Periodic Trends

Question 1

What does the period of an element show in terms of providing information of its electronic configuration?

Answer 1

The period of an element is the number of principle quantum shells of electrons.

Question 2

State the nature of the following oxides. Hence, state the bonding and structure of the oxides.
1: Na2O
2: MgO
3: Al2O3
4: SiO2
5: P4O6 and P4O10
6: SO2

Answer 2

1: basic oxide, ionic bonding
2: basic oxide, ionic bonding
3: amphoteric oxide, predominantly ionic with covalent bonding
4: acidic oxide, giant molecular structure
5: acidic oxide, simple molecular structure
6: acidic oxide, simple molecular structure

Question 3

State 3 properties that increases when going down a Group in a Periodic Table.

Answer 3

• atomic size increases
• number of principal quantum shells increases
• reactivity increases down a group of metals
• metallic character increases down a group of metals

Question 4

Which Group of elements are known as alkali metals and why?

Answer 4

Group 1 elements. They react readily with water to form alkalis.

Question 5

List the physical properties of alkali metals.

Answer 5

1: The atomic radii of Group 1 metals increase down the group.
2: The shielding effect in Group 1 metals increases down the group.
3: The density of the alkali metals increases down the group.
4: The melting and boiling points of Group 1 metals decrease down the group.
5: Group 1 metals are good conductors of electricity.
6: Alkali metals are relatively soft.

Question 6

Define shielding effect.

Answer 6

The repulsive force exerted on valence electrons by core electrons in an atom.

Question 7

State how the shielding effect changes as it
- goes down a Group.
- goes across a Period.

Answer 7

Shielding effect increases down a Group and remains constant across a Period.

Question 8

Why does the shielding effect of Group 1 metals increase down the group?

Answer 8

Group 1 metals has an increase of 1 core electron shell. The effective nuclear charge decreases and the core electrons have lesser net attraction to the valence electrons, resulting in the nucleus exerting a weaker hold on its valence electrons.

Question 9

Why does the density of alkali metals increase down the group?

Answer 9

Down the group, the atomic mass increases faster than the atomic volume.

Question 10

Why does the melting and boiling points of Group 1 metals decrease down the group?

Answer 10

As the size of the metal cations increases down the group, the electrostatic forces of attraction between the sea of delocalised electrons and the metal cations decreases. The metallic bonding weakens and less energy is required to break the weaker metallic bonds.

Question 11

Explain why the chemical reactivity of Group 1 metals increases down the group.

Answer 11

Down the group, the atomic radii of the metals increase due to more occupied electron shells. Their valence electron becomes progressively further away from the nucleus, experiencing more shielding effect between the valence electrons and the nucleus. The metal atoms lose their valence electrons more readily. The electrostatic forces of attraction between the nucleus and the valence electrons become weaker and less energy is required to remove the valence electron, thus the metals become more reactive down the Group.

Question 12

Describe the reaction when lithium reacts with cold water.

Answer 12

Lithium reacts quickly with cold water and floats on the water. No flame is seen. Effervescence is produced.

Question 13

Describe the reaction when sodium reacts with cold water.

Answer 13

Sodium reacts very quickly with cold water. It melts and burns with a yellow flame. The molten sodium darts around the surface of the water and effervescence is produced.

Question 14

Describe the reaction when potassium reacts with cold water.

Answer 14

Potassium reacts violently with cold water. It melts and burns with a lilac flame and explodes. Effervescence is produced.

Question 15

State the colour of the flame as well as the extent of reactivity when the following metals react with oxygen.
1: Li
2: Na
3: K

Answer 15

Li: crimson flame, vigorous
Na: yellow flame, explosive and dangerous
K: lilac, very explosive and very dangerous

Question 16

Which Group of elements are known as halogens?

Answer 16

Group 17 elements

Question 17

List the physical properties of halogens.

Answer 17

1: They have low melting and boiling points.
2: They are coloured.
3: They are non-conductors of electricity.
4:Their melting and boiling points increase down the group.

Question 18

State the colour of the following elements in gaseous, liquid, solid, aqueous states and in non-polar solvents.
1: F2
2: Cl2
3: Br2
4: I2

Answer 18

F2
in gaseous state: pale yellow
in liquid state: nil
in solid state: nil
in aqueous state: nil
in non-polar solvents: very pale yellow
Cl2
in gaseous state: yellow-green
in liquid state: nil
in solid state: nil
in aqueous state: colourless when dilute, pale yellow when concentrated
in non-polar solvents: very pale green
Br2
in gaseous state: reddish-brown
in liquid state: reddish-brown
in solid state: nil
in aqueous state: yellow when dilute, orange when concentrated
in non-polar solvents: reddish-brown
I2
in gaseous state: violet
in liquid state: violet-black
in solid state: shiny black
in aqueous state: pale yellow in water, brown in aqueous state
in non-polar solvents: violet

Question 19

In terms of bonding and structure, explain why the melting and boiling points of halogens increase down the group.

Answer 19

Halogens exist as simple, non-polar diatomic molecules with dispersion forces between molecules. Down the group, the size of the electron cloud increases and the polarisability of the molecule increases. Stronger dispersion forces exist between the molecules and more energy is needed to overcome the intermolecular forces of attraction.

Question 20

Why does the reactivity of halogens of group 17 decrease down the group?

Answer 20

Halogens form ions by gaining one electron. Down the group, the atomic radius increases as the number of principle quantum shells increases. The electrostatic forces of attraction between the nucleus and the valence electrons become weaker as they are further away from the nucleus. Thus, halogen atoms gain electrons less readily.

Question 21

Describe a displacement reaction.

Answer 21

A displacement reaction is a reaction in which an element takes the places of another element.

Question 22

Which two solutions, when reacted together, will not form a displacement reaction?
A: KI(aq) + Cl2(aq)
B: KI(aq) + Br2(aq)
C: KBr(aq) + I2(aq)
D: KBr(aq) + Cl2(aq)

Answer 22

Answer: C
Option A will form a brown solution.
Option B will form a brown solution.
Option D will form an orange solution.