Group 2 Elements: Alkaline Earth Metals — Revision Notes

⚡ 30-Second Revision

Electronic Config: — $[Noble Gas] ns^2$
Oxidation State: — +2
Trends Down Group:

- Atomic/Ionic Radii: $\uparrow$ - Ionization Enthalpy: $\downarrow$ - Hydration Enthalpy: $\downarrow$ - Metallic Character: $\uparrow$ - Basicity of Hydroxides ($M(OH)_2$): $\uparrow$ - Thermal Stability of Carbonates ($MCO_3$): $\uparrow$ - Solubility of Sulphates ($MSO_4$): $\downarrow$

Anomalous Be: — Covalent compounds, amphoteric $BeO/Be(OH)_2$, max C.N. 4, diagonal relation with Al, no flame colour.
Flame Colours: — Ca (brick-red), Sr (crimson-red), Ba (apple-green). Be, Mg (no colour).
Biological Importance: — $Mg^{2+}$ (chlorophyll, enzymes), $Ca^{2+}$ (bones, blood clotting, muscle contraction).
Key Compounds: — $BaSO_4$ (X-ray contrast), $Mg(OH)_2$ (antacid), $CaCO_3$ (limestone).

2-Minute Revision

The Group 2 elements, alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra), all have an $ns^2$ electronic configuration and form stable $M^{2+}$ ions. Key trends down the group include increasing atomic and ionic radii, metallic character, and basicity of hydroxides.

Conversely, ionization enthalpy, hydration enthalpy of $M^{2+}$ ions, and electronegativity decrease. The solubility of sulphates ($MSO_4$) decreases down the group, while the thermal stability of carbonates ($MCO_3$) increases.

Beryllium exhibits anomalous behavior due to its small size and high charge density, leading to covalent compounds, amphoteric oxides/hydroxides, and a diagonal relationship with Aluminium. Unlike other members, Be and Mg do not impart color to a flame; however, Ca gives brick-red, Sr crimson-red, and Ba apple-green.

Magnesium is vital for chlorophyll in plants and enzyme function in animals, while Calcium is crucial for bones, teeth, blood clotting, and muscle contraction. Remember $BaSO_4$ for X-ray imaging due to its insolubility and high atomic mass.

5-Minute Revision

Alkaline earth metals (Group 2) are Be, Mg, Ca, Sr, Ba, and Ra, characterized by their $ns^2$ valence electron configuration and a tendency to form $M^{2+}$ ions. Their reactivity, though significant, is less than Group 1 alkali metals due to higher ionization enthalpies.

Key Trends:

Atomic/Ionic Radii: — Increase down the group as new shells are added.
Ionization Enthalpy: — Decreases down the group due to increasing size and shielding, making electron removal easier.
Hydration Enthalpy: — Decreases down the group. Smaller ions ($Be^{2+}$) have higher charge density and are more hydrated.
Metallic Character: — Increases down the group as electrons are more easily lost.
Basicity of Hydroxides ($M(OH)_2$): — Increases down the group. $Be(OH)_2$ is amphoteric, $Mg(OH)_2$ is sparingly soluble, and $Ba(OH)_2$ is a strong base.
Solubility of Sulphates ($MSO_4$): — Decreases down the group ($BeSO_4 > MgSO_4 > CaSO_4 > SrSO_4 > BaSO_4$). This is due to hydration enthalpy decreasing more rapidly than lattice enthalpy for larger cations.
Thermal Stability of Carbonates ($MCO_3$): — Increases down the group. Smaller cations polarize the carbonate anion more, making $BeCO_3$ less stable than $BaCO_3$.

Anomalous Behavior of Beryllium: Due to its small size, high ionization enthalpy, and high electronegativity, Be forms covalent compounds (e.g., $BeCl_2$ is polymeric), has amphoteric oxide ($BeO$) and hydroxide ($Be(OH)_2$), and a maximum coordination number of 4. It shows a diagonal relationship with Aluminium.

Flame Colouration: Ca (brick-red), Sr (crimson-red), Ba (apple-green). Be and Mg do not show flame coloration due to tightly bound electrons.

Biological Importance: $Mg^{2+}$ is the central metal in chlorophyll and an enzyme cofactor. $Ca^{2+}$ is essential for bones, teeth, blood clotting, and muscle contraction.

