11 Chem
Chapter 8 Ionic Bonding
1
Keeping Track of Electrons
Valence electrons are the electrons responsible for the chemical properties of atoms and are those
electrons in the outermost energy level (highest number). Valence electrons are found in the s and p
sublevels in the outermost energy level (valence shell). The core electrons are those electrons in the
energy levels below the valence shell (inside energy levels).
Atoms in the same column have the same valence electron configuration. The valence electron
configuration can easily be found by looking up the group number on the periodic table.
Group number
#of valence e-
1A
1
2A
2
3A(13)
3
4A(14)
4
5A(15)
5
6A(16)
6
7A(17)
7
8A(18)
8
Electron Dot diagrams (Lewis structure)
The electron dot diagram is a way of keeping track of and presenting the valence electron structure. To
write the electron dot structure; you write the element symbol, which represents the core (inner electrons
and nucleus) and then put one dot for each valence electron. Don’t pair up until they have to
Write the electron dot diagram for
·
Na
Na
Mg
·
Mg ·
·
·C·
·
··
O
·O:
·
··
F
· F:
··
··
Ne
: Ne :
··
··
He
He Electrons are paired because the valence shell is full.
Atoms are electrically neutral because atoms have the same number of protons and electrons.
Ions are atoms, or groups of atoms, with a charge because they have different numbers of protons and
electrons. Only electrons can move, so ions are formed by the gain or lose of electrons.
C
Electron Configurations for Cations
Metals will have few valence electrons that are shielded from the nuclear charge. These valence
electrons are weakly attracted to the nucleus and will come off easily during chemical reactions. Metals
lose valence electrons to attain noble gas (stable) configuration. Metals react to make positive ions
(cations). If we look at an electron configuration, it makes sense. Na 1s22s22p63s1 loses 1 valence
electron to become Na+ with an electron configuration of 1s22s22p6 that is the same as the noble gas Ne.
Electron Configurations for Anions
Nonmetals have a many valence electrons that are poorly shielded by the core electrons and their
valence shells are almost full. Nonmetals strongly attract valence electrons and can become more stable
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Chapter 8 Ionic Bonding
2
by gaining electrons to attain noble gas configuration. Nonmetals make negative ions (anions). If we
look at the electron configuration of Sulfur this makes sense. S 1s22s22p63s23p4 has 6 valence electrons
and if it gains two valence electrons it becomes S-2 1s22s22p63s23p6 that has noble gas electron
configuration for Ar.
Stable Electron Configurations
All atoms react to achieve the stable (low energy) noble gas configuration. Noble gases have 2 s and 6 p
electrons a total of 8 valence electrons.
All atoms react by changing their number of valence electrons to acquire a complete valence shell
(called the octet rule). Not all stable ions result in the noble gas configuration; there are a few
exceptions mainly in the transition metals. Zn 1s22s22p63s23p63d104s2 loses the two valence electrons to
become Zn2+ 1s22s22p63s23p63d10 that is stable but does not have the configuration of a noble gas. It
does have a complete valence shell. This is known as the pseudo noble gas electron configuration. Other
ions like Cu+, Ag+, Au+ and Cd2+ have pseudo noble gas configurations.
Ionic Bonding
Ionic bonds occur when the reacting elements have a large difference in electronegativity values (a
difference of approximately 2). Metals are born losers and nonmetals are winners, so when they react
there is a transfer of valence electrons. Cations and anions with stable noble gas configurations result
from the transfer of the valence electrons. The resulting electro statically charged ions emit positive and
negative force fields. These ions can become more stable by neutralizing the force fields. A positive
force field neutralizes a negative force field, a force of attraction results. Anions and cations are held
together by these oppositely charged force fields. This force of attraction is called an ionic bond. Ionic
compounds contain ionic bonds between a metal ion and a nonmetal ion. Ionic compounds are called
salts. The simplest ratio of cation to anion is called the formula unit. The bond is formed through the
transfer of electrons. All the electrons must be accounted for, the total number of electrons lost must
equal the total number of electrons gained.
Na reacts with Cl. The Na loses 1 electron to form Na+ while the Cl gains 1 electron to form Cl-. The
ratio is 1:1, shown in NaCl.
