Chapter 10

Structure of Substances

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http://www.chemguide.co.uk/atoms/structsmenu.html

Structure of substances

Exist as either molecular structures or giant structuresConstruct a concept map for different structures of substances.

Concept Map

substances

molecular structures

Giantstructures

simplemolecularstructures

macromolecules

Elementse.g., oxygen, etc.

compoundse.g., carbon dioxide

plasrtic e.g., polyethene

giant ionicstructures

examplesexamples

examples

examples

giant covalentstructures

examplesexamples

examples

compoundse.g., NacCl

Elementse.g., diaomond

Compoundse.g., Silicon dioxide

giant metallic structures

Elementse.g., copper

Simple Molecular Structures

Most non-metals and covalent compounds Composed of simple discrete covalent moleculese.g., hydrogen, chlorine, and water

Structure of iodine

I2 molecules packed in a regular pattern.

Held by weak intermolecular forces (van der Waals’ forces). C rysta l S tru ctu re o f

Iod in e

Structure of dry ice (solid carbon dioxide)

Carbon dioxide molecules paced regularly.Held together by weak intermolecular forces.

= C = O

C rysta l S tru ctu re o fC arbon D iox id e

Electronic diagram of ammonia ?

NH H

H

xx

xo

xo

xo

Properties of simple molecular structures

Low melting points and boiling points.SoftUsually insoluble in water, but soluble in non-aqueous solvents.Non-conductors of electricity, whenever as solids, liquids, or in aqueous solutions.Except: aqueous solutions of carbon dioxide, ammonia & sulphur dioxide

As an electrolyte when dissolved in water

Why ???A few covalent compounds ionize when dissolved in water. Mobile ions help to conduct electricity.SO2(g) + H2O(l) H2SO3(aq)

H2SO3(aq) 2H+(aq) + SO3

2-(aq)

Why are covalent compounds volatile?

The covalent molecules are held together by weak intermolecular forces (van der Waals’ forces)Little heat energy is needed to separate the molecules.

Why are they soft?

Intermolecular forces between molecules are weak.It is wasy to separate the molecules from each other.

Why are they insoluble in water?

‘Like dissolves like’ Principle.The intermolecular forces between covalent molecules are much weaker than that between water (solvent) molecules.

Why are they non-conductors of electricity?

They are neutral molecules.They neither have mobile ions nor mobile electrons for conducting electricity.

Macromolecules

Containing thousands of atoms which are bonded together by covalent binds.Examples: carbohydrates (polysaccharides)Proteins – made of amino acids.Plastics - e.g., polyethene, polystyrene, etc.

Giant structures

Giant ionic structures – made up of oppositely charged ions. (e.g., cations and anions)Giant covalent structures – made up of atoms which are bonded together by covalent bonds in the form of a network.Giant metallic structures – consists of regularly packed positive metal ions bathed in a sea of delocalized electrons.

Giant ionic structures

How an ionic compound is formed?Usually formed by combining a metal with a non-metal.Ionic crystals consist of negative and positive ions held together by strong non-directional electrostatic attractions (ionic bonds) Ions are regularly packed in a 3-dimensional structure.e.g., sodium hydroxide, copper(II) sulphate-5-water

Sodium chloride

Electronic diagram of magnesium chloride

Mg

2+

Cl

-

Cl

- xx

xx

xx

xx

oo

oo

oo

oo

ooo

ooxo

x

Properties of giant ionic structures

Solids at room temperatureHigh melting points and boiling pointsUsually soluble in water, but insoluble in non-aqueous solvents.Conducts electricity when molten or in aqueous solution.

Why are they high melting solids?

The oppositely charged ions (caions & anions) are held together by strong electrostatic attractions (ionic bonds).,A lot of heat energy is needed to overcome the strong ionic bonds between ions.

Electrical Conductors

Definition ???An electrical conductor conduct electricity without any chemical change.e.g., copper, iron, liquid mercury, (metals) and graphite.

Electrolytes

Definition ???An electrolyte conducts electricity when molten or in aqueous solutions. It decomposes at the electrodes while conducting electricity.e.g., sodium chloride, sodium sulphateAqueous solutions of a few covalent compounds, such as carbonic acid, and ammonia solution, are weak electrolytes.

Solid Ionic compounds : Non-conductors of electricity

Why ???In solid ionic compounds, ions are held by strong electrostatic forces and are not mobile.

