S-Block Elements

s-Block Elements are the elements belonging to first ad second groups of the periodic table. The differentiating or final electron enters the s orbital in all these elements ; thus they are called as s-block elements.

Elements
The elements of the group 1 are Lithium (Li)(3), Sodium (Na)(11) , Potassium (K)(19) , Rubidium (Rb)(37) , Caesium (Cs)(55) , Francium (Fr)(87).

Occurrence
Lithium is the 35th most abundant element in earth's crust and occurs mainly in the forms of Silicates.

Sodium is the 7th most abundant element in earth's crust.

Potassium is the 8th most abundant element in the earth's crust.

Rubidium and Caesium are much less abundant. Francium is Radioactive in nature and extremely rare.

Electronic Configuration
[Noble Gas] ns1

Physical State
Alkali metals are soft, thus can be cut by knife. Lithium is the hardest alkali metal.

Atomic and Ionic Radii
The ionic radii of alkali metals are smaller than their corresponding atomic radii e.g. Na+< Na.

The ionic and atomic radii of alkali metals is highest in their respective periods. e.g. Atomic radii of Sodium is highest in Group 3.

The atomic and ionic Radii increases down the group.

Density
Alkali metals have a low density. Li, Na , K are lighter than water. Density increases down the group. But, Potassium is lighter than Sodium.

Density is low because of large size of atoms. As we go down the group, the increase in mass is more than increase in size ... Density increases down the group.

Melting and Boiling Points
Alkali metals possess low melting and boiling points. The melting and boiling points decrease down the group. As the size of atom increases, the intermolecular force decreases and ... melting and boiling points decrease down the group.

'''NOTE : The melting & boiling points in this case do not depend upon the density of the atoms, but on the extent of intermolecular forces of attraction. '''

Ionisation Enthalpy
The ionisation enthalpy of Alkali Metals is very low due to weak bond between valence electron and nucleus.

The ionisation enthalpies of Alkali metals are lowest in their respective groups. e.g. Sodium has the lowest ionisaton enthalpy in the Group 3.

The ionisation enthalpy goes on decreasing down the group, as the size of the atom increases and the nuclear charge decreases.

The second ionisation enthalpies of Alkali metals are very high because upon losing one electron, the alkali metals gain Noble gas configuration , which is the most stable configuration.

Electronegativity
Alkali metals possess the lowest values of electronegativity after noble gases. Alkali metals are highly electropositive, since thy have a high tendency to donate an electron.

Electronegativity decreases down the group, since the tendency to donate electron increases down the group , as the valence electron is losely bonded to the nucleus.

Oxidation State
All Alkali metals show only +1 oxidation state.

Hydration Energy
The energy liberated during hydration is called Hydration Energy

The Hydration Energy of Alkali Metals decreases down the group.

Flame Test
Colour of the flame darkens down the group

Li = Crimson

Na = Golden Yellow

K = Pale Violet

Rb = Violet

Cs = Violet

When an alkali metal is heated, its valence electron gets excited , due to energy provided by the heat. But, this excitation is short lived. Soon, the electron gets de-excited and emits energy in the form of light energy (photons).

Photoelectric Effect
Alkali metals are highly photosensitive, because they show a high tendency to donate electrons.

Light of energy greater than ionisation enthalpy (work function) is needed for photoelectric effect.

Nature of Compounds
Alkali metals always form ionic compound. Alkali metals are highly electropositive, this factor creates a huge electronegativity difference with the reagent.

Lattice Energy of the Compounds
Lattice energy is referred to the energy released to form crystal lattice structure of a compound. Alkali metals have a high lattice energy. The lattice energy decreases down the group.

Alkali metals form metallic bonds which are bonded together with high electrostatic force of attraction. ... A lot of energy is released during condensation of gas or liquid to crystal lattice structure.

Lattice energy decreases with increase in atomic size. ... Lattice Energy decreases down the group.

Anomalous Behavior
Lithium shows anomalous behaviour and has a diagonal relationship with Magnesium.

Important Compounds
Sodium : Sodium Carbonate, Sodium Bicarbonate , Sodium Chloride , Sodium Hydroxide

Biological Importance
Sodium : Major cation in blood plasma, reduction in fat deposits , atrophy of muscles , lung infection , retarded bone growth , reduces blood pressure.

Potassium : reduces Heart Beat, scarring of heart muscles , hypertrophy of kidney , paralysis of muscles.

Elements
Group 2 elements are called as alkaline earth metals. Berylium (Be)(4), Magnesium (Mg)(12) , Calcium (Ca)(20) , Strontium (Sr)(38) , Barium (Ba)(56) , Radium (Ra)(88) are the Group 2 Elements.

Occurrence
Berylium is the 51st most abundant element int he earth's crust.

Magnesium is the 6th most abundant element.

Calcium is the 5th most abundant element.

Strontium and Barium are less abundant.

Radium is a radioactive element and very rarely available.

Electronic Configuration
[Noble Gas] ns2

Physical State
They are more harder than alkali metals. Hardness decreases down the group due to decrease in atomic size.

Atomic and Ionic Radii
The Atomic radii of Alkaline Earth Metals is less than the corresponding alkali metals.

The atomic and ionic radii goes on increasing down the group.

Density
Alkaline Earth Metals are more dense than alkali metals. The density decreases down the group till Calcium and then sharply increases from Calcium to Barium. The density increases because the increase in mass overweighs the increase in atomic size. The irregular variation down the group is probably due to change in the crystal structure.

Alkaline Earth metals are more denser than alkali metals because their mass is more as well as their size is lesser than alkali metals.

Melting and Boiling Points
The Alkaline Earth metals have higher boiling and melting points, due to larger nuclear charge. No regular trend is seen down the group.

Ionisation Enthalpy
The Ionisation Enthalpy of alkaline earth metals is more than alkali metals because they possess a larger nuclear charge. The ionisation enthalpy decreases down the group due to increase in atomic size.

The second Ionisation enthalpy values for alkaline earth metals is lower than that of alkali metals. This is because the second electron is removed from stable noble gas core in alkali metals, while the second electron is removed from the valence shell in Alkaline Earth metals.

Electronegativity
Alkaline earth metals have low electronegativity values, but they are more electronegative than alkali metals.

The lower values of electronegativity is due to the electropositive character of alkaline earth metals.

Electronegativity decreases down the group, since metallic or electropositive character inreases down the group

Oxidation State
Alkaline Earth Metals show +1 or +2 oxidation state.

Nature of Compounds
Berylium and Magnesium form only ionic compounds. Higher elements, too mainly form ionic compounds ; but they also form covalent compounds to some extent.

Flame Test
Calcium                    Brick Red

Strontium                  Crimson

Barium                     Apple Green

Radium                     Crimson

Anomalous Behaviour
Berrylium shows anomalous behaviour and has a diagonal relationship with Aluminium.