CHEM2077: Chemistry of the Elements - Lecture 3: Group 2 Alkaline Earth Metals

Vocabulary-style flashcards covering the physical properties, chemical trends, halide structures, hydrides, and solubility thermodynamics of Group 2 Alkaline Earth Metals.

Alkaline Earth Metals

Group 2 elements consisting of $Be$, $Mg$, $Ca$, $Sr$, $Ba$, and $Ra$ characterized by an $ns^2$ valence electron configuration.

Beryllium (Be) Anomaly

The chemically unique behavior of beryllium relative to other Group 2 members due to its extremely small ionic radius ($27\,pm$) and very high charge density.

Charge Density (Be2+)

The result of the exceptionally small size of the beryllium dication which makes it exceptionally polarizing and causes covalent bonding to dominate its chemistry.

Fluorite Structure (8:4)

The crystal structure adopted by fluorides of $Ca$ to $Ba$ (e.g., $CaF_2$), where the metal cation $M^{2+?}$ is surrounded by 8 $F^-$ ions.

Rutile Structure (6:3)

The crystal structure adopted by $MgF_2$ because the $Mg^{2+}$ ion is smaller than heavier Group 2 cations.

Layered Structures

Structural formats like $CdCl_2$ adopted by $Mg$, $Ca$, $Sr$, and $Ba$ chlorides, bromides, and iodides due to the polarizability of large halide ions.

Group 2 Fluoride Solubility

These compounds are poorly soluble in water because the high lattice enthalpy is not offset by the hydration enthalpy.

Beryllium Halide solid phase structure

A 1-dimensional polymeric chain featuring bridging halogen atoms.

Lewis Acidity of $BeCl_2$

The property of beryllium chloride that allow it to act as an electron pair acceptor and dissolve in donor organic solvents like $Et_2O$ to form complexes.

Saline Hydrides

Ionic hydrides formed by metals from $Ca$ to $Ra$; they adopt distorted $hcp$ structures and serve as powerful bases and drying agents.

3-center 2-electron (3c-2e) bonds

The type of bonding found in covalent, electron-deficient compounds such as $BeH_2$ and $BeMe_2$.

Lattice Enthalpy ($\Delta_{latt}H^\theta$)

One of the two thermodynamic factors that competes with hydration enthalpy ($\Delta_{hyd}H^\theta$) to determine the solubility of ionic salts.

Solubility Rule of Thumb

The observation that ionic salts are poorly soluble if the cation and anion are approximately the same size (e.g., $BaSO_4$ or $LiF$).

Beryllium Coordination Number

Typical coordination of 4 due to the atom's small size, whereas heavier congeners in Group 2 can be 6- or 8-coordinate.

Beryllium Chloride ($BeCl_2$) Gas Phase

Exist only at very high temperatures (>900^∘C) as a linear monomeric $Cl-Be-Cl$ species.