Question

Discuss the general characteristics and gradation in properties of alkaline earth metals.

Answer 1

General characteristics of alkaline earth metals are as follows.

1. The general electronic configuration of alkaline earth metals is [noble gas] ns².
2. These metals lose two electrons to acquire the nearest noble gas configuration. Therefore, their oxidation state is +2.
3. These metals have atomic and ionic radii smaller than that of alkali metals. Also, when moved down the group, the effective nuclear charge decreases and this causes an increase in their atomic radii and ionic radii.
4. Since the alkaline earth metals have large size, their ionization enthalpies are found to be fairly low. However, their first ionization enthalpies are higher than the corresponding group 1 metals.
5. These metals are lustrous and silvery white in appearance. They are relatively less soft as compared to alkali metals.
6. Atoms of alkaline earth metals are smaller than that of alkali metals. Also, they have two valence electrons forming stronger metallic bonds. These two factors cause alkaline earth metals to have high melting and boiling points as compared to alkali metals.
7. They are highly electropositive in nature. This is due to their low ionization enthalpies. Also, the electropositive character increases on moving down the group from Be to Ba.
8. Ca, Sr, and Ba impart characteristic colours to flames.
   Ca – Brick red
   Sr – Crimson red
   Ba – Apple green

In Be and Mg, the electrons are too strongly bound to be excited. Hence, these do not impart any colour to the flame.

The alkaline earth metals are less reactive than alkali metals and their reactivity increases on moving down the group. Chemical properties of alkaline earth metals are as follows.

1. Reaction with air and water: Be and Mg are almost inert to air and water because of the formation of oxide layer on their surface.
   1. Powdered Be burns in air to form BeO and Be₃N₂.
   2. Mg, being more electropositive, burns in air with a dazzling sparkle to form MgO and Mg₃N₂.
   3. Ca, Sr, and Ba react readily with air to form respective oxides and nitrides.
   4. Ca, Ba, and Sr react vigorously even with cold water.
2. Alkaline earth metals react with halogens at high temperatures to form halides.
   \[\ce{M + X_2 -> MX_2 (X = F, Cl, Br, l)}\]
3. All the alkaline earth metals, except Be, react with hydrogen to form hydrides.
4. They react readily with acids to form salts and liberate hydrogen gas.
   \[\ce{M + 2HCl ->  MCl_2 +  H_{2(g)} ↑}\]
5. They are strong reducing agents. However, their reducing power is less than that of alkali metals. As we move down the group, the reducing power increases.
6. Similar to alkali metals, the alkaline earth metals also dissolve in liquid ammonia to give deep blue coloured solutions.
   \[\ce{M + (x - y) NH_3 -> [M(NH_3)_x]^{+2} + 2[e(NH_3)_y]^-}\]