Period 3 Elements

Question 1

Describe the structure of period 3 elements

Answer 1

• Na, Mg and Al are metals and form giant metallic structures.
• Si is metalloid and forms a giant molecular/covalent structure in which each silicon atoms is tetrahedrally bonded to four other silicon atoms by strong covalent bonds.
• P, S, Cl and Ar form simple molecular structures consisting of discreet molecules joined by weak van der waal forces but the atoms within each molecule joined by strong covalent bonds.

Question 2

What is the trend in atomic radius of period 3 elements?

Answer 2

Atomic radius decreases across the period from Na to Cl

Explanation:
- From one element to the next from Na to Cl, a proton is added to the nucleus and an electron to the same outer shell.
- This increases the nuclear charge but the screening effect remains constant.
- As a result the nuclear charge outweighs the screening effect which increases the effective nuclear charge from sodium to chlorine.
- As the effective nuclear charge increases the atomic radius reduces as the electrons are pulled closer to the nucleus.

Question 3

Describe the trend in melting point of period 3 elements

Answer 3

Melting point increases from sodium through magnesium, aluminium to silicon and generally decreases from Si to Ar with exception of sulpur that has a higher melting point than phosphorous.

Explanation
- For metals the melting point increases from sodium through magnesium to aluminium.

• From sodium to aluminium the number of delocalized electrons per atom contributed to the charge cloud increases from one to three, the charge on the metal ion increases and the ionic radius decreases.
• This increases the strength of the electrostatic attraction between the metal ions and the delocalized electrons.
• As a result the strength of the metallic bond increases and more heat energy required to break the metallic bond resulting into higher melting point from sodium to aluminium.
• There is a sharp increase in the melting point from aluminium to silicon because silicon forms a giant covalent structure consisting of very many strong covalent bonds which requires a lot of heat energy to break.
• Phosphorous, sulphur, chlorine and argon form simple molecules i.e. P4, S8, Cl2 and Ar respectively.
• The melting points of these elements depends on the strength of the Van der waal forces between their molecules which in turn depends the molecular mass; the higher the molecular mass, the stronger the Van der waal forces and the higher the melting point.
• Phosphorous has a lower melting point than sulphur because the P4 molecules have lower molecular mass and form weaker van der waal forces than the S8 molecules.
• From sulphur to argon the melting points decreases as the molecular mass and the strength of the Van der waal forces decrease.

Question 4

Why does phosphorus have a lower melting point than sulphur?

Answer 4

The P4 molecules have lower molecular mass and form weaker van der waal forces than the S8 molecules.

Question 5

What is the trend in electronegativity of period 3 elements?

Answer 5

Electronegativity increases from sodium to chlorine

Explanation:
- From one element to the next the nuclear charge increases by one unit due to an extra proton added to the nucleus while the screening effect stays constant because the extra electron is added to the same outer shell.
- As a result the nuclear charge outweighs the screening effect causing an increase in the effective nuclear charge and the power of the atom to attract bonding electrons towards its self.

Question 6

What is the trend in electropositivity of period 3 elements?

Answer 6

Electropositivity decreases across the period from sodium from chlorine.

Explanation:
- From one element to the next the nuclear charge increases by one unit due to an extra proton added to the nucleus while the screening effect stays constant because the extra electron added to the same outer shell.
- As a result the nuclear charge outweighs the screening effect causing an increase in the effective nuclear charge meaning that the valency electrons become more strongly attracted to the nucleus.
- This reduces the ability of the atoms to lose electrons.

Question 7

Define electropositivity

Answer 7

This is the tendency of an atom of an element to lose its valence electrons to become positively charged.

Question 8

How do period 3 elements react with water?

Answer 8

• Sodium reacts vigorously with cold water forming sodium hydroxide solution and hydrogen gas.

• Magnesium reacts very slowly with cold water forming magnesium hydroxide solution and hydrogen gas.
When strongly heated, magnesium burns vigorously in steam with a very bright white flame forming magnesium oxide, a white powder and hydrogen gas.