Al reacts with S. The Al loses 3e- to form the Al3+ ion while S gains 2e- to form the S2- ion. The number
of electrons lost, 3, does not equal the number of electrons gained. To get the total lost equal to the total
gained there must be 2 Al (loss of 6) and 3 S (gain of 6) so the compound would be in a ratio of 2:3
Al2S3.
Properties of Ionic Compounds
As mentioned above, an ionic compound can be more stable when the positive and negative force fields
are neutralized. Because the bonds are strong ionic compounds have high melting points and form brittle
solids.
Positive fields are neutralized when surrounded by as many negative fields as space will allow. Negative
force fields are neutralized when surrounded by as many positive fields as space will allow (closely
packed like charges repel each other). The oppositely charged ions arrange themselves in a regular
repeating pattern that neutralizes the force fields. This repeating pattern of ions creates a crystalline
structure. This crystalline structure is rigid because ions are strongly bonded.
In NaCl the Na+ is surrounded by 6 Cl- and 6 Na+ surrounds each Cl-. The coordination number gives the
number of oppositely charged ions that surround each ion in the crystal lattice (structure). The
coordination number for the Na+ ion in NaCl is 6. NaCl crystals are small cubes. This represents the face
of the cube of NaCl; the pattern would be repeated. There is a Cl- in front and behind the central Na+ and
there is a Na+ ion in each vertices of the cube face. This arrangement is called a face-centered cube.
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Chapter 8 Ionic Bonding
Na
3
+
Cl -
Cs is in the same family as Na but forms a larger ion that results in a different crystal lattice (structure).
Cl –
Cs +
Notice the Cs+ is surrounded by 8 Cl- ions so the coordination number is 8. The Cs+ is in the center of
the cube. This arrangement is called a body-centered cube.
Do ionic compounds conduct an electric current?
To conduct electricity there must be a movement of charges. In a solid the ions are locked in place, ionic
solids are non-conductors. When melted, the ions can move around so melted ionic compounds conduct.
Dissolving ionic compounds in water allows the charged ions to move, solutions of ionic compounds
conduct an electric current.
Ionic compounds made of cations and anions. Ionic compounds contain metals and nonmetals. The
electrons lost by the cation are gained by the anion. The total number of electrons lost must equal the
total number of electrons gained. The cation and anions surround each other. The Chemical Formula for
an ionic compound shows the elements present in the compound and the smallest whole number ratio of
the ions in the ionic compound. This smallest whole number ratio is a FORMULA UNIT.
There are too many compounds to remember the names of them all. IUPAC uses a systematic method in
naming compounds. We will examine the different types of compounds, starting with ionic compounds.
NOMENCLATURE - IONIC COMPOUNDS
Binary ionic compounds contain two elements and are composed of cations (metals) and anions
(nonmetals). The total number of electrons lost must equal the total number of electrons gained because
compounds are neutral. To name binary ionic compounds you first name the cation! BUT!!!!
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There two types of cations:
Chapter 8 Ionic Bonding
4
(i)
cations with a constant (fixed) charge - Group A cations have only one possible charge. The
Alkali metals form 1+ ions, Alkaline earth metals form 2+ ions. To name these cations you use
the element name. K1+ is the potassium ion, Ca2+ is the calcium ion, Al3+ is the aluminium ion.
(ii)
cations with a variable charge ( more than one charge is possible) most transition metals can
have more than one charge, Iron can form a 2+ or 3+ ion, Copper can form two ions 1+ or 2+. To
name these ions you must identify the charge that the ion has and then use a Roman numeral in
parenthesis after the element name. Fe2+ iron (II) ion, Fe 3+ iron(III) ion, Cu2+ copper(II) ion. To
name Fe2O3 you must know which iron ion is there. O is 2- but there 3 of them so the oxygen has
gained a total of 6 e-. The iron must have lost a total of 6 electrons; there are two iron ions so
each must have lost 3 e- Fe3+ iron(III); so the name is iron(III) oxide.
To complete the name of the binary ionic compound you must name the anion (nonmetal). The anion
name is derived from the root of the element name and the ending changed to ide. O2- is oxide, Cl1chloride, N3- nitride.
