Alkali Metals | General Characteristics#

s-block Elements#

s-block elements consist of group 1 and group 2 elements.
Group 1 elements are known as alkali metals. These include Lithium(Li), Sodium(Na), Potassium(K), Rubidium(Rb), Cesium(Cs) and Francium(Fr).
Group 2 elements are known as alkaline earth metals. These include Beryllium(Be), Magnesium(Mg), Calcium(Ca), Strontium(Sr), Barium(Ba) and Radium(Ra).

General electronic configuration is [noble gas]ns¹

\[Li → 1s^2\ 2s^1\ or\ [He]2s^1\]

\[Na → 1s^2\ 2s^2\ 2p^6\ 3s^1\ or\ [Ne]3s^1\]

\[K → 1s^2\ 2s^2\ 2p^6\ 3s^2\ 3p^6\ 4s^1 or\ [Ar]4s^1\]

\[Rb → [Kr]5s^1\]

\[Cs → [Xe]6s^1\]

\[Fr → [Rn]7s^1\]

Note

Alkali metals are never found in free state as they readily lose electron to form stable monovalent M⁺ ions.

Atomic and ionic radii increase down the group. It is due to increase in number of energy levels or shells (n) down the group and the shielding effect of electrons in lower energy levels which reduces the effective nuclear charge.
Alkali metals are the largest atoms of their respective period. They have the fewest number of protons relative to other elements in their respective period. From an electromagnetic standpoint, those atoms have the weakest magnet so their electrons won’t be as close to the nucleus as electrons from other atoms would be.
Radius of an atom is greater than the radius of the ion of respective atom. It is due to increase in effective nuclear charge in the ion.

The energy required to remove an electron from an isolated gaseous atom in its ground state is known as ionization enthalpy. It is depicted by Δ_iH. It is expressed in Kilojoules per mole (KJ mol^-1).
It is the quantitative measure of the tendency of an atom to lose electron.
Energy is always required to remove electron(s) from an atom. Hence, it is always positive.
Ionization enthalpy decreases down the group in alkali metals because of increase in size and outermost electrons are very well screened from nuclear charge.
Alkali metals have the lowest ionization energy in each period because they have the largest atomic size in their respective period due to which valence electron is held loosely by the nucleus and can be lost easily.

The energy required to hydrate or dilute one mole of an ion is known as hydration enthalpy.
Hydration enthalpy is inversely proportional to the radius of ion, ie, it decreases when the size of ion increases.
Effective nuclear charge is inversely proportional to size. As we move down the group effective nuclear charge decreases and hence force of attraction with water increases causing decrease in hydration enthalpy.
Hence, hydration enthalpy decreases down the group in alkali metals.
Due to the highest degree of hydration, lithium salts are mostly hydrated, e.g, LiCl.2H₂O.