Structure 2

Models of bonding
. and structure
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Structure 2.1 The ionic model

What detefnlil-the Ionic nature and propertleaof • compound?

lomc compounds are characterized by the p<esence the strong electrostatK attractions between oppos,tely
or posit,ve and negative ions, which attract each other charged rons Once liquid, however, ionic CO<npounds are
electrostatically. In solid ronK compounds, these ions electrical conductors due to the presence of mobtle ions.
are am,nged rn ngrd crystalline lattices. Melting these Due to their charge, rons rnter.,ct strongly wrth polar water
solids requires a large amount or thermal energy due to molecules, so 1ontC compounds are often water~uble.

Understandings

Structufe 2.1,1-When metal atoms lose electrons, they Structure 2.1. 3 - Ionic CO<npounds exist as three-
folm positive ions called cations. When non-metal atoms dimensional lat1ice structures, represent.ed by empirical
gain electrons, they f0<m negatnie ions called anions. f0<mulas.
Structufe 2.1.2- The ionic bond is formed by
eledrostatic attractions between oppositely charged ions.
Binary ionic c0<npounds are named with the cation first,
followed by the anion. The anron adopts the suffix "ide".

Introduction to bonds and structure

AIO<nS rarely exist ,n rsolat,on. They are connected together ,n several different
ways. Atoms can be bonded to atoms or the same type, or to atoms of different
elements The vary,ng arrangements of atoms and fearures of the bonds between
them give rise to cenain different propenies. Fot example, 7B% of the air around
us ,s nitrogen, N 2 However, ,n agriculture, nitrogen fertJl,zers are added to soils
to help crops grow Th,s rs because the structure and bonding of nitrogen ,n arr
are different to that of the nitroger>eus compounds found in fenilizers.

Atoms are held togethe< by chemical bonds. This chapter discusses three different
bonding models: ionic, covalent and metallic. These lead 10 four types of strucrure:
,onic, molecular covalent, covalent networl< and metallic. Yoo may be wondering
why the<e are four types of structure, given that there are only three types ol bonds.
This rs because covalent substances can be found rn two am>ngernents· a cononuous
3D nelwO<I<, 0< disaele groups ol atoms known as molecules.

bond metallic
ty p e sIOnlC
of~

types of . .
metallic
structure ron,c

••••
G)'G)'G),G)-metal ,on
0.G).~delocahsed

.
G),G)'(j):(j) electron

◄ Ftgure 1 Thefearethfeetypeso(
bonds and four types ol SlructUfe

.._ ________
 t b ,'ld ng and structu z
Chemical bonds
Models
Chemical bonds are strong forces of anraction that hold atoms or ions togethe<
Structure 2.1. 2.2and 2.Jdiscuss In a substance All chemical bonds occur due to electrostatic attractions between
models of bonding and structure. posilively cha1ged species and negalively charged species The type of bonding
Scientific models simplify and depends on which species are involved (table 1).
represent complex phenomena.
Sometimes models help us to
visuahze things that we cannot The (lolectrostahc attraction between ...
observe directly. This IS one of Type of bonding Positively charged Negatively charged
the reasons bonding models are species species
so useful.
ionic cations an,ons
All models have hm1tat1ons.
covalent atomic nuclei shared pa,r of electrons
This does not necessarily make
the models inadequate. but It metallic cations delocalized electrons C'l- C'l
IS important to understand the
& T•bte 1 All bonding types ,nvotve a poslt,wiy charged species and a negat...ely <Ntged
weaknesses of a model. As you speci~ that are electrost.lteally attracted to each other
wo,k through these sect,ons,
identify some of the strengths and
liml!ations of the various bonding
models Ions (Structure 2.1.7) c11 i. 5'0~,..
Sod1umu>Jo,ipc and coppe~II) sulfate are examples of ionic compounds. They
are frv·"•· I 1 1e-tand r, , pi which are properties characteristic of IonIc compounds.
lon,~AJ)CltJnds are al~f..>00~1C.d '- ,;•1Jw ll,'~ w•k • 1 ~ but ,W
e ectr1c~odJCor:, when "l10lter or d ,s i The reactions and PfOpert1es of
these ionic compounds are very different to those of !he,r constituent elements
For instance. scidium chloride, the main ingredient in table salt, is water-soluble.
However. elemental scid1um Is a soft metal that reacts V>Olently with water. and
chlo11ne Is a poisonous gas.

• F,guro 3 !;od,um chlonde crystal$ on• tree brllnct,ond coppe,111) >ullate crystal>.
Sodium chloride and copperill) sulfate a1e ionK: compounds

• F,gur• 2 Offsho1e-0tl plalfonn tn

Cahfom... USA. What examples ol wucrure Before discus!.tng ,onte bonds and the charactenstics of ionic structures. we w,11
and bond,og .,e pI.-t In the pho(ol first look mto what ,ons are

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Cations and anions

Sodium chlonde contains sodium .ans, not sodium atoms. Sodium atoms and
sodium ions have diffe<ent numbers of electrons. and therefote behave differently

You will notice three differences between Na and Na•:

I. numberofelectrons " ~r-s-.,s IO

2 electronarrangement ,,'uLi,ClJ r ve,-J\I}

(S'-Ll'lp( A F;g""' 4 (a) sod.im •tom (b) sod.im ion

3 charge 0
Sodium atoms are neutral Sodium IOfls have al+ charge. indicated by a
supe1script + sign next 10 the symbol: Na·.

Wor1led example 1
Determine the numbe< of subatomic particles to show that
a. sodium atoms are neutral
b. sodium ions have a 1+- charge.

Solution
In S11\/Cture 1.2, you learned
a. In a sodium atom lhe<e are.
thatprotonshal/ea 1+-charge
11 protons (charge = 11 + I
andelectrons~a I-charge.
11 electrons (charge = 11-) You can igno<e neutrons ,n ionic
Overall charge Is 11 - 11 = 0 charge calculations as these are
b. In a sodium ,on there are. uncha<ged
t 1 protons (charge = t t +)
10 electrons (charge= 10-)
Overall charge Is 11 - 10 = 1+

Wor1led example 2
Deduce the electron configuration of a sodium atom and a sodium ion.

Solution
Na· 1s'2s'2p•3sI
Na·. 1s2 2s'2p'

CatibnsareK>Osw1tl ,1 ,r i.•cli" tfd· ~ Th1smean.sthatcat101\Sare

t, .... t-~ ..., ~L· as the combined positNc charge of protons ,s greater than A F19ur• 5 (a) sulfur atom fb) sulfide ion
the combined negative charge of elect,ons As sodium ,ons have 11 protons and
10 electrons, the overall charge IS 1+.
Activity
Anions ar rce -l• -.~,, c • 1crl They contain a ~ P r r·1,rT)tw, 1' t, t->dr lr
·e . Figure 5 shows a sulfur atom and a sulfide ion. The sulfide ion has a Shovv that the sulfur atom is neutral
2- charge. denoted by the superscnpt in the symbol S'·. Note that anions adopt and the sulfide ,on has a charge
a shghlly different name: the fiisl part corresponds lo the name of their parent of 2- by counting the,r subatomic
atom. This is followed by the suffix ide. particles. Determine the electron
configuration of the sulfur atom
F0< novv, we Wlll only consider monatomic ions. You will look at charged groups
and sulfide ion.
of atoms (called polyatomic ions) In a late< section