• Aluminium does not react with water due to a protective layer of aluminium oxide. Silicon, phosphorus, and sulphur also do not react with water.

• Chlorine reacts with cold water forming hydrochloric acid and hypochlorous acid

Question 9

How do period 3 elements react with air/ oxygen?

Answer 9

• Sodium readily reacts with air at room temperature forming sodium oxide, a white solid.
- When ignited in limited air, it burns with a bright yellow flame also forming sodium oxide.

• In excess air/oxygen sodium burns vigorously with a bright yellow flame forming sodium peroxide, a pale yellow solid.

• Magnesium burns vigorously in air with a bright white flame on strong heating forming a white ash consisting of magnesium oxide and magnesium nitride.
(If to the products a little water is added and the mixture warmed ammonia gas is evolved)

• Aluminium forms a protective layer of aluminium oxide when exposed to air.

• Silicon: At room temperature and temperatures below 900 degrees centigrade, silicon is protected by a thin layer of silicon(IV) oxide. However if ignited at temperatures beyond 900 degrees centigrade , it burns forming a white solid of silicon(IV) oxide.

• Sulphur: When exposed to a flame sulphur burns in air with a blue flame forming white fumes of sulphur dioxide gas.

• Phosphorous readily reacts with air/oxygen.
When exposed to air phosphorous forms a dense white smoke consisting of a mixture of phosphorous(III) oxide and phosphorous(V) oxide. (If ignited in air, phosphorous burns with a bright yellow flame again forming a mixture of phosphorous (III) oxide and phosphorous(V) oxide ( P4O6 & P4O10))

Question 10

How do period 3 elements react with dilute sulphuric acid and dilute hydrochloric acid?

Answer 10

• Sodium reacts explosively with acids, so this reaction is never attempted.

• Magnesium reacts with dilute sulphuric acid and dilute hydrochloric acid with vigorous effervescence forming hydrogen gas and a colourless solution consisting of magnesium sulphate and magnesium chloride respectively.

• Aluminium reacts less vigorously with acids than magnesium forming the corresponding aluminium salt and hydrogen gas. The initial rate of reaction is low due to a protective layer of aluminum oxide.

• Silicon, phosphorous, sulphur, chlorine and argon don’t react with dilute acids.

Question 11

How do period 3 elements react with sodium hydroxide solution?

Answer 11

• Sodium and magnesium do not react with sodium hydroxide solution.

• Aluminium reacts with cold dilute sodium hydroxide with vigorous effervescence
forming hydrogen gas and a soluble complex of sodium tetrahydroxo aluminate. The reaction is highly exothermic.

• Silicon reacts with hot concentrated sodium hydroxide forming sodium silicate solution and hydrogen gas.

• Phosphorus reacts with hot concentrated sodium hydroxide forming sodium phosphinate and phosphine.

• Chlorine reacts with cold dilute sodium hydroxide forming sodium chloride, sodium chlorate(I) and water.
Chlorine reacts with hot concentrated sodium hydroxide forming sodium chloride, sodium chlorate(V) and water.

Question 12

How do period 3 elements react with chlorine?

Answer 12

• Sodium, magnesium and aluminium metals when heated and plunged into a gas jar of dry chlorine, there is a vigorous reaction, forming white crystals of sodium chloride, magnesium chloride and aluminium chloride respectively. The vigour of the reaction decreases from sodium to aluminium.

• Silicon when heated reacts slowly with dry chlorine forming silicon(IV) chloride a colourless liquid.

• Phosphorous when heated reacts slowly with excess dry chlorine forming phosphorous (V) chloride, a yellow solid.

• Sulphur when heated also reacts slowly with dry chlorine forming disulphur dichloride, a yellow liquid.

Question 13

What is the structure of period 3 chlorides?

Answer 13

• Sodium chloride and magnesium chloride have giant ionic structures
• Aluminium chloride forms simple molecular structure
• The chlorides of other elements form simple molecular structures with discrete molecules joined by weak Van der waal forces.

Question 14

How do period 3 chlorides react with water?

Answer 14

• Sodium chloride is not hydrolysed by water; it just dissolves in water forming a neutral solution of pH 7.

