Properties and Chemical Reactivity — Revision Notes

⚡ 30-Second Revision

Electronic Configuration: — $ns^2$
Ion Formation: — $M^{2+}$
Atomic/Ionic Radii: — Increase down group
Ionization Enthalpy: — Decreases down group
Electronegativity: — Decreases down group
Metallic Character/Reactivity: — Increases down group
Reducing Power: — Increases down group
Hydration Enthalpy of $M^{2+}$: — Decreases down group
Solubility of Hydroxides ($M(OH)_2$): — Increases down group
Solubility of Sulfates ($MSO_4$): — Decreases down group
Thermal Stability of Carbonates/Nitrates: — Increases down group
Beryllium: — Anomalous, covalent compounds, amphoteric $BeO/Be(OH)_2$, no flame test, diagonal relationship with Al.
Flame Coloration: — Ca (brick-red), Sr (crimson-red), Ba (apple-green). Be, Mg do not show.
Reaction with Water: — $M(s) + 2H_2O(l) \rightarrow M(OH)_2(aq) + H_2(g)$ (reactivity increases down group, Be unreactive, Mg slow with cold water).

2-Minute Revision

Alkaline earth metals (Group 2) are characterized by their $ns^2$ electronic configuration, leading to the formation of $M^{2+}$ ions. Key trends down the group include increasing atomic size, decreasing ionization enthalpy, and increasing metallic character and reactivity.

They are strong reducing agents. Their hydration enthalpy decreases down the group, which is crucial for solubility trends. Hydroxides ($M(OH)_2$) show increasing solubility down the group, while sulfates ($MSO_4$) show decreasing solubility.

Thermal stability of carbonates and nitrates increases down the group due to decreasing polarizing power of the cation. Beryllium exhibits anomalous behavior due to its small size and high charge density, forming covalent and amphoteric compounds, and showing a diagonal relationship with Aluminium.

Only Ca, Sr, and Ba impart characteristic colors to a flame. They react with water to form hydroxides and hydrogen, with reactivity increasing from Mg to Ba, while Be is unreactive.

5-Minute Revision

Alkaline earth metals (Be, Mg, Ca, Sr, Ba) are s-block elements with $ns^2$ valence electrons, forming $M^{2+}$ ions. Their properties are governed by this configuration and periodic trends. Atomic and ionic radii increase down the group, while ionization enthalpy and electronegativity decrease, leading to an increase in metallic character and reducing power.

Hydration enthalpy of $M^{2+}$ ions decreases down the group due to increasing size. This influences solubility: $M(OH)_2$ solubility increases (e.g., $Mg(OH)_2$ sparingly soluble, $Ba(OH)_2$ soluble), while $MSO_4$ solubility decreases (e.

g., $MgSO_4$ soluble, $BaSO_4$ insoluble). Thermal stability of $MCO_3$ and $M(NO_3)_2$ increases down the group as the polarizing power of the cation decreases. For example, $BeCO_3$ is unstable, while $BaCO_3$ is very stable.

Chemically, they react with oxygen to form oxides ($MO$), and sometimes peroxides ($BaO_2$) or nitrides ($M_3N_2$). They react with water to form hydroxides and hydrogen gas, with reactivity increasing down the group ($Be$ unreactive, $Mg$ slow with cold water, $Ca, Sr, Ba$ react readily).

They form ionic halides ($MX_2$) with halogens, though $BeX_2$ is covalent. Beryllium is anomalous: it forms covalent compounds, its oxide ($BeO$) and hydroxide ($Be(OH)_2$) are amphoteric, it doesn't react with water, and it doesn't show flame coloration.

It also exhibits a diagonal relationship with Aluminium, sharing properties like amphoteric oxides and covalent compound formation. Calcium, Strontium, and Barium impart characteristic colors to a flame (brick-red, crimson-red, apple-green respectively) due to electron excitation.

Prelims Revision Notes

1
Electronic Configuration: — All Group 2 elements have $ns^2$ valence electrons. They readily lose these two electrons to form $M^{2+}$ ions, achieving a stable noble gas configuration. This makes them strong reducing agents.
2
Atomic & Ionic Radii: — Increase down the group ($Be < Mg < Ca < Sr < Ba$).
3
Ionization Enthalpy (IE): — $IE_1$ and $IE_2$ are relatively low but higher than Group 1. Both decrease down the group ($Be > Mg > Ca > Sr > Ba$). The sum $IE_1 + IE_2$ is crucial for $M^{2+}$ formation.
4
Electronegativity: — Decreases down the group ($Be > Mg > Ca > Sr > Ba$).
5
Metallic Character & Reactivity: — Increases down the group ($Be < Mg < Ca < Sr < Ba$).
6
Hydration Enthalpy: — High for $M^{2+}$ ions, decreases down the group ($Be^{2+} > Mg^{2+} > Ca^{2+} > Sr^{2+} > Ba^{2+}$).
7
Physical Properties: — Silvery-white, lustrous, harder than alkali metals. Melting/boiling points are higher than Group 1. Density generally increases down the group.
8
Flame Coloration: — Ca (brick-red), Sr (crimson-red), Ba (apple-green). Be and Mg do not show flame coloration due to high excitation energy.
9
Reactivity with Air/Oxygen: — Form oxides ($MO$). Mg burns brilliantly. Ba can form peroxide ($BaO_2$). All form nitrides ($M_3N_2$) at high temperatures.
10
Reactivity with Water: — $M(s) + 2H_2O(l) \rightarrow M(OH)_2(aq) + H_2(g)$. Reactivity increases down the group. Be does not react. Mg reacts slowly with cold water, vigorously with steam. Ca, Sr, Ba react readily with cold water.
11
Reactivity with Halogens: — Form halides ($MX_2$). $BeX_2$ is covalent and polymeric.
12
Reactivity with Hydrogen: — Form hydrides ($MH_2$). $BeH_2$ is covalent and polymeric.
13
Solubility Trends:

* **Hydroxides ($M(OH)_2$):** Solubility increases down the group. $Be(OH)_2$ (amphoteric) $ightarrow Ba(OH)_2$ (strong base). * **Sulfates ($MSO_4$):** Solubility decreases down the group. $BeSO_4, MgSO_4$ (soluble) $ightarrow BaSO_4$ (insoluble). * **Carbonates ($MCO_3$):** Generally insoluble, solubility decreases down the group.

1
Thermal Stability:

* **Carbonates ($MCO_3$):** Increases down the group. $BeCO_3$ (unstable) $ightarrow BaCO_3$ (stable). Decomposition: $MCO_3 \xrightarrow{Delta} MO + CO_2$. * **Nitrates ($M(NO_3)_2$):** Increases down the group. Decomposition: $M(NO_3)_2 \xrightarrow{Delta} MO + 2NO_2 + \frac{1}{2}O_2$.

1
Anomalous Behavior of Beryllium: — Smallest size, highest IE, high polarizing power. Forms covalent compounds, amphoteric $BeO/Be(OH)_2$, does not react with water, does not show flame test, max coordination number 4.
2
Diagonal Relationship: — Be with Al (similar charge/radius ratio, amphoteric oxides, covalent compounds, complex formation).

Vyyuha Quick Recall

Bright Metals Can Shine Brilliantly: Be, Mg, Ca, Sr, Ba (Group 2 elements).

Solubility Hydroxides Increase, Sulfates Decrease: Remember the opposite trends for solubility of hydroxides and sulfates down the group.

Be Always Anomalous: Beryllium is Anomalous and has a diagonal relationship with Aluminium.