Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 110	General Chemistry	Fall 2003
Lecture Notes::Lec 25_29 October	© R. Paselk 2003
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The Chemistry of the Elements

The Representative Elements

Group II - the Alkaline Earth Metals

Chemistry

• Reactive metals with electronic configurations of: [noble gas]ns²
  • Readily lose outermost electrons to attain a noble gas configurations.
  • Much too reactive to be found free in nature.
  • Reactivity generally increases from Be to Ba.
• Group II elements other than Be are very metallic.
  • Relatively soft metals, but not as soft as Alkali metals.
  • Note properties in table below, discuss mp, density, ionization energies, reduction potentials and chemistry etc.

Properties of Group II - the Alkaline Earth Metals

Property		Be	Mg	Ca	Sr	Ba	Ra
Valence-shell electron configuration		2s2	3s2	4s2	5s2	6s2	7s2
Melting point (°C)		1280	651	851	800	850	960
Density (g/cm3)		1.86	1.75	1.55	2.6	3.6	5.0
Ionization energies - 1st ,2nd & 3rd (kJ/mol) M(s) Æ M2+(aq) + 2 e-		899 1757 14849	738 1450 7730	590 1145 4941	549 1064 4207	503 965 3420	509 978 -
Standard Reduction Potentials (V, 25°C) M2+(aq) + 2 e- Æ M(s)		-1.69	-2.37	-2.87	-2.89	-2.91	-2.92
Electronegativity		1.5	1.2	1.0	1.0	0.9	0.9

Beryllium is exceptional, as are other Period 2 elements as a result of small radii and consequently strong bonding.

• Be²⁺ ionic radius is similar to that of Al³⁺, giving them some chemical similarities
  • This is a common phenomena on the Periodic table, with a diagonal relationship occurring between the first member of a group and the diagonal member of the third group.
• All Be compounds have significant covalent bonding.
• Be²⁺ does not exist as such in either ionic crystals or in solution.
• Be is fairly unreactive at room temperature.
• BeH₂ forms a polymer, with two hydrogens bridging each Be in Be-H-Be bonds (tetrahedral).
  • The Be-H-Be bond is known as a three center bond. Only one electron pair is involved in this bond! (Can only accommodate one pair around H.)
  •  
• Be forms halogen compounds of the formula BeX₂. As an example;
  • BeCl₂ is linear.
  • Note Be has only a tetrad of electrons around it, not the classic octet!
  • As expected from VSEPR theory BeCl₂ is linear, however, in its crystal structure a chain is formed with each Be in e center of a tetrahedron with Cl's at the corners (see figure in text, p 902)
  • Be in BeCl₂ is very electron deficient, and is thus a good Lewis acid, reacting with Lewis bases such as ammonia: BeCl₂ + NH₃ Æ Cl₂Be-NH₃
• Be is found in mineral beryl (gem quality = emerald). Used to harden copper alloys. Toxic as element and in compounds.

Properties of the remaining Alkaline Earths are more similar.

• Magnesium has both covalent and ionic compounds:
  • Grignard Reagents in OChem: R-Mg-Cl, where R- = a carbon aduct such as CH₃CH₂-
  • Magnesium is found in nature as dolomite, CaMg(CO₃)₂, as well as carbonates and silicates and in sea water.
  • It is an essential element in biology as a cofactor for NTP metabolism.
  • Valuable metal in light-weight alloys.
• Calcium and the remaining Group II elements form ionic compounds with oxidation states of +2.
  • Calcium is found in vast deposits of limestone and marble formed from ancient calcium carbonate plankton shells.
  • Calcium and "hard water."
    • Soap scum: Divalent ions such as calcium can "cross-link" soap molecules, e.g with sodium stearate:
  • 2CH₃(CH₂)₁₅CH₂COO^-Na⁺_(aq) + Ca²⁺_(aq) Æ CH₃(CH₂)₁₅CH₂COO^-Ca^2+-OOCCH₂(CH₂)₁₅CH_3(s)
    • Boiler scale: Ca²⁺_(aq) + 2HCO₃^-_(aq) Æ CaCO_3(s) + CO_2(g) + H₂O_(l)
  • Also found as gypsum (CaSO₄*2H₂O) and apatite (Ca₁₀(OH)₂(PO₄)₆), a major component of bone and tooth enamel.
• Strontium and Barium: Like Ca, compounds are ionic with oxidation state of 2+
  • Strontium has similar characteristics to calcium, however it forms more insoluble precipitates with phosphate. This can be very important in that Sr replaces Ca, for example in bone. This partially explains the particular danger of the radioisotope ⁹⁰Sr, which is produced in atomic bomb tests.
  • Ba, which is quite toxic as an ion, forms a salt, BaSO₄, which is harmless because it is so incredibly insoluble.

© R A Paselk

Last modified 30 October 2003