Chemical Bonding
86 CHEMICAL BONDING [CHAP. 5
5.7 PREDICTING THE NATURE OF BONDING IN COMPOUNDS
Electronegativity
Elecrronegativity is a qualitative measure of the ability of an atom to attract electrons involved in covalent bonds. Atoms with higher electronegativities have greater electron-attracting ability. Selected values of electronegativity are given in Table 5- 1. The greater the electronegativity difference between a pair of elements, the more likely they are to form an ionic compound; the lower the difference in electronegativity, the more likely that, if they form a compound, the compound will be covalent.
Table 5-1 Sclected Electroncgativities
F
CHAP. 51 CHEMICAL BONDING 89
You need not memorize values of electronegativity (although those of the second-period elements are very easy to learn). You may generalize and state that the greater the separation in the periodic table, the greater the electronegativity difference. Also, in general electronegativity increases to the right and upward in the periodic table. Compounds are generally named and formulas are written for them with the less electronegative element first. (Some hydrogen compounds are exceptions. The symbol for hydrogen is written first only for acids. Hydrogen combined with a halogen atom or with a polyatomic anion forms an acid. In NH,, the H is written last despite its lower electronegativity because NH, is not an acid.) You may not even have to consider electronegativity in making deductions about chemical systems-for example, in naming compounds (Chap. 6). In fact, the following generalizations about bonding can be made without reference to electronegativity.
Most binary compounds (compounds of two elements) of metals and nonmetals are essentially ionic. All compounds involving only nonmetals are essentially covalent except for compounds contain- ing the N H 4 + ion.
Practically all tertiary compounds (compounds of three elements) contain covalent bonds. If one or more of the elements is a metal, there is likely to be ionic as well as covalent bonding involved in the compound.
Formation of Ions in Solution
When some molecules containing only covalent bonds are dissolved in water, they react with the water to produce ions in solution. For example, pure hydrogen chloride. HCI, and pure ammonia, NH,, consist of molecules containing only covalent bonds. When cooled to sufficiently low tempera- tures (-33°C for NH,, -85°C for HCI) these substances condense to liquids. However, the liquids do not conduct electricity, since they are still covalent and contain no ions. In contrast, when HCI is dissolved in water, the resulting solution conducts electricity well. Aqueous solutions of ammonia also conduct, but poorly. In these cases, the following
r~
reactions occur to the indicated extent to yield ions:
+ :&-
(100%)+
+
:O:H- (approximately 1%) H 30+ is often abbreviated H +.Solved Problems
THE OCTET RULE
5.1. Arrange the electrons in each of the following atoms in shells: ( a ) Li, (6) Na, ( c ) K, and ( d ) Rb.
Shell Number
1 2 3 4 5
( a ) Li 2 I
( h ) Na 2 8 1
( C ) K 2 8 8 1
( d ) Rb 2 8 18 8 1
90 CHEMICAL BONDING [CHAP. 5
5.2.
5.3.
5.4.
5.5.
5.6.
5.7.
5.8.
5.9.
Arrange the electrons in each of the following atoms in shells: ( a ) F, ( h ) C1, and ( c ) Br.
.4 ! 2s.
Shell Number
1 2 3 4 5
How many electrons are there in the outermost shell of each of the following‘? ( a ) Mg, ( h ) Si, ( c ) P, (ci) Br. and ( e ) Kr.
/Ins. ( a ) 2, ( h ) 4, ( ( 3 ) 5 , ( t i ) 7, and ( e ) 8.
Which electron shell is most important to the bonding for each of the following atoms? ( a ) H, ( h ) Li, (c.1 Na, ( d ) K, ( e ) Rb, and (f’) Cs.
/-111.s. In each case the outermost shell is the niost important. ( a ) 1 , ( h ) 2. ( c ‘ ) 3, ( c l ) 4, ( e ) 5 , and ( f ) 6.
Explain why uncombined atoms of all of the elements in a given main group of the periodic table will be represented by a similar electron dot notation.
A m Thcy all have thc same numbcr of valence electrons.
How many electrons can fit into an atom in which the outermost shell is the ( U ) first shell?
