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Ionic compounds

Chapter 21

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sodium and chlorine reaction

Ionic bonding

Ionic bonds form between metals and non-metals.

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Na atom.It has 1 electron in its outer shell

Cl atom.It has 7 electrons in its outer shell

Ionic bonding

• Salt or Sodium Chloride (NaCl) is a good example of a ionic bonding. Sodium (Na) has 1 electron in the outer shell and chlorine (Cl) has 7 electrons in its outer shell.

• If Sodium lost this electron, its next shell will be full, but that would also make sodium a positive ion. If chlorine gained 1 electron, its shell would be full with a maximum of 8 electrons, and it would then be a negative ion.

• Thus Sodium Chloride (NaCl) is a bonding of the Na+

ion and the Cl- ion.

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Ionic bonding

Sodium will become Na+, chlorine will become Cl–. The attraction between the opposite charges is an ionic bond.

Sodium gives its outer electron to chlorine

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Ionic bonding

sodium 10 electrons = 10- chlorine 18 electrons = 18-

11 protons = 11+ 17 protons = 17+

1+ 1-

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The atoms, which we now call ions, become charged: Na+ and Cl-.the attraction sticks the ions together, and is called an ionic bond.

More ionic compounds

• Magnesium is in Group 2 of the Periodic Table. Therefore it has 2 electrons in the outer shell.

• Magnesium loses 2 electrons when it reacts. The its ion has a 2+ charge, Mg2+. You can see that each magnesium atom can “satisfy” 2 chlorine atoms.

• So the formula of magnesium chloride is MgCl2.7


• Now let’s look at sodium reacting with oxygen. An oxygen atoms has 6 electrons in its outer shell.

• The oxygen atoms gains 2 electrons, each with a negative charge. Therefore its ion has a 2- charge, O2-. It takes 2 sodium atoms to “satisfy” 1 oxygen atom.

• So the formula of sodium oxide is Na2O.

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1. What is the charge on a halide ion?

2. What type of ion do all metals form?

3. The charge on a sulphide ion is 2–. What is the formula of aluminium sulphide?

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1. What is the charge on a halide ion?

1–

2. What type of ion do all metals form?

Positive or a cation

3. The charge on a sulphide ion is 2–. What is the formula of aluminium sulphide?

Al2S3

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Working out the formula

Read page 265 and then try these examples:

a. magnesium oxide

b. aluminium oxide

c. magnesium bromide

d. aluminium chloride

e. aluminium bromide

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Properties of ionic compounds

Metals always form positive ions, and non-metals form negative ions

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alum crystals copper sulphate crystals


Ionic compounds:

• are made of crystals (which can be split along certain angles)

• have high melting points

• are often soluble in water

• conduct electricity when molten or dissolved in water, but not when solid

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Experiment 21.1 Testing ionic compounds

1. Spread a few grains of sodium chloride on a microscope slide.Focus your microscope on the grains.

• What shape are the grains?• Are the angles at the surface of the grains similar?

We call solids with regular angles crystals.

2. Heat some sodium chloride crystals strongly in a test-tube.• What happens? Does sodium chloride have a high melting point?

3. Add 2 spatulas of sodium chloride to half a beaker of water.Stir it with a glass rod.

• What happens? Is sodium chloride soluble in water?

4. Set up the circuit as shown:5. Dip the electrodes into some solid sodium chloride.• Does the solid conduct?

Now half-fill the beaker with water, and stir.• Does the bulb light up now?• Does the solution conduct electricity?

Switch off you power pack as soon as the test is completed.14

carbonelectrodes

test solidsodiumchloridethen itssolution

4 V

chlorine gas


Metals always form 1…. ions, and non-metals form 2…. ions.

Ionic compounds are made of 3…. which can be 4…. along certain angles. Ionic compounds have 5…. melting points. Many are soluble in 6…. and will conduct electricity if the ions are free to 7….(i.e. when molten or in 8…. solution).

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Metals always form positive ions, and non-metals form negative ions.

