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Trend 2.4 of the Beryllium Group (Group IIA) Trends is that Group II metal oxide basicity and hydroxide solubility in water increase as you go down the column.

BeO and Be(OH)2 are amphoteric and react with acids and strong bases such as NaOH. MgO is basic and Mg(OH)2 is weakly basic and do not dissolve in NaOH solution. The oxides of calcium, strontium, and barium are basic and the hydroxides are strongly basic. The solubilities of the hydroxides in water follow the order: Be(OH)2 < Mg(OH)2 < Ca(OH)2 < Sr(OH)2 < Ba(OH)2

Metal Oxide Basicity[]

Group II metal oxides become more basic as you go down the column. This trend is easily seen if you compare the electronegativity of the group II metal to the electronegativity of oxygen.

Element Electronegativity[1] Electronegativity
O 3.44
Be 1.57 1.87
Mg 1.31 2.13
Ca 1.00 2.44
Sr 0.95 2.49
Ba 0.89 2.55

As you can see the electronegativities[2] of the metals decrease down the column making the change in electronegativities increases down the group. The greater the difference in electronegativity the more ionic the metal-oxygen bond becomes. The more ionic the metal-oxygen bond the more basic the oxide is (see Figure 11.12 from Rodgers[3]).

Rodgers 11.12






Metal Hydroxide Solubility[]

Group II metal hydroxides become more soluble in water as you go down the column. This trend can be explained by the decrease in the lattice energy of the hydroxide salt and by the increase in the coordination number of the metal ion as you go down the column.

Element Lattice Energy (kJ/mol) [4] Coordination Number
Be 3620 4
Mg 2998 6
Ca 2637 6
Sr 2474 8
Ba 2330 8


The larger the lattice energy the more energy it takes to break the lattice apart into metal and hydroxide ions. Since the atomic radii [5] increase down the group it makes sense that the coordination numbers also increases because the larger the metal ion the more room there is for water molecules to coordinate to it. See Figure 11.8 from Rodgers[3] of water interacting with the NaCl lattice to see water molecules breaking apart the ionic lattice and coordinating to ions.

Rodgers 11.8








References[]

  1. CHEMIX School - Periodic table with electronegativity chart http://www.standnes.no/chemix/periodictable/electronegativity-chart.html
  2. Creative Chemistry. “Trend in electronegativity of Group 2 elements.” http://www.creative-chemistry.org.uk/alevel/module1/trends3.html
  3. 3.0 3.1 Rodgers, Glen E. “Descriptive Inorganic Coordination, and Solid-State Chemistry.” Ch. 11, pg. 297.
  4. Mu, Lai-Long, He, Hong-Mei, Feng, Chang-Jun. “Lattice Energy Estimation for Complex Inorganic Ionic Crystal.” Chinese Journal of Chemistry 24 (2006): 855-861. http://onlinelibrary.wiley.com/doi/10.1002/cjoc.200690163/abstract
  5. Creative Chemistry. “Trend in atomic radius of Group 2 elements.” http://www.creative-chemistry.org.uk/alevel/module1/trends1.html
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