S block Elements
The elements in which the last electron enters in s-orbital are called s-block elements. These elements are also known as alkali metals.
Alkali metals
Group 1 elements are collectively called alkali metals. They dissolve in water to form hydroxides. They are strongly alkaline in nature.
Occurrence of Alkali Metals
- Highly reactive.
- Found in a combined state only.
- Sodium and potassium are the seventh and eighth abundant metals found in the earth crust.
General and Physical Properties
Electronic configuration
The outermost electronic configuration of group 1 l elements is ns^-1. They commonly form univalent, ionic, and colourless compound (except Li).
Physical Appearance
- Silvery white in colour.
- Very soft.
- Acquire a wax-like appearance.
- Easily cut with a knife.
- The softness of alkali metals is due to weak metallic bonding in them because of the large size of the atoms.
Thermal and Electrical Conductivity
- Alkali metals are good conductor of heat and electricity.
- Atom is loosely bound, due to their less effective nuclear charge.
Density
The density of alkali metals are low and increases down the group. The low density of alkali metals is attributed to their large atomic size and weak metallic bond. Density increases with potassium as an exception. Its density is less than that of sodium.
Melting and Boiling points
Alkali metals have low melting and Boiling points because of their larger atomic size. Melting and Boiling points decreases on moving down the group.
Flame Colouration
All alkali metals give characteristic colours in Bunsen flame.
| Elements | Flame Colour |
| Lithium (Li) | Crimson Red |
| Sodium (Na) | Golden Yellow |
| Potassium (K) | Purple |
| Rubidium (Rb) | Red Violet |
| Caesium (Cs) | Blue |
Atomic Size and Mass
Atomic size of alkali metals is largest in the respective group. As we move down in a group atomic size increases without any exception.
Ionisation Enthalpy
Ionisation enthalpy of alkali metals is lowest in its respective period. Their second ionisation enthalpy is very high because of the inert gas configuration. Lowest members like K and Cs emit electrons even when exposed to light, this effect is called the photoelectric effect.
Electropositive Character
They are strongly electropositive or metallic in nature. The electropositive Character increases from Li to Cs.
Hydration Enthalpy and Hydration of Ions
The process of dissolution of a substance in water in which the substance absorbs water molecules on its surface by weak Vander Waal’s forces is called hydration.
The amount of energy released when one mile of gaseous ions combine with water to form hydrated ions is called the hydration enthalpy.