Hot molten ionic compounds or its aqueous solutions

Why ???Ions become mobile when dissolved in water or in hot molten state.Can be electrolyzed / decomposed when conducting electricity.

Ionic Compounds –usually soluble in water

Why ?Is water a covalent compound?Why water is an universal solvent?Water – a polar solvent Benzene, chloroform, absolute alcohol, etc – non-aqueous solvent.

Water – a polar solvent

A polar covalent compound ???Is chorine molecule polar? ?Why ?In chlorine molecule, 2 chlorine atoms link together by a non-polar covalent bond.The 2 chlorine atoms (identical atoms) share the 2 bonding electrons equally. (Each with 50 % share.)

Water – a polar solvent

Is the OH bond of the H2O molecule non-polar?

Are the 2 atoms identical?The bonding electrons are not equally shared by the two atoms. (oxygen & hydrogen)The oxygen atom has a greater share of the bonding electrons.

Water – a polar solvent

Oxygen atom thus has a partial negative charge ( -).Hydrogen atom has a partial positive charge (+).

H

O

H

-

++

How sodium chloride dissolves in water?

How sodium chloride dissolves in water?

When the attractive forces between ions and water molecules are greater than that between oppositely charged ions, the ionic compound dissolves in water. However, some ionic compounds are insoluble in water. (i.e., Attractive forces between ions and water are smaller than that between oppositely charged ions.)Examples ???Limestone (calcium carbonate), copper (II) oxide

Giant covalent structures

Millions of non-metal atoms link together by extensive strong covalent bonds.Link together by a network of covalent bonds.e.g., diamond, silicon(IV) oxide, etc.2 different crystals of carbon: diamond & graphite.

Diamond

Each carbon atom links to 4 other carbon atoms in a tetrahedral arrangement.

carbon atom

covalent bonds

Silicon(IV) oxide

Each silicon atom joins to 4 oxygen atoms

Each oxygen atomjoins to 2 silicon atoms.

represents an oxygen atom

represents a silicon atom

Key:

Properties of giant covalent structures

Solids with very high melting points and boiling points.Hard (except graphite)Insoluble in any solvent.Non-conductors of electricity (except graphite)

Why are they high melting solids?

Atoms are bonded extensively by a network of strong covalent bonds. A lot of heat energy is needed to to break the extensive covalent bonds.

Why are they non-conductors of electricity?

Giant covalent structures have neither mobile ions nor mobile electrons. They are non-conductors of electricity.

Giant metallic structures

Metal atoms are packed closely together in a regular pattern to form giant metallic structure.

Metallic Bond ??

What is the electronic structure of sodium?2,8,1The outermost shell electrons of sodium atoms – loosely held by the nucleus.Can be lost (given up) easily.Move freely and randomly in all directions.Known as ‘mobile’ / ‘delocalized’ electrons.

Metallic Bond

A piece of metal consists of a giant lattice of positive metallic ions surrounded by a ‘sea’ of mobile electrons.Definition ?The electrostatic attraction between a sea of mobile electrons and positive metal ions is known as the metallic bond.Non-directional.

Metallic Bond

Metallic structure

Properties of Metals

Good conductors of heat and electricitySolids with high melting and boiling pointsWith high densitiesMalleable and ductile ???Malleable – rolled into sheetsDuctile – drawn into thin wires.High tensile strength

Why are they good conductors of heat & electricity?

There are mobile electrons in giant metallic structures. The mobile electrons are responsible for the conduction of electricity. When a piece of metal is connected to battery, the mobile electrons will flow in one direction only (from the negative pole of the battery to the positive pole). Thus, metals are good conductors of electricity. Metals are also good conductors of heat. When one end of the metal is heated, the delocalized electrons there become more energetic. They move faster, collide more frequently with neighbouring electrons. Heat is thus transferred from one end to the other. Heat is also conducted along the metal by the vibration of metal ions.

Electrical Conduxtion

Electrical Conduction

Why metals are high melting solids?

In giant metallic structures, atoms are regularly packed and held strongly by metallic bonds. A lot of energy is needed to overcome the strong metallic bonds. Therefore, metals usually have high melting points.

Why they have high densities?

Metals have high densities as their atoms are closely packed in giant metallic structures

Why metals are malleable and ductile?

When a force is applied to a piece of metal, the layers of atoms can slip over one another. There is no breaking of the metallic bond. The metal atoms are still held together by metallic bond. Metals are malleable and ductile.