• Magnesium chloride is slightly hydrolysed by water forming a slightly acidic solution with pH of about 6.5.

• Aluminium chloride is hydrolysed by water forming an acidic solution with pH of about 3.
Explanation;
The aluminium ion has a small ionic radius and carries a big charge (3+) resulting into high charge density and polarizing power. The aluminium ion therefore exerts a powerful attraction for water molecules forming a soluble complex of hexaaquaaluminium ion, [Al(H2O)6]3+. The high polarizing power of the aluminium ion allows it to form a strong aluminium-oxygen bond in the complex which weakens the oxygen-hydrogen bond and releasing hydrogen ions. This makes the solution acidic.

• Phosphorus(III) chloride and Phosphorus(V) chloride are hydrolysed by water forming phosphorous acid (H3PO3) and phosphoric acid (H3PO4) respectively together with white fumes of hydrogen chloride.

• Disulphur dichloride is hydrolysed by water forming sulphurous acid, a yellow precipitate of sulphur and moist fumes of hydrogen chloride.
• Chlorine reacts water forming hydrochloric acid and hypochlorous acid.

Question 15

Why does the hydrolysis of AlCl3 by water form an acidic solution.

Answer 15

• The aluminium ion has a small ionic radius and carries a big charge (3+) resulting into high charge density and polarizing power.
• The aluminium ion therefore exerts a powerful attraction for water molecules forming a soluble complex of hexaaquaaluminium ion, [Al(H2O)6]3+.
• The high polarizing power of the aluminium ion allows it to form a strong aluminium-oxygen bond in the complex which weakens the oxygen-hydrogen bond and releasing hydrogen ions.
• This makes the solution acidic.

Question 16

Covalent or ionic bonding?
NaCl
MgCl2
AlCl3
SiCl4
PCl3, PCl5
SCl2, S2Cl2
Cl2

Answer 16

NaCl - ionic
MgCl2 - ionic
AlCl3 - predominantly covalent
SiCl4 - covalent
PCl3, PCl5 - covalent
SCl2, S2Cl2 -covalent
Cl2 -covalent

Question 17

Covalent or ionic bonding?
Na2O
MgO
Al2O3
SiO2
P4O6, P4O10
SO2, SO3
Cl2O, ClO2, Cl2O6, Cl2O7

Answer 17

Na2O- ionic
MgO - ionic
Al2O3 - covalent
SiO2 - covalent
P4O6, P4O10 -covalent
SO2, SO3 - covalent
Cl2O, ClO2, Cl2O6, Cl2O7 -covalent

Question 18

Acidic, Basic or Amphoteric?
Na2O
MgO
Al2O3
SiO2
P4O6, P4O10
SO2, SO3
Cl2O, ClO2, Cl2O6, Cl2O7

Answer 18

Na2O- Basic
MgO- Basic
Al2O3 -Amphoteric
SiO2- Acidic
P4O6, P4O10- Acidic
SO2, SO3- Acidic
Cl2O, ClO2, Cl2O6, Cl2O7- Acidic

Question 19

What structure do the following have?
Na2O
MgO
Al2O3
SiO2
P4O6, P4O10
SO2, SO3
Cl2O, ClO2, Cl2O6, Cl2O7

Answer 19

Na2O- Giant ionic
MgO- Giant ionic
Al2O3- Giant ionic
SiO2- Giant molecular
P4O6, P4O10- Simple molecular
SO2, SO3- Simple molecular
Cl2O, ClO2, Cl2O6, Cl2O7- Simple molecular

