Ci 5.36 Bonding and Structures

8/9/2019 Ci 5.36 Bonding and Structures

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BONDING AND

STRUCTURES


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Bonding

Intra

(strong)

Inter

(weak)

Ionic(metal + non-metal)

Covalent(non-metals)

Hydrogen(H attached to

N,O or F)

Dipole forces

Permanent

Dipole

Induced/

InstantaneousDipoles

Metallic

(metals)

TYPES OF BONDING


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Hydrogen Bonding Hydrogen is an exceptional element in that when

it forms a covalent bond its electron is held toone side of the nucleus leaving the other siderelatively bare. Any approaching negatively

charged group can get very close to thehydrogen nucleus and produce an unexpectedlylarge electrostatic attraction.

These electrostatic attractions are exaggeratedwhen H is bonded to a more electronegativeelement e.g. N, F or O. Such electrostaticattractions are called hydrogen bonds.


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Both are explained bythe increased

attraction betweenmolecules caused byhydrogen bondingmaking it more

difficult to separatethem.

For example, in

water. Black dotsrepresent oxygenatoms and white dotsrepresent hydrogenatoms.


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Permanent Dipole Forces

Permanent Dipole

-Dipole Interactions

One part of the molecule is always slightly positive or

negative compared with another part

When molecules that have permanent dipoles cometogether, they will arrange themselves so that the

negative and the positive ends of the molecules attract

one another


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The attractions

are called dipole-dipoleinteractions.

The moleculeseventually align

in order to findthe bestcompromise

betweenattraction andrepulsion.


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Instantaneous Dipole Forces

These are small electrostatic forces that arecaused by movement of electrons within thecovalent bonds of molecules that would

otherwise have no permanent dipole.

As one molecule approaches another theelectrons of one or both are temporarily

displaced owing to their mutual repulsion. Thismovement causes small, temporary dipoles tobe set up which attract one another.


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These dispersion forces

increase with the sizeof the molecule and

with its surface area.

Large molecules with

big surface areas havemore electrons, and

more dispersion forces

and greater attraction

and therefore higherboiling points.


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Question

Which type of dipole do the following

molecules have (explain your reasoning)

Cl2

HCl

CH4

CH3Cl

Poly(ethene)

Poly(propene)

Poly(chloroethene) (PVC)


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Structures

The properties of substances are decided by

theirbonding and structure.

Structure The way atoms are arranged

relative to one another, e.g. Giant Lattice,

Molecular(Simple and Macro).

How does bonding and structure decide

properties?


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For a solid substance

there are three main factors

Type of particle it contains

Atoms, ions or molecules. If a substance contains ions

or polar molecules, it may dissolve in water.

How the particles are bonded together

Ionic, covalent, metallic or weak intermolecular bonds.

The stronger the bonds, the higher the mp/bp of the

substance, and the greater the hardness.

How the particles are arranged relative to each other

One-dimensional chains, two-dimensional sheets or one

of many three-dimensional arrangements.


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Classification of Substances According to Structures (CI 5.6)


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Structure and the Periodic Table

As you go across a period of the PeriodicTable, there is a trend in the structures ofthe element. (fig 37 p.118 CI).

Metallic covalent network covalentmolecules monatomic

Note Carbon exists in two different forms;covalent network (graphite & diamond)and covalent molecules (fullerenes).


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Trends in reactions across a period

Tables 12, 14, 15 p.121 CI 5.6

Reactions with oxygen

In period 3 all elements except argon form oxidesNa2O, MgO, Al2O3, SiO2, P4O10/P4O6, SO3 /2, Cl2O/ Cl2O7

Reactions with chlorine

In period 3 all elements except argon form chlorides

NaCl, MgCl2, AlCl3, SiCl4, PCl5/PCl3, S2Cl2, Cl2


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Reactions with water

Na Vigorous, forms NaOH and H2Mg Only with steam, forms Mg(OH)2 and H2

Al No reaction unless protective oxide layer

removed from surface, forms Al(OH)3 and H2Si No reaction

P No reaction

S No reaction

Cl Dissolves in water (some reacts to form anacidic solution (HCl + HClO))

Ar No reaction


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Acid-base properties of oxides

Acid-base character of oxides is linked to theirstructure.

Giant ionic lattices are basic (Na2O & MgO).

Covalent oxide structures are basic (P4O6, SO3& Cl2O7).

Bonding in Al2O3 has both ionic and covalent

character so it can behave as both an acid and a

base it is Amphoteric.


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Behaviour of chlorides in water

Behaviour of chlorides in water is linked to theirstructure.

Chlorides with giant ionic lattices simply dissolve in water

with no chemical reaction (NaCl & MgCl2).

Chlorides with covalent molecular structures react

(hydrolysed), producing fumes of HCl and forming acidic

solutions (AlCl3, SiCl4, PCl5, S2Cl2).

Complete CI 5.6 q2, 4,5

Ci 5.36 Bonding and Structures

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