( h ) second shell? (c’> third shell?
h s . ( ( 1 ) 2. The first shell is filled. ( h ) 10. The first and second shells are both filled. ( c ) 18. The first and second shells arc filled, and there arc eight electrons in the third shell (the maximum number before the fourth shell starts tilling).
Compare the number of electrons in Problem 5.6 with the atomic numbers of the first three noble gases.
,411s. Thcy itrc the same--2, 10, and 18.
How does lithium achieve the “octet” configuration?
Arzs. Li loses an electron, lcaving it with the electron configuration o f He. A configuration of two electrons in its outermost shell corresponds t o the octet because the outermost shell is the first shell, which can hold only two electrons.
Which elements acquire the electron configuration of helium by covalent bonding?
~ Z S . Only hydrogen. Lithium and beryllium iire mctals, which tend t o lose electrons (and form ionic bonds) rather than share. The resulting configuration of two electrons in the first shell, with no other shells occupied, is stable, and therefore is also said t o satisfy the octet rule. Second-period clcmcnts of higher atomic number tend to acquire the clcctron configuration of neon. If the outermost shell o f an atom is the first shell. the maximum number of electrons in the atom is 2.
IONS
5.10. What is the difference between NO, and NO,-?
,4m.
5.11. ( a ) What is the charge on the sodium ion? ( h ) What is the charge on the sodium atom?
( c ) What is the charge on the sodium nucleus?
A t i s .
The first is ;I compound and the wcond is ;in ion-a part of ;I compound.
( ( 1 ) I + . 0 7 ) 0, and ( c ) 1 1 + . Notc ho% important it is to read the questions carefully.
CHAP. 51 CHEMICAL BONDING 91
5.12.
5.13.
5.14.
5.15.
5.16.
5.17.
5.18.
5.19.
You have been given a stack of $3.00 gift certificates for a store which gives no change for these certificates. What is the minimum number of $2.00 items that you can buy without wasting money? How many certificates will you use?
Am. You can buy three items (for $6) with two certificates (worth $6).
What is the formula of the compound of aluminum and sulfur?
Am. AI,S,. Aluminum has three electrons in the valence shell of each atom, and sulfur requires two.
This problcm involves the same reasoning as the last problem.
When a sodium atom loses an electron to form Na’, how many electrons are there in what is now the outermost shell? in the valence shell?
Ans. There are eight electrons in the second shell, which is now the outermost shell. since the one electron in the third shell has been lost. There are now zero electrons in the valence shell.
Arrange the electrons in each of the following ions in shells: ( a ) Mg”, ( h ) Ca”, and ( c ) 0’-.
Am.
Shell Number
1 2 3 4
Write the formulas for the compounds of ( a ) N a + and C10,- and (6) ME2+ and C10-.
Explain why one of the formulas requires parentheses.
Atis. NaCIO, and MgtCIO),. Thc parentheses mean two CIO- ions: no parentheses means two 0
atoms in one CIO, ion.
Write formulas for the compounds formed by the following pairs of ions: ( a ) N a + and Cl-, ( b ) N a + and S2-, ( c ) Ba2+ and S2-, ( d ) AI3+ and S2-, (e) Mg2+ and N”, and (f) CO’+ and
c10
-.Write formulas for the compounds formed by the reaction of ( a ) sodium and sulfur, ( b ) barium and bromine, (c) aluminum and oxygen, ( d ) lithium and nitrogen, (e) magnesium and nitrogen,
< f >
aluminum and fluorine, and ( g ) magnesium and sulfur.Ans. Crc> Na,S, ( h ) BaBr,, (c) AI,O,, ( d ) Li,N, ( e ) Mg,N,, (f) AIF,, and ( g ) MgS.
What ions are present in each of the following compounds?
( a ) FeCI, ( d ) Ba(ClO),
( b ) c u , s ( e ) (NH&PO,
( c ) CUO ( f ) CaCO,
A m . (11) F e 2 + and Cl- t h ) C u ‘ and S‘
(c) CU” and 0’
( d ) Ba’+ and CIO- ( e ) NH,’ and PO,, ( f ) Ca’+ and CO,’-