Ionic compounds are made of crystals which can be split/cleaved along certain angles. Ionic compounds have high melting points. Many are soluble in water and will conduct electricity if the ions are free to move (i.e. when molten or in aqueous solution).

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Giant ionic structures

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• Dorothy Mary Hodgkin was a British chemist.• Using X-ray crystallography, a method used to

determine the three dimensional structures of biomolecules, she confirmed the structure of vitamin B12, and was awarded the Nobel Prize in Chemistry.

• In 1969, after 35 years of work and five years after winning the Nobel Prize, Hodgkin was able to decipher the structure of insulin.

• X-ray crystallography became a widely used tool and was critical in later determining the structure of many biological molecules such as DNA.

Giant ionic structures

Part of the giant ionic structure of sodium chloride. It is called a giant ionic lattice.

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Millions of positive and negative ions are bonded together to form a 3D network.

An ionic compound dissolving in water

The oxygen end of a water molecule is slightly negative compared to the hydrogen end. The electrons in water are not evenly spread.

The water molecules are attracted to the ions and pull them from the giant structure (lattice).

The compound dissolves. Its ions are then free to move around.

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-

+H H

O

Chemistry at work: Ionic compounds –Halides

1. What charge do halide ions have?

2. Why is fluoride added to toothpaste and some drinking water supplies?

3. Which halide compounds are used in photography?

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Chemistry at work: Ionic compounds – Halides

1. What charge do halide ions have?

1–

2. Why is fluoride added to toothpaste and some drinking water supplies?

To protect against tooth decay

3. Which halide compounds are used in photography?

Silver halides

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SummaryWhen metals react with non-metals they form ionic compounds.Metals give electrons to non-metals.This makes positive metal ions, and negative non-metal ions.The oppositely charged ions are attracted to each other by strong electrostatic forces. This is called ionic bonding.

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Summary

The ions are arranged in huge lattices, called giant ionic structures.Ionic compounds :

• are made of crystals• have high melting points• are often soluble in water• conduct electricity when molten or dissolved in

water (when free ions are present), but not when solid.

(continued)

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Covalent bonding

Chapter 22

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Shared electrons form covalent bonds between non-metal atoms.

Carbon has 4 electrons in its outer shell. It needs to gain 4 electrons to get a full outer shell.(Remember that the 2nd shell can hold 8 electrons.

Hydrogen has just 1 electron.If it can gain 1 more electron, it will fill its shell.(The 1st shell is filled by just 2 electrons.)

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The Cl2 molecule

• Three ways to show the covalent bond in Cl2. The right-hand one is a ”dot and cross” diagram (showing the outer electrons only).

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Bonding in HCl

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Bonding in H2O

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Bonding in NH3

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Double bonds

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= =

Giant covalent structures

• The giant covalent structure of diamond.• It is made from only carbon atoms.• Each atom forms 4 strong covalent bonds with its

neighbours.31

Carbon in the form of graphite

• Diamond and graphite are allotropes of carbon. • Allotropes are different forms of the same element (in the same state.)• Graphite feels smooth and slippery. Your pencil contains graphite.• The 4th electron from each carbon atom is found in the gap between the

layers. • Electrons can move along between the layers in graphite.• These electrons hold the layers together by a weak force.• Graphite turns straight into a gas at 3000 °C. This is called sublimation.

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1. What is an allotrope?

2. What special property does the non-metal graphite have?

3. What is the hardest substance on Earth?

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1. What is an allotrope?

Different forms of the same element in the same state

2. What special property does the non-metal graphite have?

It is a conductor of electricity (and heat)

3. What is the hardest substance on Earth?

Diamond

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Covalent bonding

Draw a diagram to show the bonding in a hydrogen fluoride molecule (HF). Show all the electron shells.