Name the following binary ionic compounds:
CuO
KCl
Na3N
AlN
Ca3P2
PbO
PbO2
Na2S
CoCl2
Ternary ionic compounds have more than two elements. Ternary compounds usually contain a cation
and a polyatomic ion. Polyatomic ions are groups of atoms that stay together, act as a single unit and
have a charge. When a polyatomic ion is taken more than once in a compound the ion is placed within
parenthesis and a subscript is used. Mg is 2+ NO3 is 1- so magnesium nitrate is Mg(NO3)2.
You must memorize the polyatomic ions (more listed on the back of your periodic table handout)
Ethanoate (acetate) CH3COO- or C2H3O2-1
Nitrate NO3-1
Nitrite NO2-1
Hydroxide OH-1
Permanganate MnO4-1
Cyanide CN-1
Sulfate SO4-2
Sulfite SO3-2
Carbonate CO3-2
Chromate CrO4-2
Dichromate Cr2O7-2
Phosphate PO4-3
Phosphite PO3-3
The only polyatomic cation is Ammonium NH4+1
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Chapter 8 Ionic Bonding
Naming Ternary Ionic Compounds
1. Name the cation as shown above
2. Name the polyatomic anion
Examples:
CaSO4 cation is Calcium; the anion is sulphate. The compound is calcium sulphate.
Fe2(SO4)3 SO4 is 2- x 3 = 6-; Fe total = 6+ so each Fe must be 3+. The name is iron(III) sulfate.
Name the following ternary ionic compounds:
NaNO3
CaSO4
CuSO3
(NH4)2O
LiCN
Fe(OH)3
(NH4)2CO3
NiPO4
Write Formulas for Ionic compounds
For all ionic compounds the charges have to add up to zero; that is, the total number of electrons lost
must equal the total number of electrons gained. To write the formula of a compound:
1. Write the cation symbol with the charge.
2. Write the anion symbol with the charge.
3. Write the subscript for each so that the total e- lost = total e- gained
Put polyatomics in parenthesis when the subscript is greater than one.
Write the formula for calcium chloride.
Calcium is Ca+2 loss of two
Chloride is Cl-1 gain of one
Need 2 x Cl-1 for a gain of two. Ca+2 Cl2-1 the formula unit is CaCl2
Write the formulas for these ionic compounds:
Lithium sulfide
tin(II) oxide
Magnesium fluoride
Copper(II) sulfate
Iron(III) phosphide
Sodium nitrate
Iron(III) sulfide
Ammonium sulfide
Barium nitrate
Tin(IV) oxide
Ammonium phosphate
Metallic Bonds
Metals hold onto their valence electrons very weakly. Valence electrons are free to move through the
solid. Metals can be described as a group of cations tightly packed together and are surrounded by
valence electrons, which are in continuous motion around the cations. It is therefore very hard to
determine which electron belongs to which cation. This attraction of “free-floating” valence electrons
around the cation is called a metallic bond. Because the valence electrons do not belong to any one
particular metal atom metals can be viewed as metallic cations held together by a common cloud of
valence electrons. Metal atoms are held together in the solid as positive ions floating in a sea of
5
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Chapter 8 Ionic Bonding
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electrons. The loosely held valence electron cloud allows for the flow of charge. Thus metals are
conductors of electricity. The valence electron cloud also isolate the cations from one another as they
“roll” around in the structure. This ensures a great mobility for the cations as well. So, it is possible to
make the cations slide on top of one another so that we can change the shape of their structure without
affecting the composition of the metal.. Metals are malleable that is they can be hammered into different
shapes (can be bent). Metals are ductile that is they can be drawn into wires (valence electrons allow
atoms to slide by each other).
Metallic bonding may be viewed as
e
-
metal cation
Metals have closest packing patterns and when melted they will mix to form solutions which cooled
solidify to form what we refer to as alloys. Gold jewelry is a common alloy of gold, copper and silver.
The new mixture will not have the same properties as the original respective metals.
