Electronic Configurations & Oxidation States

Akaline-earth metals subgroup elements are s-elements with two electrons at the external energy level, n:

ns²,

Therefore ns²-electrons are easily removed and Met⁺² state is energetically favourable. Alkaline-earth metals always have the oxidation state +2 in compounds.

The name alkaline-earth elements are attributed to those metals whose oxides (“earths” of alchemists) produce an alkaline reaction in water.

Ca, Sr, Ba and Ra form a series of elements, within which their chemical and physical properties change regularly, similarly to the s-elements of the first group behavior. The examples of such systematic variation of properties in the Ca—Sr—Ba—Ra series are:

• diminishment of solubility of sulfates, nitrates, chlorides and others;

• increase of thermal stability of carbonates, nitrates, peroxides;

• increase of reaction rates of those metals interaction with hydrogen.

Like Fr, all isotopes of Ra are radioactive. Most long-living isotopes ₂₂₆Ra have the half-decay period Т_1/2  1600 years.

Physical properties

Be and Mg are lustrous metals that are quickly covered by thin coating layer of oxides, which gives them opaque appearance. Be is a metal of light-gray color, it is hard and fragile, and Mg is silverish-white, considerably more soft and more plastic metal. Mechanical strength of Be considerably exceeds mechanical strength of other metals and their alloys, but due to large fragility its application as a construction metal is complicated and limited.

Be (unlike many metals) is diamagnetic, while Mg is a paramagnetic material. Beryllium transparency to X-rays is its unique property; it is used for making windows in X-ray emission tubes.

At fresh cut Ca, Sr, Ba and Ra are silvery white metals, whose fresh cut dim on air quickly as a result of interaction with О₂ and N₂.

According to physical properties they stand nearer to the alkali metals, than Mg, which stands at the top of the group:

0. they are considerably softer than Mg;

1. they also should be kept under the layer of kerosene;

2. Ca, Sr, Ba, Ra are easily oxidized at the surface, but are heavier, than alkali metals;

3. they self-ignite easily (Ва ignites on air already at its squashing).

Three allotropes of cаlcium are known. They can be prepared by heating of the lower temperature allotrope named α-Ca. Alkaline-earth elements in nature contain a mixture of stable isotopes (only Ве is solely ⁹Be).

Compounds of alkaline-earth metals impart specific colours to the flame: Са gives orange-red color, Sr and Ra is carmine-red, Ba is yellowish-green.

Physical Properties. Summary Some Alkaline-Earth Metals Subgroup Trends

	Be	Mg	Ca	Sr	Ba
Electronic configuration	[He]2s2	[Ne]3s2	[Ar] 4s2	[Kr]5s2	[Xe]6s2
Atomic radius, nm	113	160	197	215	221
Ionic radius М2+, nm	34	74	104	120	138
Potential of ionization, eV I1 : M0  М+ + e	9.30	7.65	6.10	5.70	5.21
Electronegativity	1.5	1.2	1.0	1.0	0.9
Melting point, С	1283	650	847	770	718
Density, g/cm3	1.85	1.74	1.54	2.63	3.76
Hydration heat, H298, kJ/mol	-2456	-1954	-1615	-1477	-1339
Ео М2+/М0, V	-1.847	-2.363	-2.866	-2.888	-2.906
Conductivity (Hg = 1)	23	22	22	4	2

• Melting points of Groups I and II elements are the function of metallic bonds strength. The stronger is the bond, the higher is the melting point.

• Metallic bond strength depends on: (1) ionic radius, (2) number of valence electrons

• Down the group, the size of cations increases

Þ polarizing power decreases

Þ compound with large anion becomes more stable

∴ thermal stability of carbonates & hydroxides of Group I metals increases down the group