Question 20

Describe the trend in the melting point of period 3 oxides

Answer 20

• Sodium oxide, magnesium oxide and aluminium oxide have giant ionic structures with strong electrostatic forces of attraction between the positively charged metal ions and the oxide ions.
• The magnesium ion (Mg2+) carries a bigger charge and has a smaller ionic radius than the sodium ion (Na+). - Therefore magnesium oxide has higher lattice energy due to stronger electrostatic attraction between its ions resulting into a higher melting point than sodium oxide.
• The aluminium ion exerts high polarizing power on the oxide ion due the high charge-ionic radius ratio of the aluminium ion. This distorts the electron cloud of the oxide ion more resulting into a decrease in the ionic character in the bond. Therefore the melting point of aluminium oxide is less than that of magnesium oxide.
• The melting point of silicon(IV) oxide is high because it forms a giant molecular structure with a three – dimension network of very many strong covalent bonds. To break the bonds a lot of heat energy is supplied resulting into high melting point.
• Oxides of phosphorous, sulphur and chlorine are small discrete molecules attracted to each other by weak Van der waal forces which require less heat to break and resulting into low melting points. However, the larger the molecular mass, the stronger the Van der waal forces and the higher the melting points.

Question 21

Why is the melting point of aluminium oxide is less than that of magnesium oxide?

Answer 21

The aluminium ion exerts high polarizing power on the oxide ion due the high charge-ionic radius ratio of the aluminium ion. This distorts the electron cloud of the oxide ion more resulting into a decrease in the ionic character in the bond.

Question 22

How do period 3 oxides react with water?

Answer 22

• Sodium oxide reacts vigorously with water forming sodium hydroxide, a strongly an alkaline solution.

• Magnesium oxide only slightly dissolves in water forming magnesium hydroxide solution.

• Aluminium oxide and silicon(IV) oxide are insoluble in water.

• Phosphorus(III) oxide and phosphorous(V) oxide are white solids that dissolve in water forming phosphorous acid (H3PO3) and phosphoric acid (H3PO4) respectively.

• Sulphur dioxide and sulphur trioxide dissolve in water forming sulphurous acid (H2SO3) and sulphuric acid respectively.

• Oxides of chlorine dissolve in water forming chloric acids.
✓ Chlorine(I) oxide, an orange gas, dissolves in water forming chloric(I) acid.
𝐶𝑙2𝑂(𝑔) + 𝐻2𝑂(𝑙) → 2𝐻𝑂𝐶𝑙(𝑎𝑞)
✓ Chlorine(IV) oxide, a yellow gas, is a mixed anhydride and dissolves in water forming a mixture of chloric(III) acid and chloric(V) acid.
𝐶𝑙𝑂2(𝑔) + 𝐻2𝑂(𝑙) → 𝐻𝐶𝑙𝑂2(𝑎𝑞) + 𝐻𝐶𝑙𝑂3(𝑎𝑞)
✓ Chlorine(VI) oxide, a red liquid, is a mixed anhydride and dissolves in water forming a mixture of chloric(V) acid and chloric(VII) acid.
𝐶𝑙2𝑂6(𝑙) + 𝐻2𝑂(𝑙) → 𝐻𝐶𝑙𝑂3(𝑎𝑞) + 𝐻𝐶𝑙𝑂4(𝑎𝑞)
✓ Chlorine(VII) oxide, a colourless liquid, dissolves in water forming chloric(VII) acid.
𝐶𝑙2𝑂7(𝑙) + 𝐻2𝑂(𝑙) → 2𝐻𝐶𝑙𝑂4(𝑎𝑞)

Question 23

How do period 3 oxides react with acids?

Answer 23

• Sodium oxide and magnesium oxide are basic and readily react with acids forming the corresponding metal salt and water only.
• Al2O3 is amphoteric and reacts with acids forming a salt and water only.
• Oxides of silicon, phosphorus, sulphur and chlorine are acidic and therefore do not react with acids.

Question 24

How do the group 3 oxides react with sodium hydroxide?

Answer 24

• Sodium oxide and magnesium oxide are basic oxides and do not react with sodium hydroxide solution.

• Aluminium oxide is amphoteric and reacts with hot concentrated aqueous sodium hydroxide forming a soluble complex of tetrahydroxoaluminate ions.

• Silicon(IV) oxide is an acidic oxide and dissolves in concentrated sodium hydroxide forming sodium silicate and water.

• Oxides of chlorine react with cold aqueous sodium hydroxide forming chlorates and water.
✓ Chlorine(I) oxide forms sodium chlorate(I) and water.

