CHAPTER 4: CHEMICAL BONDING
Bonding and the Periodic Table Electronegativity and Electropositivity Predicting bond type plus definitions Other types of bond Characteristics of the different bond types Intermediate types of bond and Fajan's ules Predicting the number of bonds The octet fable !ybrid orbitals "hapes of molecules
4.1. INTRODUCTION
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When it comes to on!in" & perhaps the most important section in chapter (as section $)$)$* it is essential to imagine the electrostatic forces involved in bonding$ Energy is also a useful concept& but on its o(n& it does not al(ays lead to such a clear picture of the (ay things are$ It is electrostatic attraction (hich holds atoms and ions together& not energy$ !o(ever& comple+ energy calculations provide essential information ,for e+ample the shapes of atomic and molecular orbitals-$ .e do not need to e+amine the calculations in detail& but (e do need to include in our models the information they provide ,section #$/$-$ 0oreover& energy gives us a 1uantifiable measure of the forces$ 2 measure (hich is easily determined from e+periments ,chapter 3-$ "ee also sections 4
and %#$#$$v$
4.#. BONDING AND THE PERIODIC TABLE
#$$%$ $ection #.4.#. (as also important preparation for this chapter$ There are fe( facts at this level (hich must simply be learned$ !o(ever& you must be able to instantly picture the place in the periodic table of any element mentioned in your syllabus$ If you have these positions stamped on your mind as the starting point for all your inorganic theory& there (ill be far fe(er other facts that you simply need to remember$ !ere are the elements (hose positions you must 5no( for the majority of syllabi at this level ,FI6$ #$%$-*
2 valuable part of inorganic chemistry is learning ho( to relate a vast mass of theory to t(o simple observations& and ho( to use the relationship to ma5e predictions* i% &i'st ose'()tion* Physical and chemical properties of elements and their compounds follo( largely predictable patterns do(n groups in the periodic table$ ii% $econ! ose'()tion * Physical and chemical properties of elements and their compounds follo( largely predictable patterns across periods in the periodic table$
If you come to grips (ith this approach to inorganic theory& you (ill have mastered a po(erful& and far more (idely applicable& techni1ue$ #$$$ P)tte'ns !o*n ) "'o+, * Changes do(n a group follo( from the increasing number of electron shells do(n a group$ This causes the outer electrons to become further ,7and more screened from- the nucleus$ These changes are abbreviated as being due toinc'e)sin" si-e$ ,7 8uantum mechanics reveals that& surprisingly& screening does not increase do(n a group as (e might predict from the simple 9shell9 model of atomic structure$ !o(ever& this fact is not (idely accepted at this level& and the shell model prevails$#$$)$ P)tte'ns )c'oss ) ,e'io!* Changes across a period follo( from the increasing number of protons in the nucleus (hich have to hold only the same number of shells in place$ These changes are summarised as being due to increasingeecti(e n+c/e)' ch)'"e$
There are also more obvious changes associated (ith the increasing number of outer electrons$ #$$#$ L)o+'in" the ,oint * These patterns are fundamental$ Inorganic facts should be learned only as e+amples of these patterns& or should at least be understood and related to each other in the light of the patterns$ There are unlimited pieces of information you could learn$ :ote also& that decreasing si;e across a period should never be given as a fundamental cause of any other pattern$
4.0. ELECTRONEGATIIT2 AND ELECTROPO$ITIIT2
#$)$%$ The t3,e o on!in" bet(een t(o elements can be conveniently predicted from their 'e/)ti(e e/ect'one")ti(ities $ In this conte+t& e/ect'one")ti(it3 can be regarded as the ability of an atom to gain electrons and form negative ions$ Conversely&e/ect'o,ositi(it3 can be regarded as the ability of an atom to lose electrons and form positive ions$ Often the single characteristic of electronegativity is used* an electropositive element has a very lo( electronegativity$ Electronegativity follo(s group and periodic trends and changes$ #$)$$ E/ect'one")ti(it3 !o*n ) "'o+, *
4.4. PREDICTING BOND T2PE
#$#$%$ B3 cominin" our understanding of the periodic table and of electronegativity& (e can predict the type of bonding bet(een pairs of elements$ #$#$$ Bon!in" et*een e/ements *i!e/3 se,)')te! in the ,e'io!ic t)/e * Elements to(ards the left of the table and to(ards the bottom (ill tend to be the ,least electronegative and- the most electropositive$ They (ill be the most li5ely to lose electrons and form positive ions$ Conversely& those to(ards the top right hand corner of the periodic table (ill be the most electronegative ,and least electropositive-& and most li5ely to form negative ions$ These opposite types of element& (hich differ (idely in electronegativity& are therefore most li5ely to forme/ect'o()/ent ionic% bonding (ith each other$ Deinition: ionic on!in" is the electrostatic attraction bet(een oppositely charged ions (hich are arranged in a crystal lattice and (hich are formed by the transfer of electrons from one atom ,giving positive ions- to another ,giving negative ions-$
