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Inorganic Graphite, the hardest synthetic substance Inorganic graphite, the hardest synthetic substance Boron nitride (BN)x, the hardest synthetic substance is known as ′inorganic graphite′. It has same number of boron and nitrogen atoms. It is isoelectronic to a similarly structured carbon lattice and thus exists in various crystalline forms. Its most stable polymorph also called h–BN, A–BN, or g–BN (graphitic BN) has hexagonal layered structure, which is similar to graphite. The hardest polymorph is wurtzite BN. On Mohs scale w–BN has relatively equal hardness (∼10 points) with diamond.

Learning Objectives

After completing the topic, the student will be able to:

  • Give an account for variation in properties of p–block elements.
  • List the elements in group 13 or group IIIA in periodic table and write their electronic configurations.
  • Explain the general trends in periodic properties of group 13 elements.
  • Discuss in detail about chemical and physical properties of elements of boron family.
  • Compare and examine the properties of boron with other elements in the group.
  • Give the reasons for anomalous behavior of boron and its similarity with silicon.
  • Discuss in detail about boron, aluminium and their compounds.
Periodic table The p–block of the periodic table consists of the elements of group 13, 14, 15, 16, 17 and 18, except helium (which belongs to s-block).
Introduction to p-block elements

Elements having outer electronic configuration of the type ns2 np1 to ns2 np6, with all other inner orbitals completely filled, are termed p–block elements. In other words, the elements in which the last electron enters into any of the outermost p–orbitals are called p–block elements. Since there are three p–orbitals and six electrons can be accommodated in these orbitals, there are six groups of p–block elements – Groups 13 to 18. Boron, carbon, nitrogen, oxygen, fluorine and helium head the groups. Their valence shell electronic configuration is ns2 np1−6(except for He).

The variation in properties of the p–block elements due to the influence of 'd' and 'f' electrons in the inner core of the heavier elements makes their chemistry interesting.

Electronic configuration and Oxidation states

In p–block elements the last electron enters the outermost 'p' orbital. The inner core of the electronic configuration may, however, differ. The difference in inner core of elements greatly influences their physical properties (such as atomic and ionic radii, ionization enthalpy, etc.) as well as chemical properties. Consequently, a lot of variation in properties of elements in a group of p–block is observed.

The maximum oxidation state shown by a p–block element is equal to the total number of valence electrons (i.e., the sum of the s and p–electrons). Clearly, the number of possible oxidation states increases towards the right of the periodic table. In addition to this p–block elements may show other oxidation states which normally, but not necessarily, differ from the total number of valence electrons by unit of two.

The important oxidation states exhibited by p–block elements are shown in the Table on right side. In boron, carbon and nitrogen families the group oxidation state is the most stable state for the lighter elements in the group. However, the oxidation state two unit less than the group oxidation state becomes progressively more stable for the heavier elements in each group. This is sometimes attributed to the 'inert pair effect'. The relative stabilities of these two oxidation states, group oxidation state and two unit less than the group oxidation state, may vary from group to group. It is interesting to note that the non–metals and metalloids exist only in the p–block of the periodic table.