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Chemistry at work: Carbon

1. What is the name of the allotrope of carbon, containing 60 C atoms per molecule?

2. How are industrial diamonds made?

3. Why can graphite be used as a lubricant?

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Chemistry at work: Carbon

1. What is the name of the allotrope of carbon, containing 60 C atoms per molecule?

Buckminster-fullerene, or a bucky-ball

2. How are industrial diamonds made?

Graphite is compressed and heated

3. Why can graphite be used as a lubricant?

The layers of carbon atoms slide over each other easily

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Sir Harry Kroto’s bucky balls

1996 was a big year for Harry Kroto.

Not only was he knighted (Sir Harry!), but he also received the Nobel Prize for Chemistry.

Read more at: link

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Giant Covalent Structure Simple Molecular Structure

Substance Boiling point (°C) Substance Boiling point (°C)

diamond 4830 methane -164

silicon 2355 water 100

sand (silica) 2230 chlorine -35

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Simple molecular structures

Substance, with low melting points and boiling points have simple molecular structures.


Substances, with 1…. melting points and 2….points have simple 3…. structures.

They have 4…. covalent bonds between atoms in the molecule, but 5…. forces of attraction between 6…..

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Substances, with low melting points and boilingpoints have simple molecular structures.

They have strong covalent bonds between atoms in the molecule, but weak forces of attraction between molecules.

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Experiment 22.1 Heating iodine

Use tweezers to put a few crystals in a flask. Replace the bung.

Hold the base in you hands for a few minutes, as shown:

Look carefully through the flask, against a white background.

• What can you see?

• Do you think it is easy or hard to separate iodine molecules

from each other? do you think it has a giant structure?

• Iodine turns straight from a solid into a gas.

What is this type of change called? (see the bottom of page 273).

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iodine

Experiment 22.2 Heating sulphurCollect 2 spatulas of powdered sulphur in a test-tube.Warm it slowly and gently over a Bunsen flamein a fume-cupboard.• What happens? Describe the changes you see as it melts.

When it has melted, heat the sulphur more strongly.Keep the flame away from the mouth of the test-tube.It could ignite any sulphur vapour escaping.• How does the sulphur change?

When the liquid is about to boil, remove the mineral wool plug.Pour it quickly into a beaker of cold water.• What happens?

Get the solid formed out of the beaker.Pull at it gently.• What happens? Is the solid stretchy?

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sulphur dioxide gascan be formed

moltensulphur

coldwater

sulphur

Heating sulphur

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cold water plastic sulphur

Chemistry at work: Ozone

1. What is the formula of ozone?

2. What does the ozone layer do?

3. Where is the hole in the ozone layer?

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Chemistry at work: Ozone

1. What is the formula of ozone?

O3

2. What does the ozone layer do?

Ozone molecules absorb harmful ultraviolet light from the Sun

3. Where is the hole in the ozone layer?

Over Antarctica

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Answers

1 2

3 4

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7

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Summary

Non-metal atoms are bonded to each other by covalent bonds.

A covalent bond is a shared pair of electrons.

Covalent substances can have either:

• a giant covalent structure, or

• a simple molecular structure.

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Summary

Giant covalent structures are huge 3-dimensional networks of atoms.

Millions of atoms are all joined by strong covalent bonds.

This means that they have high melting points and boiling points.

(continued)

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Summary

Simple molecular substances are made of small molecules.

The atoms in each molecule are joined together by strong covalent bonds. However, there are only weak forces of attraction between molecules.

Neither structure conducts electricity (except for graphite).

(continued)

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Metals and structures

Chapter 23

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Metals

1. Which group of metals is the most reactive?

2. Give three examples of magnetic metal elements.

3. List common metal properties.

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Metals

1. Which group of metals is the most reactive?

Group 1

2. Give three examples of magnetic metal elements.

Cobalt, nickel and iron

3. List common metal properties.

Conductors of heat and electricity; sonorous; ductile; malleable; lustrous; dense; high melting and boiling points

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Metallic bonding

Use this diagram to explain metallic bonding.

1.

2.

