SPM Form 4 Chemistry Notes — Chapter 4『4.5』Elements in a Period

4.5 Elements in a Period

 Period is the horizontal rows of elements in the Periodic Table.

 The modern periodic table has 7 periods.

 The period number indicates the number of electron shell.

 Elements in the same period have same number of electron shells.

 The proton number of elements increases from left to right crossing the period.

 The number of electrons is also increases from left to right crossing the period.

Properties of elements across Period 3

Elements 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
Metal properties	Metal	Metal	Metal	Semi-metal	Non-metal	Non-metal	Non-metal	Non-metal
Electronegativity	0.9	1.2	1.6	1.8	2.1	2.5	3.0	-

Physical change across a period

❶ The state of matter

The state of matter of the elements in period 2 and period 3 change from solid to gas across the period (at room temperature)

❷ The atomic size

The atomic size of elements decreases across a period.

This is because：

 The number of proton in the nucleus increases. 『一定要写"in the nucleus"』

 As the number of proton in the nucleus increases, the positive charge of the nucleus also increases.

 Thus, the attraction force between the nucleus and the valence electrons is getting stronger and stronger across the period.

 This force will pulls the valence electron closer to the nucleus and thus reduces the atomic radius.

 Therefore the size of the atom decreases across a period from left to right.

❸The electronegativity

 The electronegativity increases

This is due to the increasing nuclei attraction on the valence electrons and the decrease in atomic size.

 The atom will attract electrons to achieve a stable octet electron arrangement more easily.

Changes in oxide properties of Period 3

 Across period 3, the property of the oxide of element changes from basic to amphoteric and then to acidic.

 Metal oxides are basic (react with acids)

 Oxides of non-metal are acidic (react with alkalis)

 Amphoteric oxide show both basic and acidic properties (react with both acids and alkalis)

Oxide of elements	Na2O	MgO	Al2O3	SiO2	P4O10	SO2	Cl2O7
Name	Sodium oxide	Magnesium oxide	Aluminium oxide	Silicon(IV) oxide	Phosphorus pentoxide	Sulphur dioxide	Dichloride heptoxide
pH value when dissolves in water	13-14	8-9	7-8	6-7	3	3	1
Acid-base properties	Base	Base	Amphoteric	Acid	Acid	Acid	Acid
With dilute nitric acid, HNO3	Dissolves	Dissolves	Dissolves	No  change	No change	No change	No  change
With sodium hydroxide, NaOH solution	No change	No change	Dissolves	Dissolves	Dissolves	Dissolves	Dissolves

Uses of semi-metals (Metalloids)

 Semi-metals or metalloids are weak conductors of electricity

 The most widely used semi-conductors are silicon and germanium

 It is used to make diodes and transistors in electronic industry.