✓ Chlorine(IV) oxide forms sodium chlorate(III), sodium chlorate(V) and water.

✓ Chlorine(VI) oxide forms sodium chlorate(V), sodium chlorate(VII) and water.

✓ Chlorine(VII) oxide forms sodium chlorate(VII) and water.

Question 25

What is the bonding of group 3 hydrides?

Answer 25

The hydrides of sodium, magnesium and aluminium are ionic while the hydrides of silicon, silicon, sulphur and chlorine are covalent.

Question 26

How do period 3 hydrides react with water?

Answer 26

• Sodium hydride, magnesium hydride and aluminium hydride react with water at room temperature with vigorous effervescence forming metal hydroxides and liberating hydrogen gas.
• Silicon tetrahydride is hydrolyzed by water forming silicic acid( H2SiO3) and liberating hydrogen gas.
• Phosphine is non-polar and therefore not hydrolysed by water, a polar solvent. (Phosphine is non-polar because the electronegativities of phosphorous (2.2) and hydrogen (2.2) are the same resulting into a zero net dipole moment.)
• Hydrogen sulphide partially ionizes in water forming a weak acid called hydrogen sulphide.
• Hydrogen chloride is completely ionized in water forming hydrochloric acid, a strong acid.

Question 27

What is the main ore of aluminum?

Answer 27

Bauxite, Al2O3.2H2O

Question 28

What are the major impurities in bauxite?

Answer 28

Silicon(iv) oxide, SiO2 and oxides of iron especially iron(iii) oxide, Fe2O3

Question 29

Describe the process of purification of the ore for aluminum extraction

Answer 29

• Aluminium is extracted from its ore called Bauxite which contains silicon(IV) oxide and oxides of iron as the main impurities.
• The bauxite is roasted at low temperature to convert all the iron impurities into iron(III) oxide.
• The roasted ore is then crushed into powder and then heated with concentrated sodium hydroxide solution.
• Aluminium oxide which is amphoteric and silicon(IV) oxide which is acidic dissolve in the sodium hydroxide forming sodium aluminate and sodium silicate respectively.
• Iron(III) oxide which is basic remains undissolved in sodium hydroxide and it is removed by filtration.
• A current of carbon dioxide gas is blown through the remaining solution to precipitate aluminium hydroxide from sodium aluminate solution.
• Sodium silicate remains in solution. The aluminium hydroxide precipitate is filtered off, washed with water, dried and then heated strongly to form pure aluminium oxide.

Question 30

Why is bauxite roasted at low temperature?

Answer 30

To convert all the iron impurities into iron(III) oxide.

Question 31

Why is the roasted bauxite crushed into powder and then heated with concentrated sodium hydroxide solution?

Answer 31

To eliminate Iron(III) oxide which is basic and remains undissolved in sodium hydroxide (removed by filtration) while aluminum oxide which is amphoteric and silicon(IV) oxide which is acidic dissolve in the sodium hydroxide forming sodium aluminate and sodium silicate respectively.

Question 32

Describe the process of electrolysis of aluminium oxide

Answer 32

• The pure aluminium oxide is dissolved in molten cryolite (Na3AlF6) to lower its melting point and then electrolysed in a steel tank lined with a graphite cathode and graphite rods to act as the anode.
• Molten aluminium is formed at the cathode according to the following equation:
  Al3+ (l) + 3e− → Al(l)
• The molten aluminium collects at the bottom of the tank where is tapped off at intervals.
• Oxygen gas is evolved at the anode according to the following equation:
  2O2− (g) → O2 (g) + 4e−
• Oxygen slowly oxidizes the hot graphite anode forming carbon dioxide gas according to the following equation:
  C(s) + O2 (g) → CO2 (g)
• Therefore the anode needs to be replaced from time to time.

Question 33

What are the uses of aluminium in relation to its properties?

Answer 33

• Used to make overhead electricity cables because it is a good conductor of electricity, it is light, resists corrosion and it is ductile.

• Used as CD and DVD coating because its shiny surface reflects laser beam that reads CDs and DVDs.

