.8: Naming Compounds and Writing Formulas
The formal system for binary molecular compounds
 Lower group number to the left, higher group number to the right
 If bonding is within the same group, higher atomic number first, lower
atomic number second
Question: Is this system consistent with naming ionic compounds cations
first, anions second?
1
Curiously, HF is one of the strongest anhydrous acids known
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 Notice that – oddly – hydrogen is placed between groups 15 and 16
Common names that have replaced formal nomenclature
Gaseous vs. aqueous haloacids
Question: What is the structural formula for HCl in the gas state? What is
the structural formula for aqueous HCl?
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The relation of polyatomic oxoanions to their corresponding acids – an
introduction to polyatomic ions
Notice that while the above is an accurate representation of the formal
system for naming binary molecular species, one typically sees sodium
hypochlorite (bleach) as NaOCl (presumably due to the great number of
compounds that have halogens in the trailing position)
The major polyatomic ions
For our purposes
NH3 
H2CO3 
H2SO4 
HNO3 
H3PO4 
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Naming Simple (binary) ionic compounds
Remember: cation first, anion second
Question: LiH is named lithium hydride, whereas HCl is named hydrogen
chloride. What is implied by the –ide ending?
When dealing with transition metals, note that the old –ous and –ic endings
have largely been replaced with roman numerals
Question: Name Fe2O3 and FeO using the Roman numeral and –ous/-ic
systems
Question: What is the ionic formula for the dental caries prophylactic
stannous fluoride?
More Examples:
Magnesium oxide
Sodium phosphate
Chromium (III) chloride
Calcium sulfate
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