Properties And Trends Of Compounds Of S And P Block Elements - Session 2

S block and P block elements form compounds that might behave as acids or bases or both. The compounds that behave both as acids and bases are called amphoteric. We shall discuss the acidic, basic and amphoteric nature of the different compounds formed by S and P block elements. We shall also discuss the trend of acidic and basic nature of these compounds in periods and groups of periodic table.

Halides are the binary compounds that consist of halogen atom combined with any other atom. Halogens are the group 7A elements which include Fluorine, Chlorine, Bromine and Iodine. Some examples of the halides formed by the s block elements are Sodium Chloride, Magnesium Chloride, Calcium Chloride and Potassium Chloride. Some of the halides formed by p block elements are Carbon Tetrachloride, Phosphorous Trichloride and Phosphorous Pentachloride.

Basicity of the halides decreases downwards in a group. The decreasing order of basicity of halide ions is illustrated. Fluoride ion is most basic and Iodide ion is least basic in nature.

The electropositive and electronegative nature of oxide’s central atom determines whether the oxide is acidic or basic. If central atom of oxide is highly electropositive, the oxide is basic in nature. If central atom of oxide is highly electronegative, the oxide is acidic in nature. We know that S block elements are highly electropositive and this electropositivity decreases as we move from left-to-right in a periodic table. Electronegativity rises from left-to-right in a periodic table.

We can conclude that the acidic characteristics of oxides rises from left-to-right in a periodic table. On the other hand, the basic characteristics of oxides decreases from left-to-right in a periodic table. For example, the oxides of s block elements are basic in nature. Basic oxides react with water to form basic solution. Magnesium oxide reacts with water to form Magnesium Hydroxide.In same way Potassium Oxide reacts with water to form Potassium Hydroxide.

Oxides of p block elements form acidic oxides. An oxide that gives off an acid when combined with water is called an Acidic oxide. Some of the examples of acidic oxides are SO₂, CO₂, SO₃, and NO₂. Sulphur Dioxide combines with water to form sulphurous acid. Carbon dioxide combines with water to form carbonic acid. Sulphur trioxide combines with water to form sulphuric acid. Nitrogen dioxide combines with water to form Nitric acid.

As we know electronegativity of atoms decreases downwards in a group. Due to decrease in electronegativity of central atom of oxide, the acidic characteristics of oxides decreases downwards in a group. The basic characteristics of oxides rises in a group because electropositivity of central atom of oxide rises downwards in a group. For example, barium hydroxide is more basic than beryllium hydroxide.

Hydrides are the class of chemical compound in which hydrogen is combined with other element. Some examples of hydrides are Hydrochloric acid, Lithium hydride and Methane.Hydrides formed by s block elements are basic in nature. While hydrides formed by p block elements are acidic in nature. This shows that basic nature of hydrides decreases from left-to-right in a periodic table. Acidic nature of hydrides rises from left-to-right in a periodic table.

Acidic nature of hydrides rises from top to bottom in a group because bond formed between larger atom and hydrogen atom is weak and hydrogen can be easily donated in water.For example, H₂Te is more acidic than H₂O. This is because size of Te is larger than oxygen atom.Water is an amphoteric hydride. Amphoteric means a specie can behave both as an acid and a base.When water reacts with an acid it acts as base. When it reacts with a base it acts as an acid.

This groups is called the hydroxide group. When a metal combines with hydroxide it forms metal hydroxide. Metal hydroxides are formed mostly by s block elements. Some examples of the metal hydroxides are Sodium Hydroxide, Magnesium Hydroxide and Potassium Hydroxide. The non-metal hydroxides are formed by p block elements. The compound Hydrochlorous Acid is a non-metal hydroxide.

We shall now discuss the acidic and basic nature of hydroxides. Elements of the s block form basic hydroxides. This is because of high electropositive characteristics of metals of s block elements. For example, sodium hydroxide ionizes in water to form sodium ion and hydroxide ion. The presence of hydroxide ion in water makes the solution basic.

The elements of the p block mostly form acidic hydroxides. This is because of high electronegativity of p block elements. For example, HOCl is acidic in nature. This is because the chlorine atom is highly electronegative. It attracts shared pair of electrons from hydrogen atom and separates the hydrogen atom as a hydrogen ion. The presence of hydrogen ions in solution makes the solution acidic.

Basicity of hydroxides decreases from left-to-right in a periodic table. Acidity of hydroxides rises from left-to-right in a periodic table. This is due to rise in the electronegativity from left-to-right in a periodic table. For example, HOCl is more acidic as compared to NaOH due to to high electronegativity of chlorine atom as compared to sodium atom. Similarly, NaOH is more basic as compared to HOCl due to high electropositivity of sodium atom as compared to chlorine atom.

Now lets discuss trend of acidity or basicity of hydroxides in a group. The basicity of hydroxides rises from top to bottom in a group. This is because as the size of atom attached to hydroxide group rises downwards in a group, the internuclear distance between cation atom and hydroxide ion grows. Therefore, hydroxide ion is easily ionized which makes the solution more basic. For example, Potassium Hydroxide is more basic than Lithium hydroxide. This is because of large atomic size of Potassium which results in rise in internuclear distance between potassium and hydroxide ion. As a result Hydroxide ion in Lithium Hydroxide is easily ionized.