Group 2 elements (alkaline earth metals) all have two electrons in their outermost principal quantum shell. All Group 2 metals can form ionic compounds in which they donate their two outermost electrons (acting as reducing agents) to form a +2 charge ion (so they become oxidized).
Alkaline Earth Metals include Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra).
Metals become more reactive as they move down the group.
Interesting Science Videos
Trends in Thermal Stability of Group 2 Metals
- As two electrons are lost from their valence shells, all Group 2 metals form 2+ ions.
- The top metal cations in the group are smaller in size than those at the bottom. For example, beryllium (the first element in Group 2) has an atomic radius of 112 pm, whereas calcium (further down the group) has an atomic radius of 197 pm.
- The metal cations at the top of Group 2 have the highest charge density because the same charge (2+) is packed into a smaller volume. As a result, smaller Group 2 ions polarize neighboring negative ions more strongly.
- When a carbonate or nitrate ion comes into contact with a cation, it becomes polarised. This is because the metal cation attracts the electrons in the carbonate or nitrate ion.
- The less heat required to thermally decompose an anion, the more polarised it is. As a result, the thermal stability increases as one moves down the group.
- As one moves down the group, the cation becomes larger, resulting in a lower charge density and a lower polarising effect on the carbonate or nitrate anion. As a result, the anion is less polarized, and more heat is needed to thermally decompose them.
Trends in Enthalpy Change of Lattice Energy
The lattice energy of the ionic compound decreases as one moves down the group because the positively charged cations become larger as one moves down the group. Also, there is more space between the negatively and positively charged ions in the ionic compound, and the attractive forces between the ions are weaker.
Additionally, the electrostatic forces between the ions are weaker, and less energy is released during the formation of the ionic compound from its gaseous ions. As a result, the lattice energy becomes less exothermic.
Trends in the Solubility of Group 2 Elements
Group 2 hydroxides become more soluble as they move down the group.
In contrast, the solubility of Group 2 sulfates decreases as one moves down the group.
Sparingly soluble compounds are those with extremely low solubility. Except for barium sulfate, which is insoluble in warm water, most sulfates are soluble in warm water.
| Elements of Group 2 | Hydroxides [M(OH)2] | Sulfates [MSO4] |
| Magnesium (Mg) | Least Soluble | Most Soluble |
| Calcium (Ca) | ↓ | ↓ |
| Beryllium (Be) | ↓ | ↓ |
| Barium (Ba) | Most Soluble | Least Soluble |
Trends in Enthalpy Change of Hydration Energy
The hydration energy decreases as one moves down the group, and the positively charged ions become larger. As a result, the ion-dipole bonds formed between cations and water molecules weaken.
This means that less energy is released when the gaseous Group 2 ions become hydrated. As a result, the standard enthalpy change of hydration becomes less exothermic.
Hydroxides and Sulfates of Group 2
| Hydroxide ions | Sulfate ions |
| Hydroxide ions are relatively small ions in Group 2 hydroxides. | Sulfate ions are quite large ions. |
| The lattice energy falls more quickly than the standard enthalpy change of hydration. | The lattice energy falls more slowly than the standard enthalpy change of hydration. |
| As a result, as one moves down the group, the enthalpy change of the solution becomes more exothermic. | As one moves down the group, the standard enthalpy of solution becomes more endothermic. |
The compound is more soluble when the standard enthalpy of solution is more exothermic.
This is why sulfates become less soluble as they move down the group and hydroxides become more soluble.
Effect of heat on the Group 2 carbonates
All of the carbonates in this group decompose thermally to metal oxide and carbon dioxide gas. The term “thermal decomposition” refers to the process of breaking down a compound by heating it.
All of the Group 2 carbonates and their oxides are white solids.
If “X” represents any of the elements, this decomposition is as follows:
XCO3 (s) → XO (s)+CO2 (g)
Carbonates require more heat to decompose as they move down the group. The carbonates become more thermally stable as they progress through the group.
Effect of heat on the Group 2 Nitrates
Thermal decomposition of Group 2 nitrates results in metal oxide, nitrogen dioxide, and oxygen gas. When heated, these compounds emit white solids as well as brown nitrogen dioxide and oxygen gases.
Magnesium and calcium nitrates typically crystallize with water, and the solid may dissolve in its own crystallization water to form a colorless solution before decomposing.
If “X” represents any of the elements, this decomposition is as follows:
2X(NO3)2 (s) → 2XO (s) + 4NO2 (g) + O2 (g)
The nitrates further down the chain must also be heated more vigorously before they decompose.
Group 2 nitrates also become more thermally stable as they progress through the group.
Other Compounds of Group 2 Elements
Carbides: Except for beryllium, alkaline earth metals and their oxides react with carbon to form carbides. Carbides react with water to produce acetylene gas, which is then used as a source of the gas.
M + 2C → MC2 MC2 + 2H2O → M(OH) 2 + C2H2
Oxides: Beryllium only reacts with oxygen above 600°C. Magnesium and strontium burn in oxygen to form oxides, while Barium forms peroxides.
BeO and MgO are more covalent than the other oxides. Beryllium oxide is amphoteric, while magnesium and calcium oxide is weakly basic, and other oxides are basic.
Bicarbonates: Bicarbonates are water soluble and exist only in solution. Carbonate solubility decreases from Be to Ba. Carbonates dissolve in the presence of carbon dioxide by forming bicarbonates.
Halides: All halogens react with alkaline earth metals ranging from calcium to barium to form solid ionic halides with a distinct crystal structure. From fluorine to iodine, reactivity decreases. Because of the high polarization of the small covalent ion on the electron cloud of the halogen anion, as indicated by Fajan’s rule, beryllium halides have more covalent bonding.
References
- https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_and_Websites_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/1_s-Block_Elements/Group__2_Elements%3A_The_Alkaline_Earth_Metals/1Group_2%3A_Chemical_Reactions_of_Alkali_Earth_Metals/The_Thermal_Stability_of_the_Nitrates_and_Carbonates
- https://byjus.com/question-answer/discuss-the-trend-of-the-following-i-thermal-stability-of-carbonates-of-group-2-elements/
- https://www.savemyexams.co.uk/a-level/chemistry/cie/22/revision-notes/6-inorganic-chemistry-a-level-only/6-1-group-2-a-level-only/6-1-1-similarities-trends–compounds-of-magnesium-to-barium/
- https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/Supplemental_Modules_and_Websites_(Inorganic_Chemistry)/Descriptive_Chemistry/Elements_Organized_by_Block/1_s-Block_Elements/Group__2_Elements%3A_The_Alkaline_Earth_Metals/1Group_2%3A_Chemical_Reactions_of_Alkali_Earth_Metals/The_Solubility_of_the_Hydroxides_Sulfates_and_Carbonates
