Chapter 3
Ionic compounds
3.1 Ions
Atoms have the ability to gain or loose electrons and become ions.
Ion: an electrically charged atom or group of atoms.
Cation: an atom that has lost one or more electrons
(a positively charged ion).
Anion: an atom that has gained one or more electrons
(a negatively charged ion.)
• Atoms tend to gain or loose electrons in order to attain a noble gas
electron config.
− Metals tend to become cations.
− Cations are smaller than the parent element,
•
• Sodium atom
vs
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sodium ion
− Nonmetals tend to become anions.
− Anions are larger than the parent element.
chlorine atom
vs
chloride ion
Writing Ions
We write the charge for an ion in the upper right-hand corner of the
atomic symbol.
- One (+) for each electron lost
- one (-) for each electron gained.
The number comes before the +/- symbol.
Problem: An aluminum atoms loose three electrons when it becomes an
ion.
a) Write the symbol for the ion that results. Al3+
b) Is it a cation or an anion? cation
.
Problem: An oxygen atoms gains two electrons when it becomes an ion.
a) Write the symbol for the ion that results. O2b) Is it a cation or an anion? anion
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3.2 Periodic Properties and Ion Formation
Ionization Energy (IE)
The ionization energy is the energy required to remove one
from an atom or ion in the gaseous state.
electron
Atom(g) + ENERGY Cation + electron
THE LOWER THE IE, THE EASIER IT IS TO REMOVE AN ELECTRON!
Trends in Ionization Energy
a) I.E. increases as you go
right
across the periodic table.
b) I.E.
decreases as you go
down the group or family.
Problem: Which element in each of the following pairs is likely
to loose an electron more easily?
a) Be or B? Be (i.e. farther to the left)
b) Ca or Mg?
Ca (i.e. lower)
Electron Affinity
Electron afinity is defined as the energy released when an electron is
added to the lowest energy unoccupied orbital in a gaseous atom.
Atom + electron Anion + ENERGY
THE HIGHER THE EA, THE EASIER IT IS TO ADD AN ELECTRON!
a) E.A. increases as you go
right
across the periodic table.
b) E.A.
decreases as you go
down the group or family.
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Relative ionization energies
(red) and electron affinities
(blue) for elements in the first
four rows of the periodic
table.
Take-away lesson:
• Halogens gain electrons most easily.
• Alkali metals loose electrons most easily.
• Noble gases neither loose nor gain electrons.
• Nonmetals in groups 14, 15, 16 do not find it easy to gain or loose e− They sometimes form ionic compounds or ionic bonds.
− Frequently they form covalent bonds, which share electrons. (We
will see this in the next ch.)
Problem: Which element in each of the following pairs is likely
to gain an electron more easily?
a) F or Br? (F because higher)
b) Cl or P? (Cl because farther right)
• Elements that lose an electron, and those that gain an electron will
react with each other by transfer of an electron from the metal to
the nonmetal.
• The product that results is electrically neutral.
• Ion-transfer reactions of metals and nonmetals form products
unlike either element.
− When iron reacts with oxygen to form rust, the product rust
in not like iron or oxygen.
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3.3 Ionic Bonds
In substances made up of positively charge cation and negatively
charged anions, the + and - are attracted to each other like the poles of a
magnet.
Ionic Bond: The electrical forces of attraction between ions of opposite
charge in a crystal.
Ionic bonds are formed by a transfer of electrons.
Ionic Solid: A crystalline solid that is held together by ionic bonds.
Ionic Compound: A compound that contains ionic bonds.
3.4 Some Properties of Ionic compounds
• Ions in each compound settle into a pattern that efficiently fills space
and maximizes ionic bonding.
• Ions in an ionic solid are held rigidly in place by attraction to their
neighbors.
• Each ion in NaCl is attracted to 6 others.
•
•
• The ions in the solid are
packed in a 3-D array where
each cation is attracted to
multiple anions (show NaCl
model).
•
•
• Electricity can only flow through a medium containing
a) charged particles
b) that are free to move.
• Once an ionic solid is dissolved in water, the ions separate.
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• They can move freely, and conduct electricity. (known as electrolytes)
Ionic Compounds
• Have high melting points and boiling points.
o NaCl melts at 801 °C and boils at 1413°C.