• Used as food foils because it is non-toxic, resistant to corrosion and can easily be rolled into thin sheets.

• Used to manufacture saucepans because it a good heat conductor, resists corrosion and non-toxic.

• Used to coat suits for fire fighters because it reflects heat and this keeps them cool.

• It is used to manufacture aeroplane bodies because it is light.

Question 34

How does aluminum react with oxygen?

Answer 34

When exposed to air, aluminium forms a protective layer of aluminium oxide which prevents it from further reaction and protects it from corrosion.

Question 35

How does aluminum react with acids

Answer 35

• With dilute hydrochloric and dilute sulphuric acids
Aluminium reacts with hydrochloric acid and dilute sulphuric acid forming hydrogen gas and the corresponding aluminium salt. The initial rate of reaction is low due to a protective layer of aluminum oxide.

• With dilute nitric acid
Aluminium is rendered passive by nitric acid due to formation of a protective layer of aluminium oxide.

• With concentrated sulphuric acid
With concentrated sulphuric acid aluminium forms aluminium sulphate, sulphur dioxide and water.

Question 36

What is the reaction of aluminum and aqueous sodium hydroxide?

Answer 36

Aluminium reacts vigorously with cold dilute aqueous sodium hydroxide forming a soluble complex of tetrahydoxo aluminate ions and hydrogen gas.

Question 37

How does aluminum react with chlorine and hydrogen chloride?

Answer 37

Anhydrous aluminium chloride is prepared by passing dry chlorine gas or hydrogen chloride gas over heated aluminium.

Question 38

How does heating affect aluminium chloride?

Answer 38

• When heated to 180 degrees centigrade aluminium chloride (AlCl3) dimerises through coordinate bonding forming a compound of formula Al2Cl6 in vapour state at temperatures between 180 and 400 degrees centigrade
• If heated beyond 400 degrees centigrade, the coordinate bonds break and the dimer (Al2Cl6) dissociates back into molecules with formula AlCl3 also in gaseous state.

Question 39

Why is an aqueous solution of aluminium chloride acidic?

Answer 39

• The aluminium ion has a small ionic radius and carries a big charge (3+) resulting into high charge density and polarizing power.
• The aluminium ion therefore exerts a powerful attraction for water molecules forming a soluble complex of hexaaquaaluminium ion, [Al(H2O)6]3+.
• The high polarizing power of the aluminium ion allows it to form a strong aluminium-oxygen bond in the complex which weakens the oxygen-hydrogen bond and releasing hydrogen ions.
• This makes the solution acidic.

Question 40

How does aqueous sodium carbonate affect aqueous solutions of Al3+ ions

Answer 40

Observation: A white precipitate and effervescence of a colourless gas.

Explanation: Because of its high charge density and the polarizing power of the aluminium ion, the aluminium ion exerts a powerful attraction for water molecules forming a complex of hexaaquaaluminium ion, [Al(H2O)6]3+.

When the carbonate ion, a strong base, is added, it completely hydrolyses the complex by removing enough hydrogen ions from it forming aluminium hydroxide, a white precipitate and evolving carbon dioxide gas.

Question 41

How is Aluminum powder used in detection of nitrate ions?

Answer 41

On a nitrate solution with a mixture aluminum powder and aqueous sodium hydroxide, ammonia gas is liberated.

The gas turns moist red litmus blue and forms dense white fumes with concentrated hydrochloric acid.

Question 42

What is observed when aluminum ions react with sodium hydroxide solution?

Answer 42

A white precipitate soluble in excess forming a colourlesss solution containing a soluble complex of tetrahyroxoaluminate ions.

Question 43

What is observed when aluminum ions react with ammonia solution?

Answer 43

A white precipitate insoluble in excess ammonia.

Question 44

What is observed when aluminum ions
react with sodium carbonate solution?

Answer 44

A white precipitate accompanied by evolution of a colourless gas.

Question 45

What is observed when aluminum ions react with litmus solution?

Answer 45

A blue lake solution is formed if to a solution containing aluminium ions about 3 drops of litmus solution are added followed by addition of aqueous ammonia dropwise until in excess.