"uch a transfer of electrons to form ions can be represented for sodium chloride as follo(s*
. :ote that (e cannot define a single ionic bond in a lattice& because an ion is attracted to all the surrounding ,oppositely charged- ions in the lattice& as seen in the sodium chloride lattice ,FI6$ #$$-*
#$#$)$ Bon!in" et*een e/ements c/ose in the t)/e * i% The mo'e e/ect'o,ositi(e e/ements * .hen t(o elements occur in the more electropositive region of the periodic table& neither is able to completely gain electrons from the other$
0oreover& neither is able to maintain sufficient control over the bonding electrons to form a covalent bond ,section #$#$#$-$
In fact& those outer electrons (hich are available for bonding& form a cloud of electron density in (hich the resultant positive ions are arranged as a lattice$ The bonding is met)//ic bonding& and it occurs bet(een atoms of the same element as (ell as in alloys ,see belo(-$ Deinition: met)//ic on!in" is the electrostatic attraction of positive ions arranged in a lattice for the electrons in a single metallic bonding orbital (hich permeates the lattice ,FI6$ #$)$-
This type of bonding& unli5e electrovalent or covalent bonding& does not re1uire atoms of different elements to be present in fi+ed& (hole number& ,stoichiomet'ic- ratios$ True compounds are not formed& but alloys ,section %/$=$-$ A//o3s are intimate associations& some(here bet(een mi+tures and
compounds& in (hich a degree of separation can sometimes be achieved by physical means$ #$#$#$ Bon!in" et*een e/ements c/ose in the t)/e * ii% The mo'e e/ect'one")ti(e e/ements * .hen t(o atoms occur in the more electronegative part of the periodic table& neither (ill be able to completely lose control of electrons to the other$
0oreover& neither is able to lose sufficient control over the available bonding electrons to form a metallic bonding orbital$ In fact& the bonding electrons are held in a (ell defined space bet(een the t(o bonding atoms$ The precise definition of a covalent bond may surprise you& because at this stage it is important to emphasise that all bonds are electrostatic$ Too often& covalent bonds are made to sound li5e diplomatic pacts based on an agreement bet(een t(o atoms to share a pair of electrons$ Deinition: A co()/ent on! bet(een t(o atoms is the electrostatic attraction of the t(o nuclei for a shared pair of electrons in a bonding orbital formed by the overlap of t(o singly occupied atomic orbitals ,or& in the case of a!)ti(e covalent bond& sometimes 5no(n as acoo'!in)te bond& formed by the overlap (ith a vacant orbital of an atomic orbital containing a pair of electrons-$
:ote that for the first time (e have been able to define a single bond$ :ote also that the 5ey difference bet(een one type of bond and another is (here the electrostatic forces occur$ 2ll bonds are electrostatic forces of attraction bet(een positive charge and negative charge& and therefore ,ultimately- bet(een protons and electrons$ 2 simple e+ample of a covalent bond is sho(n for hydrogen in FI6$ #$#$
The formation of a dative covalent bond is illustrated by the follo(ing e+ample*
The arro( in the ammonium ion represents the direction in (hich the electrons have been donated to the bond$ In practice& once formed& the ne( bond is indistinguishable from the other three covalent bonds$
$ When t*o h3!'o"en )toms co//i!e they do not al(ays form a covalent bonding orbital li5e that sho(n in FI6$ #$#$ For e+ample& if the t(o atomic orbitals contain electrons (ith parallel spins& they (ill tend to repel each other$ If they have opposite spins& their magnetic moments cancel out in a bonding orbital$ .hen repulsion occurs& it is sometimes said that an)nti5on!in" o'it)/ has formed$ This may seem li5e an elaborate construct for a non?event& and you (ill have to read else(here if you (ant to be convinced other(ise$ Certainly& at this level& the concept is li5ely to lead to meaningless statements li5e* 9!e molecules therefore do not e+ist because their formation is energetically unfavourable9$ This leaves unans(ered real 1uestions li5e& 9.hat ma5es their formation energetically unfavourable@9
2n e+planation in terms of electrostatic forces (ould be closer to e+perience$ If you could get do(n to atomic level and pull an electron a(ay from an atom you (ould e+perience forces due to electrostatic attraction& not energy ? at least& not in models (hich are easy to graspA The concept of energy is more useful in chemistry (hen considering something you (ould e+perience as energy& such as heat$ It also provides a unifying concept& for e+ample bet(een electrostatic forces and heat or light$ 0oreover& it is useful as a tool for developing better models& as mentioned in section #
& and as (e shall see in the section on hybrid orbitals ,section #$/$-$ Energy is also an e+tremely important concept for chec5ing predictions& and for ma5ing them (hen further e+planation ,or more detailed description- is unnecessary ,section %#$#$$v$-$