• Shatter if struck sharply
• Dissolve in water if
the attraction between water and the ions
is greater than
the attraction of the ions for each other.
o Most (but not all) ionic compounds are water soluble.
3.5 Ions and the Octet Rule
Main Group Ions (Groups 1,2 & 13-18)
Main Group elements tend to gain or loose electrons in order to obtain
an electron configuration similar to a stable noble gas.
Octet Rule: Main group elements tend to undergo reactions that leave
them with 8 valence electrons.
Na [Ne] 3s1 + F [He] 2s2 2p5 ⇒ Na+ [Ne] + F- [Ne]
• Alkali metals have 1 more electron than a noble gas.
• Halogens have 1less electron than a noble gas.
• When halogens and alkali metals react, an electron is transferred.
• Each now has a noble gas electron configuration.
Ca [Ar]4s2 + O[He] 2s2 2p4 ⇒ Ca2+ [Ar] + O2- [Ne]
Al [Ne] 3s2 3p1 + N [He] 2s2 2p3 Al3+ [Ne] + N3- [Ne].
F-, O2-, and N3- all have the same electron configuration [Ne].
These ions are isoelectric.
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3.6 Ions of Some Common Elements
E.g. Group 1 (1A) lose 1 e- to become 1+ ions (Li, Na, K, Rb, Cs)
Group 2 (2A) lose 2 e- to become 2+ ions (Be, Mg, Ca, Sr, Ba)
Group 13 (3A) lose 3 e- to become 3+ ion (Al)
Group 17 (7A) gain 1 e- to become 1– ions (F, Cl, Br, I)
Group 16 (6A) gain 2 e- to become 2– ions (O, S, Se)
Group 15 (5A) gain 3 e- to become 3– ions (N, P)
Group 14 (4A) gain 4 e- to become 4– ions (C)
3.7 Naming Ions
Naming Cations
1. Fixed Charge Cations
Groups 1, 2, Al3+, and a few transition elements, which only form 1
valence state, (e.g. Zn 2+ Cd2+ and Ag+) form only one ion. They
simply have the word "ion" added after their name.
Problem: Write the formula and give the name for the cation that can
form from the following metals.
– Na
Na+ sodium ion
– Mg Mg2+ magnesium ion
– Al
Al3+ aluminum ion
2. Variable Charge Cations
• Transition metals (most) can have multiple different charges.
• New Stock System uses metal name with the valence shown as a
Roman Numeral in parenthesis.
• Old Greek/Latin System uses "ic" ending for the higher common
charge and "ous" ending for the lower common charge.
These just need to be memorized!
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Practice: give the New and Old System names for the following ions.
New System
(Stock System)
Old System
Fe2+
Fe3+
iron(II) ion
iron(III)ion
ferrous ion
ferric ion
Sn2+
Sn4+
tin(II) ion
tin(IV)ion
stannous ion
stannic ion
Hg2+
mercury(II) ion
mercuric ion
Many others that
are on a list for
you to memorize.
3. Polyatomic cations (only 3 common ones)
Hg22+
mercury (I) ion
mercurous ion
NH4+
H 3O +
ammonium ion
hydronium ion
Naming Anions
1. Monatomic anions
Named by replacing the ending of the element’s name with “ide”.
Group 17 anions (of halogens) are called halides
Practice: Write the formula and give the name for the anion that
forms when the following elements gain enough electrons to fill their
octet.
Cl
Cl-
chloride
F
F-
fluoride
O
O2- oxide
S
S2-
N
N3- nitride
sulfide
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3.8 Polyatomic Anions
Polyatomic ion: An ion that is composed of more than one atom.
Most polyatomic anions contain oxygen.
The formulas includes subscripts to tell us how many of each type of
atom is present in the one polyatomic ion
The atoms in a polyatomic ion are held together by covalent bonds.
(a sharing of electrons)
The entire group of atoms act as a single unit.
Ions ending in “ate” indicates the base number of oxygens
(typically 3 but sulfate and phosphate have 4)
Practice: write names for the following polyatomic ions:
• SO42-
sulfate ion
.
SO32- sulfite ion (after series intro)
• PO43-
phosphate ion
.
