Alkaline Earth Metals

Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra) are IIA elements. The oxides of these elements (CaO (Lime), SrO (strontia), BaO (Baryta)) are thermally very stable and basic in nature & hence, they are called alkaline earths.

Like alkali metals, these are reactive but less reactive than alkali metals. The outer electronic configuration of these metals is ns2.

As atomic number increases, the physical properties show a gradation down the group. Due to similar outer electronic configuration, they show similar chemical properties.

Atomic volume, atomic and ionic radii

Because of the addition of an extra shell of electrons to each element from Be to Ra, the atomic volume increases from Be to Ra. The atoms of these elements although fairly large, are smaller than those of the corresponding elements of the group IA.

This is due to higher nuclear charge of these atoms which tends to draw the orbital electrons towards the nucleus. The ions are also large but smaller than those of the elements of group 1. This is again due to the fact that removal of the two valence electrons to from M2+ ions increases the effective nuclear charge which pulls the electrons inwards and thus reduces the size of the ion. Atomic as well as ionic radii increase on moving down the group on account of presence of an extra shell at each step.

Ionisation energy

Since the alkaline earth metals possess smaller size and greater nuclear charge than the alkali metals, the electrons are more tightly held and hence the first ionisation energy is greater than that of the alkali metals.

The first ionisation energy represents the energy required to remove an electron from a neutral atom (M) while the second ionisation energy represents the energy required to remove an electron from positive ion (M+) which, of course, is difficult than the former case. After the removal of one electron, the effective nuclear charge increases and hence the remaining electron is held even more tightly leading to very high ionisation energy.

Oxidation state

Due to the presence of two s-electrons in the outermost orbit, high heat of hydration of the dipositive ion and comparatively low values of IE2, the alkaline earth metals are bivalent.

Moreover, since the bivalent ions have an inert gas configuration, it is very difficult to remove the third electron from the element and hence oxidation state higher than 2 are not encountered. Further, the bivalent ion has no unpaired electron hence their compounds are diamagnetic and colourless provided their anions are also colourless.

Electropositive character

Due to their large size and comparatively low ionisation energies, the alkaline earth metals are strongly electropositive elements. However, these are not as strongly electropositive as the alkali metals because they have smaller size and higher ionization energies than the alkali metals, and hence, unlike alkali metals, these elements do not emit electrons on exposure to light. On moving down the group, the electropositive character increases.

Metallic character

The alkaline earth elements are metals and their metallic character increases down the group. Metallic nature is again due to low ionization energy and since the ionization energy decreases down the group, their metallic character increases.

Heat and electrical conductivity

Due to the presence of two loosely held valence electrons per atom which are free to move throughout the crystal structure, the alkaline earth metal, are good conductors of heat and electricity.

Reducing properties

Alkaline earth metals have two electrons outside the noble gas configuration. Due to their large size and low ionisation energy, they can easily lose outermost electrons and hence undergo oxidation (loss) of electrons easily.

Chemical Properties

The chemical properties of the alkaline earth metals are comparable to those of alkali metals. But due to smaller size, greater charge and high ionisation energy, these are much less reactive than the corresponding alkali metals. Further since their ionisation energies decrease with increase in atomic number, their reactivity increases from Be to Ba.

Action of air

Their less reactivity than the alkali metals is evident by the fact that they are only slowly oxidised on exposure to air. When burnt in air they form ionic oxides of the type MO, however the higher members (Sr to Ra) form peroxides.

Action of water

These metals react slowly with water liberating hydrogen and forming metal hydroxides.

Action of hydrogen

All elements except beryllium, combine with hydrogen to form hydrides, MH2. Magnesium hydride (like BeH2) is covalent while other hydrides are ionic.

Action of halogens

All these elements combine with halogens at elevated temperature forming halides, MX2. Beryllium halides are covalent while the rest are ionic and thus dissolve in water and conduct electricity in aqueous solution and in molten state.