Group 2

10 Group 2 10.1 Selected Group 2 elements and their compounds		10  7	Candidates should be able to:  (a) describe the trends in physical properties of Group 2 elements: Mg, Ca, Sr, Ba; (b) describe the reactions of Group 2 elements with oxygen and water; (c) describe the behaviour of the oxides of Group 2 elements with water; (d) explain qualitatively the thermal decomposition of the nitrates, carbonates and hydroxides of Group 2 elements in terms of the charge density and polarisability of large anions; (e) explain qualitatively the variation in solubility of sulphate of Group 2 elements in terms of the relative magnitudes of the enthalpy change of hydration for the relevant ions and the corresponding lattice energy.
10.2 Anomalous behaviour of beryllium	2			Candidates should be able to: (a) explain the anomalous behaviour of beryllium as exemplified by the formation of covalent compounds; (b) describe the diagonal relationships between beryllium and aluminium; (c) explain the similarity of aqueous beryllium salts to aqueous aluminium salts in terms of their acidic property.
10.3 Uses of Group 2 compounds	1			Candidates should be able to: (a) state the uses of Group 2 compounds in agriculture, industry and medicine.

Physical properties of group 2 elements

1. Group 2 are also known as alkali earth metal.

2. All have valence shell electronic configurations of ns²

3. They are all reactive metals and are not found in the free elemental states in nature.

4. In their pure state, they have a silvery colour but tarnish rapidly in air due to the formation of an oxide layer on the metal’s surface.

For instance ;

                                        2Mg(s)   + O₂(g) à 2MgO(s)

5. They are soft and can be easily cut with a knife

1. Group 2 are also known as alkali earth metal.

2. All have valence shell electronic configurations of ns²

3. They are all reactive metals and are not found in the free elemental states in nature.

4. In their pure state, they have a silvery colour but tarnish rapidly in air due to the formation of an oxide layer on the metal’s surface.

For instance ;

                                        2Mg(s)   + O₂(g) à 2MgO(s)

5. They are soft and can be easily cut with a knife

6. Group 2 elements give characteristic flame test as below;

Element	Magnesium	Calcium	Strontium	Barium
Color  of the flame	Brilliant white	Brick red	Crimson	Apple green

7. The elements of Group 2 and some basic physical properties are described as below;

Name , symbol	Proton number	Atomic radius/ nm	Melting point (oC)	1st ionizationenergy (kJ/mol)
Beryllium , Be	4	0.11	1287	900
Magnesium, Mg	12	0.16	650	738
Calcium, Ca	20	0.197	842	590
Strontium, Sr	38	0.215	777	550
Barium, Ba	56	0.218	727	503

Atomic radius

1.  Atomic radius depend on 2 factors

a)  Nuclear charge

b)  Screening effect

2. When going down to Group 2, both screening effect and nuclear charge increase.

3. However, the increase in screening effect is greater as more shell is used to filling in the electrons.

4. This will cause the effective nuclear charge to decrease causing the outermost electron to be further away from the nucleus.Hence atomic radius increase.

Melting point

1. The melting point of the Group 2 generally decrease when goes down to group.

2. As goes down to Group 2 the metallic bonding become weaker as the delocalized electrons became further than nucleus.

3. The only thing is that, magnesium, which had a melting point lower than expected. This is one of the anomaly of Group 2 as  magnesium has a diagonal relationship with lithium. So the metallic behavior of magnesium will be more or less similar to that of lithium

Ionizations Energy

1. The 1st ionization energy ................. when goes down to Group 2.

2.  Atomic size when goes down to Group 2 which contribute the decrease in ionization energy. Furthermore, with the increase in atomic size, the number of shell also increase thus causing the screening effect to increasae.

3. This may decrease the effective nuclear charge as the distance between the electron and the nucleus is getting further.decrease

4. The third ionization energy of Group 2 elements are extremely high, which suggested that the 3rd electron the withdrawn from an inner shell. Thus Group 2 elements only goes through 2nd ionization energy and form a stable M2+.

 First Ionization energy                 : M (g) → M⁺ (g) + e-

 Second Ionization energy           : M⁺ (g) → M²⁺ (g) + e

Element	Be	Mg	Ca	Sr	Ba
1st ionization energy (kJ/mol)	900	740	590	550	500
2nd ionization energy (kJ/mol)	2700	2190	1740	1610	1470

Electronegativity

1. Going down the group the electronegativity decreases

Element	Electronegativity
Be	1.50
Mg	1.30
Ca	1.00
Sr	0.97
Ba	0.93

Chemical Properties of Group 2 elements.

1. When goes down to Group 2, the ionization energy decrease.

2.  Meaning that, it is easier to remove 2 electrons.

3. In the angle of electrochemistry, when goes down to Group 2, the reducing ability increased, meaning when goes down to

Group 2, it increase as a reducing agent

Element	Be	Mg	Ca	Sr	Ba
E0/V	1.85	-2.37	-2.87	-2.89	-2.9

The more negative the value of E^o value . the stronger the reducing power/agent

Exam Question

1.(a) Group 2 elements and compounds show some marked trends in physical and chemical properties within the group with increasing atomic number.

(i) State and explain the trend in solubility of the Group 2 sulphates with increasing atomic number.                [4]

2.Anhydrous beryllium chloride has polymeric structure in the solid state. In the vapour state, it forms dimer

at 405 °C and monomer at 900 °C. [STPM 2013 Semester system]

(a) Draw the structures for the monomer and dimer of beryllium chloride.                                                   [2]

Monomer	Dimer

(b) All the chlorides of Group 2 elements of the Periodic Table are ionic compounds except beryllium chloride.Explain why beryllium chloride is an exception.                                                                      [3]

(c) Beryllium shows a diagonal relationship to aluminium in Group 13 of the Periodic Table.

Write chemical formulae of the beryllium compound and aluminium compound which resemble each other,

and state their property.                                                                                                                                                     [3]

Beryllium compound	Aluminium compound	Acid-base property

(d) Compare the thermal stability of barium carbonate with magnesium carbonate. Explain your answer. [3]