Ionic and Metallic Bonding Review Questions
1. How many valence electrons does each of the following atoms have?
a. Potassium
b. carbon
c. magnesium
d. oxygen
2. State how many electrons will be gained or lost for the following elements in forming an ion
a. calcium
b. fluorine
c. aluminum
d. oxygen
3. Write the formula for the ion when the element becomes isoelectronic with a Noble gas.
a. sulphur
b. sodium
c. fluorine
d. barium
4. Use electron dot symbols (Lewis structure) to determine the formula unit and then name the
ionic compounds formed when the following elements combine.
a. potassium and iodine b. calcium and chlorine
c. aluminum and sulfur d. magnesium and phosphorus
5. Define the term valence electrons.
6. Write the electron dot structures for each of the following elements.
a. S
b. C
c. Al
d. Be
e.
Cl
7. The atoms of the noble gas elements are stable. Explain.
8. Write the complete electron configuration for the following atoms and ions. Comment on the
results:
Ar, K+, Ca2+
9. What are cations and anions? How and why are cations and anions produced?
10. Why are ionic compounds electrically neutral?
Chapter 8 Ionic Bonding
11 Chem
7
11. Write the correct chemical formula for the compound formed by the following pairs of ions:
a. Na+ and Fb. K+ and S2c. Ca2+ and N3d. Al3+ and O212. Which of the following pairs of elements are most likely to form ionic compounds?
a. chlorine and bromine b. potassium and helium
c. lithium and fluorine
d. iodine and sodium
13. Write the formula for the ions in the following compounds.
a. LiF
b. BaO
c. Na2S
d. Al2O3
e. Ca3N2.
14. In your own words describe a crystal.
15. Most ionic compounds are brittle. Why?
16. Why does molten MgCl2 conduct an electric current although crystalline MgCl2 does not? How
else can we get MgCl2 to conduct a current?
17. In your own words define a metallic bond.
18. Describe what is meant by the terms ductile, and malleable. Explain why metals exhibit these
properties.
19. Explain why it is possible to bend metals but not ionic crystals?
20. Explain why metals are good conductors of electricity.
Nomenclature
Name the following binary ionic compounds:
Write a formula for the binary ionic compounds:
1) MgS
__________________________
1) magnesium oxide
__________________
2) KBr
__________________________
2) lithium bromide
__________________
3) Ba3N2
__________________________
3) calcium nitride
__________________
4) Al2O3
__________________________
4) aluminum sulfide
__________________
5) NaI
__________________________
5) potassium iodide
__________________
6) SrF2
__________________________
6) strontium chloride
__________________
7) Li2S
__________________________
7) sodium sulfide
__________________
8) CaO
__________________________
8) radium bromide
__________________
9) K2S
__________________________
9) magnesium sulfide
__________________
10) LiBr
__________________________
10) aluminum nitride
__________________
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Chapter 8 Ionic Bonding
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Name for the following, use the Stock system.
Use the Stock system to write the formula for:
1) CuS
_____________________________
1) iron(II) chloride
________________
2) Fe2O3
_____________________________
2) copper(I) sulfide
________________
3) FeI2
_____________________________
3) lead(IV) iodide
________________
4) Cu2S
_____________________________
4) tin(II) fluoride
________________
5) SnCl2
_____________________________
5) chromium(III) oxide
________________
6) CuO
_____________________________
6) iron(II) nitride
________________
7) PbF2
_____________________________
7) cobalt(III) phosphide
________________
8) CuCl2 _____________________________
8) iron(III) chloride
________________
9) CuBr
_____________________________
9) copper(II) sulfide
________________
10) PbO
_____________________________
10) lead(II) bromide
________________
Ternary Compounds
Ternary Compounds
Write the correct chemical name for:
Write the correct chemical formula for:
1) AlPO4
_____________________
1) potassium phosphate
___________
2) CaCO3
_____________________
2) aluminium hydroxide
___________
3) Mg(OH)2
_____________________
3) sodium hydrogen carbonate
___________
4) K2SO4
_____________________
4) calcium nitrate
___________
5) Na2CO3
_____________________
5) sodium carbonate
___________
6) NH4NO3
_____________________
6) tin(II) carbonate
___________
7) Sn(NO3)2
_____________________
7) lead(II) sulfate
___________
8) FePO4
_____________________
8) iron(III) carbonate
___________
9) PbCO3
_____________________
9) potassium hydroxide
___________
10) Cu2CO3
_____________________
10) ammonium sulfate
___________