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 .

## **Group 1**

### **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 .

### **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 .

## Group 2

### **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 .

### **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 .