PERIODIC TABLE OF ELEMENTS

ELEMENTS IN A PERIOD[PERIOD 3]

PROPERTIES OF ELEMENTS IN PERIOD 3

Element of Period 3

Na Mg Al Si P S Cl Ar

Proton number 11 12 13 14 15 16 17 18

Electron arrangement

2.8.1 2.8.2 2.8.3 2.8.4 2.8.5 2.8.6 2.8.7 2.8.8

Atomic radius (pm)

186 160 143 118 110 104 100 94

Physical state at room temperature

Solid Solid Solid Solid Solid Solid Gas Gas

Electronegativity 0.9 1.2 1.5 1.8 2.1 2.5 3.0 -

Electronegativity of an element refers to the measurement of the strength of an atom in its molecule to attract electrons towards its nucleus.

PERIODIC CHANGE IN PROPERTIES TO THE ELEMENTS ACROSS PERIOD 3Across Period 3,

• The proton number increases by one unit from one element to the next element

• All the atoms of the elements have three shells occupied with electrons

• The number of valence electrons in each atom increases from 1 to 8

• All the elements exist as solid except chlorine and argon which are gases

• The atomic radius of elements decreases – this is due to the increasing nuclei attraction on the valence electrons

• The electronegativity increases – this is also due to the increasing nuclei attraction on the valence electrons and the decrease in atomic size

ACIDIC AND BASIC PROPERTIES OF OXIDES• Oxides can be classified into basic oxide, acidic oxide

and amphoteric oxide:

• Metal usually form basic oxides.

• Non-metals usually form acidic oxides.

• A few metals form amphoteric oxides.

Basic oxide An oxide that dissolves in acid only

Acidic oxide An oxide that dissolves in alkali only

Amphoteric oxide An oxide that dissolves in both acid and alkali

[PEKA 2] EXPERIMENT 4.3

AIM

To study the properties of the oxides of elements in Period 3.

PROBLEM STATEMENT

How do the acid-base properties of the oxides of elements change across Period 3?

HYPOTHESIS

Acidic properties of the oxides of elements increase whereas the basic properties of the

oxides of elements decrease across Period 3.

VARIABLES

Manipulated Variable: Oxides of elements of Period 3.

Responding Variables: pH values in water and solubility in acid or

alkali.

Fixed Variables: water or nitric acid, HNO3 or sodium hydroxide, NaOH solution

OPERATIONAL DEFINITION

1. Oxide that dissolves in water to form solution with pH less than 7 is acidic and pH more than 7 is alkaline.

2. Oxide that can dissolve in an acid exhibits basic properties.

3. Oxide that can dissolve in an alkali exhibits acidic properties.

4. Oxide that can dissolve in both acid and alkali exhibits amphoteric properties.

APPARATUS

Boiling tubes, test tubes, test tube rack, test tube holder, 10 cm3 measuring cylinder, Bunsen burner, rubber stopper, dropper, glass rod and spatula.

MATERIALS

Magnesium oxide, aluminium oxide, silicon (IV) oxide, phosphorus pentoxide, Universal Indicator, 2 mol dm-3 nitric acid, 2 mol dm-3 sodium hydroxide solution and distilled water.

PROCEDURE

A. Reaction with water

1. Add half a spatula of magnesium oxide powder into 10 cm3 of distilled water in a test tube.

2. Close the test tube tightly with a rubber stopper. Shake the mixture.

3. Add two drops of universal indicator to the mixture using a dropper. Shake the mixture and record the pH value.

4. Repeat the experiment with aluminium oxide, silicon (IV) oxide and phosphorus pentoxide in place of magnesium oxide respectively.

A. Reaction with nitric acid or sodium hydroxide solution

1. Add half a spatula of magnesium oxide powder into two separate boiling tubes.

2. Add 5 cm3 of nitric acid and 5 cm3 of sodium hydroxide solution separately to the contents in each boiling tube.

3. Heat the contents in each boiling tube slowly while being stirred with a glass rod.

4. Record the solubility of magnesium oxide in the two solutions.

5. Repeat the experiment with aluminium oxide, silicon (IV) oxide and phosphorus pentoxide in place of magnesium oxide respectively.

DATA AND OBSERVATION

OXIDESolubility in

waterpH

value

Solubility in sodium

hydroxide solution

Solubility in nitric acid

MgO Dissolves slightly in water to form a colourless solution

8 – 9 Does not dissolve in sodium hydroxide solution

Dissolves in nitric acid to form a colourless solution

Al2O3Does not dissolve in water

None Dissolves in … colourless solution

Dissolves in … colourless solution

SiO2Does not dissolve in water

None Dissolves in … colourless solution

Does not dissolve in …

P2O5Dissolves in water to … colourless solution

2 – 3 Dissolves in … colourless solution

Does not dissolve in …

INTERPRETING DATA

1 (a) Magnesium oxide dissolves slightly in water to form a solution with pH 8-9, a weak alkali. Hence, magnesium shows metallic properties.

(b) Phosphorus pentoxide dissolves in water to form a solution with pH 2-3, a strong acidic solution. Hence, phosphorus shows non-metallic properties.

(c) Magnesium oxide dissolves in nitric acid but does not dissolve in sodium hydroxide solution. Thus, it shows basic properties.

(d) Aluminium oxide dissolves in nitric acid and sodium hydroxide solution. Thus, it shows amphoteric properties.

(e) Silicon (IV) oxide and phosphorus pentoxide dissolve in sodium hydroxide solution but do not dissolve in nitric acid. Thus, they show acidic properties.

2 When going across Period 3 from left to right (magnesium to phosphorus), the acidic properties of the oxides of elements increase whereas the basic properties of the oxides of elements decrease.

3 The metallic properties of the elements across Period 3 also decrease.

DISCUSSION

Q1: All acidic oxides can dissolve in alkalis whereas all basic oxides can dissolve in acids. Amphoteric oxides can dissolve in both acids and alkalis. Nitric acid, HNO3 and sodium hydroxide, NaOH solution are used to determine the acidic / basic / amphoteric properties of the oxides of elements.

Q2: CHEMICAL EQUATIONS

Magnesium oxide dissolves slightly in water to form an alkaline solution:

MgO (s) + H2O (l) Mg(OH)2 (aq)

Phosphorus pentoxide dissolves in water to form an acidic solution:

P4O10 (s) + 6H2O (l) 4H3PO4 (aq)

Q3: CHEMICAL EQUATIONS

Magnesium oxide dissolves in nitric acid only:

MgO(s) + 2HNO3(aq) Mg(NO3)2(aq) + H2O(l)

Aluminium oxide can dissolve in both nitric acid and sodium hydroxide solution.

Al2O3(s) + 6HNO3(aq) 2Al(NO3)3(aq) + 3H2O(l)

Al2O3(s) + 2NaOH(aq) 2NaAlO2(aq) + H2O(l)

CONCLUSION

When going across Period 3 from left to right (magnesium to phosphorus) the

acidic properties of the oxides of elements increase whereas the basic properties of the oxides of elements

decrease.

The hypothesis can be accepted.