Alkali Metals or Group-I elements have one electron in their valence shell. They do not occur in the native or free state as they are very reactive. These elements are collectively known as alkali metals because their oxides and hydroxides form strong alkalies like NaOH, KOH, etc.
General Characteristics of Alkali Metals
(i) Electronic configuration ns1
(ii) Atomic radii
The alkali metals have the biggest atomic radii in their respective periods.
Atomic radii increases as we go down the group due to the addition of a new shell in each subsequent step.
(iii) Ionic radii
Ionic radii of the alkali metals are much smaller than their corresponding metals due to lesser number of shells and increased nuclear charge.
The ionic radii of all these alkali metal ions go on increasing on moving down the group.
(iv) Density
These are light metals with low densities. Lithium is the lightest known metal. On moving down the group, ‘density increases from Li to Cs.
This is because, down the group, both the atomic size and atomic mass increases but the effect of increase in atomic mass is more as compared to increase in atomic size.
(v) Melting and boiling points
The melting and boiling points of alkali metals are quite low and decrease down the group due to weakening of metallic bond.
(vi) Softness
These are soft. malleable and ductile solids which can be cut with knife.
(vii) Atomic volume
Atomic volume of alkali metals is the highest in each period and goes on increasing down the group from top to bottom [Li to Cs].
(viii) Ionisation enthalpy
The ionization energy is the amount of energy required to remove the most loosely bound electron, the valence electron, of an isolated gaseous atom to form a cation .
The ionisation enthalpy of alkali metals is the lowest amongst the elements because they readily loose a electron to gain the stable noble gas electronic gas configuration.
(ix) Electropositive character
The elements that can easily lose electrons to form positive ions are called electropositive elements and this character is known as electropositive character.
Due to low ionisation enthalpies alkali metals are strongly electropositive or metallic in nature. The electropositive nature increases from Li to Cs due to decrease in ionization enthalpy.
(x) Oxidation state
oxidation number is a number assigned to an element in chemical combination which represents the number of electrons lost or gained, by an atom of that element in the compound.
The alkali metal atoms show only +1 oxidation state, because their unipositive ions attain the stable noble gas configuration.
(xi) Hydration of ions
A hydration reaction is a chemical reaction in which a substance combines with water.
The degree of hydration depends upon the size of the cation. Smaller the size of a cation greater is its hydration enthalpy
Relative degree of hydration,
Li+ > Na+ > K+ > Rb+ > Cs+
(xii) Flame colouration
Alkali metals and their salts impart characteristic colours to the flame because the outer electrons get excited to higher energy levels, When the electron return to the original state it releases visible light of characteristic wavelength which provides a colour to the flame.
(xiii) Photoelectric effect
The photoelectric effect is the emission of electrons or other free carriers when light is thrown on a material
Due to very low ionisation enthalpy, alkali metals specially ‘Cs’ exhibit photoelectric effect i.e., eject electrons when exposed to light. so it is used in photoelectric cells.
(xiv) Electrical conductivity
Due to the presence of loosely held valence electrons which are free to move throughout the metal structure the alkali metals are good conductors of heat and electricity. Electrical conductivity increases from top to bottom in the order
Li+ < Na+ < K+ < Rb+ < Cs+
(xv) Reducing character
A reducing agent is an element or compound that loses or donates an electron to another chemical species in a redox chemical reaction. This character is known as reducing character.
All the alkali metals are good reducing agents due to their low ionisation energies. Their reducing character follows the order
Na < K < Rb < Cs < Li
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