The elements of Group 1 and Group 2 of the modern periodic table are called S-block elements. The two types of s-block elements are possible i.e. the elements with one electron (s1) or the elements with two electrons (s2) in their s-subshell.
The s-block elements having only one electron in the s-orbital are called group one or alkali metals whereas the s-block elements having two electrons filling the s-orbital are called group two or alkaline earth metals.
What are the S-Block Elements?
The electrons present in an atom occupy various sub-orbitals of available energy levels in the order of increasing energy. The last electron of an atom may find itself in either of the s, p, d and f subshells. Accordingly, the elements of the atom having their last valence electron present in the s-suborbital are called the s-block elements.
S-block comprises of 14 elements, namely, hydrogen (H), lithium (Li), helium (He), sodium (Na), beryllium (Be), potassium (K), magnesium (Mg), rubidium (Rb), calcium (Ca), cesium (Cs), strontium (Sr), francium (Fr), barium (Ba), and radium (Ra).
Electronic Configuration of S-Block Elements
The electronic configuration of s-block elements is explained below.
The alkali elements in s-block consist of a single valence electron in their outermost shell. This outermost electron is loosely held which makes these metals highly electropositive. Due to which they are not available in the free state in nature. The general electronic configurations of s-block elements – group 1 are as shown in the table below:
The electronic configuration of s-block elements (Group 2) is shown below:
Properties of S-Block Elements
Both alkali and alkaline earth elements show a regular gradation in their properties among their respective group elements. But the first member of both s-block elements, namely, Lithium and Beryllium differ much from the rest of their members but at the same time, they resemble more with the diagonal element present in the next column.
The anomaly of these s-block elements is due to,
- Low atomic and ionic size
- Greater charge density (charge/volume of the atom)
- Greater polarization
- Absence of d-orbitals.
Greater polarization of s-block elements makes the first element more covalent and differentiates them from the rest, which are ionic.
The similarity in size and charge density makes them resemble the element diagonally placed in the next group (diagonal relationship).
It is observed that the physical and chemical properties of these s-block elements changes in a particular trend as the atomic number of the elements increases. Changes in the various properties of the group are as mentioned below:
Chemical Properties of S-Block Elements
Atomic and Ionic Radii
When the s-block elements of modern periodic table are observed it is seen that the size of the alkali metals is larger compared to other elements in a particular period. As the atomic number increases the total number of electrons increases along with the addition of shells.
On moving down the group the atomic number increases. As a result, the atomic and ionic radius of the alkali metals increases.
As we go down the group the size of the atoms increases due to which the attraction between the nucleus and the electrons in the outermost shell decreases. As a result, the ionization enthalpy decreases. The ionization enthalpy of the alkali metals is comparatively lesser than other elements.
As the ionic sizes of the elements increases the hydration enthalpy decreases. Smaller the size of the ion the hydration enthalpy is high as the atom has the capacity to accommodate a larger number of water molecules around it due to high charge/radius ratio and hence gets hydrated.
Physical Properties of S-block elements
- In the s-block elements, the density of the alkali metals increases down the group. Exception: the density of potassium is less than the density of sodium.
- The alkali metals have a low melting and boiling point due to the weak metallic bonding.
- Alkali metals and its respective salts have the capability to impart color to the oxidizing flame due to the heat generated from the flame which excites the valence electrons from one energy level to another energy level. This helps in the detection of alkali metals during the flame test.
Diagonal Relationship within S-Block Elements
A diagonal relationship in s-block elements exists between adjacent elements which are located in the second and third period of the periodic table. For example, Lithium of group 1A and second period shows similarities with the properties of magnesium which are located in the 2nd group and 3rd period.
Similarly, properties of beryllium which are located in the 2nd group and 2nd period show a likeness with properties of aluminium which is located in the third period and third group. The two elements which show similarities in their properties can be called a diagonal pair or diagonal neighbours.
The properties of s-block elements vary significantly when compared to the other elements of the sub-group they belong to. The diagonal neighbors show a lot of similarities. Such a relationship is exhibited as you move left to right and down the group; the periodic table has opposing factors.
For example, the electronegativity of the s-block elements increases as we go across the period and decreases as we go down the group. Therefore when it is moved diagonally the opposite tendencies cancel out and the value of electronegativity almost remains the same.
Similarities between Lithium and Magnesium
- The hardness of lithium and magnesium is higher than the other elements in their respective groups.
- Chlorides of lithium and magnesium have the capability to be soluble in ethanol.
- They are lighter when compared to other elements in their groups.
- Lithium and magnesium react gently with water. The oxides and hydroxides are less soluble.
- In the presence of nitrogen, lithium and magnesium form their respective nitrides.
- Superoxides are not formed when lithium and magnesium react with excess oxygen.
- Carbon dioxide and their respective oxides are formed when carbonates of magnesium and lithium are heated.
Similarities between Beryllium and Aluminum
- Aluminum hydroxide and beryllium hydroxide react with excess alkali to form their respective ions.
- Both these elements have the capacity to withstand the acid attack due to the presence of an oxide film on the surface of the metal.
- Both these metals have the tendency to form complexes.
- Chlorides of both these metals possess the capacity to be soluble in organic solvents.