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Metallic bonding

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Arrange 4 pieces of wood in a square. You could use4 books with a square hole between them.The size of the square depends upon the size andnumber of model metal atoms you can use.Your model atoms must all be the same size,just like a metal element.(You can use marbles, table-tennis balls, polystyrene sphere, etc.)

Fill the bottom of the square with model atoms.Make sure that there are no gaps.Not sit the second layer on top. Again, thereshould be no gaps.

Now make a third layer. The atoms can sit in two possible positions,1) directly above the centres of the atoms in the first row, or2) with their centres not in line with the first row.

Try to make both structures.

Experiment 23.1 Close-packed structures

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Close-packed structures

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Both of these giant, close-packed structures are shown below:

For example, gold or copper For example, zinc or nickel

Structures of metals

Metals are usually 1…., suggesting that their atoms must be packed 2…. together.

Most metals also have 3…. melting points, which suggests their atoms are arranged in 4….structures.

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Structures of metals

Metals are usually dense, suggesting that their atoms must be packed closely together.

Most metals also have high melting points, which suggests their atoms are arranged in giantstructures.

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Alloys

Al alloy is a mixture of metals.

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• If small amounts of another metal are added to aluminium, for example, it becomes a lotstronger.

• Pure aluminium cannot cope with the stress puton wings during flight, with the strength anaeroplane needs.

• Alloying affects the structure of a metal.

Using the apparatus shown, make rows ofsmall bubbles in the dish. The plunger should bepushed in slowly and steadily. This will makesure that the bubbles are the same size.

The bubbles represent the atoms in a metal.Remember their close-packed structures?Fill the dish with bubbles.

• Do the bubbles line up in rows?• What happens when a bubble bursts? Can you see•how easily the rows of bubbles slide past each other?

Now inject a larger bubble into the middle of the dish.This is like adding an atom of a different metal.In other words, you have made an alloy!

• Can you see how this disturbs the regular pattern of bubbles?

Experiment 23.2 Bubble bath

regular patternof bubbles

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AlloysUse this diagram to explain why alloys are stronger than pure metals.

1.

2.

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Alloys

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The layers of atoms can’t slide over each other as easily now.They get “jammed” in place.The alloy is a lot harder and stronger thanthe original metal.

Chemistry at work: Alloys

1. Which elements do all coinage alloys have in common?

2. Why is copper added to gold jewellery?

3. What is solder used for?

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Chemistry at work: Alloys

1. Which elements do all coinage alloys have in common?

Copper and nickel

2. Why is copper added to gold jewellery?

To make the metal harder and more durable

3. What is solder used for?

To join up parts of an electrical circuit

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Summary table 1Substance Electrical

conductivity State at room temperature

Melting point / oC

Likely type of structure

Calcium chloride Good when aqueous or in liquid state. Poor as a solid.

Solid

Sucrose Poor Solid

Sand Poor Solid

Ethane Poor Gas

Brass Good (even as a solid)

Solid

Summary table 1Substance Electrical

conductivity State at room temperature

Melting point / oC

Likely type of structure

Calcium chloride Good when aqueous or in liquid state. Poor as a solid.

Solid

Sucrose Poor Solid

Sand Poor Solid

Ethane Poor Gas

Brass Good (even as a solid)

Solid

ionic

simple molecular

giant covalent

simple molecular

giant metallic

772

186 decomp

1650(±75)

-182

900 to 940

Summary table 2

Giant

Graphite

Water

Yes

No

No

High

Low

Solid

Covalent

Fill in the gaps.

Answers

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Summary

• Metal atoms are bonded together by a ‘sea’ of electrons.

• The electrons are free to drift between the atoms.

• They move in one direction when an electric current flows.

• Metal atoms are arranged in giant structures.• This explains why most metals have high

melting points.

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Summary

• The atoms are usually packed closely together in these giant structures (lattices).

• This is why most metals have a high density.

• Mixtures of metals are called alloys.

• An alloy is stronger than the metals used to make it.

(continued)

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Documents

Please read through these slides and the textbook to