PO33- phosphite ion (after series intro)
• CO32-
carbonate ion
.
• NO3-
nitrate ion
.
• ClO3-
chlorate ion
.
• BrO3-
bromate ion
.
• IO3-
iodate ion
.
NO2- nitrite ion
(after series intro)
Some can form a series,
− ClO4-
perchlorate ion “per” means one more__________
“O” than "ate"
− ClO3-
chlorate ion
− ClO2-
chlorite
“ite” means one less__________
“O” than “ate”
− ClO-
hypochlorite
“hypo” means one less__________
“O” than "ite"_______
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Other polyatomic ions you should know:
Ion Name
Thiocyanate
Chromate
Dichromate
Hydrogen
Carbonate
(Bicarbonate)
Oxalate
Formula
SCNCrO42Cr2O72-
HCO3
-
C2O42-
Cyanide
CN-
Nitrite
NO2-
Ion Name
Hydroxide
Permanganate
Acetate
Hydrogen
phosphate
Dihydrogen
phosphate
Hydrogen sulfate
(Bisulfate)
Hydrogen sulfite
Formula
OHMnO4CH3COOaka CH3CO2aka C2H3O2HPO42H2PO4HSO4HSO3-
3.9-10 Formulas and Naming of Ionic Compounds
Naming Ionic Compounds
− Name of cation followed by name of anion
− Do not include the work "ion" in the name of the compound
− Do not name quantities of each ion.
Formulas of Ionic Compounds
• All compounds are neutral.
• The total positive charge (from the cations)
must equal
the total negative charge (from the anions).
• Polyatomic ions are treated as a single unit.
Case 1: If the charge of the cation and the anion are the same:
1) Write the cation followed by the anion.
2) Do not show charges of the ions in the final formula
3) Do not use parentheses.
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Practice:
Formula
Name
K+ + F– →
KF
potassium fluoride
Ca2+ + O2- →
CaO
calcium oxide
Al3+ + N3- →
AlN
aluminum nitride
SrSO4
strontium sulfate
Sr2+ + SO42- →
.
NH4+ + Br- →
NH4Br
ammonium bromide
Case 2: If the charges on the anion and cation are different,
1) Use the charge of the cation as a subscript for the anion.
2) Use the charge of the anion as a subscript for the cation.
3) Include polyatomic ions in parentheses if the subscript is >1.
4) Do not show charges of the ions in the final formula.
Note: If both charges are even, divide both numbers by 2.
−
(Note: Reactants are not balanced.)
−
Practice – using fixed charge cations
−
Ions
Formula
Name
−
Li+ + O2- →
−
Mg2+ + Cl- → MgCl2
magnesium chloride
−
Al3+ + S2- →
aluminum sulfide
−
Cd2+ + NO3- → Cd(NO3)2
cadmium nitrate
−
NH4+ + N3- → (NH4)3N
ammonium nitride
−
Sr2+ + S2- →
strontium sulfide
Li2O
Al2S3
Sr2S2→SrS
lithium oxide
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.
More Practice – using variable charge cations:
− Formula
new name
old name
− Cu2SO4
cuprous sulfate
copper(I) sulfate
− Sn(NO3)4
stannic nitrate
tin(IV)nitrate
− PbCO3
plumbous carbonate
lead(II) carbonate
− Hg2Br2
mercurous bromide
mercury(I) bromide
− FePO4
ferric phosphate
iron(III) phosphate
Naming Binary Compounds made of Two Nonmetals (Covalent
Compounds)
Rule: Greek prefixes are used to designate the number of atoms of each
element present.
− Prefixes are mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca…
− The prefix mono is omitted for the first element
− The 'o' in mono and the 'a' in penta are dropped when the prefix is attached
to a name starting with the letter 'o', e.g., CO is carbon monoxide not carbon
monooxide
− The second element gets the suffix "ide"
− The IUPAC ordering scheme is:
B>Si>C>P>N>H>S>I>Br>Cl>O>F
− Examples: N2O3 dinitrogen trioxide
CO carbon monoxide
P2O5 diphospherous pentoxide
SF6 sulfur hexafluoride
CCl4 carbon tetrachloride
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