Important Compounds: $BaSO_4$ is used as a non-toxic X-ray contrast agent (barium meal) due to its insolubility. $Mg(OH)_2$ is an antacid. $CaCO_3$ is limestone/marble.

Prelims Revision Notes

Group 2 Elements: Alkaline Earth Metals (Be, Mg, Ca, Sr, Ba, Ra)

1. Electronic Configuration & Oxidation State:

General configuration: $[Noble Gas] ns^2$.
Always form +2 ions ($M^{2+}$) by losing two valence electrons.
Strong reducing agents.

2. Physical Properties & Trends:

Atomic/Ionic Radii: — Increase down the group (new shells).
Ionization Enthalpy ($IE_1, IE_2$): — Decrease down the group (larger size, more shielding).
Hydration Enthalpy of $M^{2+}$ ions: — Decrease down the group (larger ions, lower charge density, weaker hydration). $Be^{2+}$ is most hydrated.
Metallic Character: — Increases down the group (easier electron loss).
Electronegativity: — Decreases down the group.
Density: — Irregular trend (Mg > Ca).
Melting/Boiling Points: — Generally higher than Group 1 (stronger metallic bonding), but irregular trend.
Flame Colouration: — Ca (brick-red), Sr (crimson-red), Ba (apple-green). Be and Mg do NOT show flame colour (tightly bound electrons).

3. Chemical Properties & Reactivity:

Reactivity with Air/Oxygen: — Form oxides (MO) and nitrides ($M_3N_2$). Be forms protective oxide layer. Mg burns brilliantly. Reactivity increases down the group.
Reactivity with Water: — Form hydroxides ($M(OH)_2$) and $H_2$. Be does not react. Mg reacts with hot water. Ca, Sr, Ba react vigorously with cold water. Reactivity increases down the group.
Reactivity with Halogens: — Form halides ($MX_2$).
Reactivity with Hydrogen: — Form hydrides ($MH_2$). $BeH_2$ is covalent/polymeric; others are ionic.
Reactivity with Acids: — Liberate $H_2$.

4. Anomalous Behavior of Beryllium:

Smallest size, highest IE, highest EN in the group.
Forms predominantly covalent compounds (e.g., $BeCl_2$ is polymeric).
Oxide ($BeO$) and hydroxide ($Be(OH)_2$) are amphoteric (react with both acids and bases).
Maximum coordination number of 4 (no d-orbitals).
Does not react with water/steam.
Shows diagonal relationship with Aluminium (similar charge/size ratio).

5. Important Compounds & Their Properties:

Oxides (MO): — $BeO$ (amphoteric), others are basic. Basicity increases down the group.
Hydroxides ($M(OH)_2$): — Solubility and basic strength increase down the group. $Be(OH)_2$ (amphoteric), $Mg(OH)_2$ (sparingly soluble, antacid), $Ca(OH)_2$ (slaked lime).
Carbonates ($MCO_3$): — Thermal stability increases down the group ($BeCO_3$ least stable). Solubility generally decreases.
Sulphates ($MSO_4$): — Solubility decreases down the group ($BeSO_4 > MgSO_4 > CaSO_4 > SrSO_4 > BaSO_4$). $BaSO_4$ is highly insoluble (used as X-ray contrast).
Nitrates ($M(NO_3)_2$): — All soluble. Decompose to MO, $NO_2$, $O_2$ on heating.

6. Biological Importance:

Magnesium ($Mg^{2+}$): — Central atom in chlorophyll, essential for photosynthesis. Activator for many enzymes, involved in nerve and muscle function.
Calcium ($Ca^{2+}$): — Most abundant mineral in body. Essential for bones, teeth, blood clotting, muscle contraction, nerve impulse transmission.

Vyyuha Quick Recall

Be Mag Can Store Bananas Radically

Be: Beryllium (Anomalous, covalent, amphoteric)
Mag: Magnesium (Chlorophyll, no flame)
Can: Calcium (Bones, brick-red flame)
Store: Strontium (Crimson-red flame)
Bananas: Barium (Apple-green flame, $BaSO_4$ X-ray)
Radically: Radium (Radioactive)