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The Quintuple Bond: The quintuple bond Single bond or double bond or triple bond are the common bonds between atoms. Chemists are also familiar with quadruple(4 bonds) bonds which are rarer but occur especially for certain complexes of Cr, Mo, W and Re. Ex: [Mo2Cl8]4− and [Re2Cl8]2−. Recently an unusual type of quintuple (5 bonds) bond was reported for dichromium and dimolybdenum compounds. The five (n−1) d−orbitals of metals involved in bond formation in these complexes. Metal‐metal quintuple bonds have a σ2π4δ4 configuration. This means that the five bonds present between the metal centers are; one sigma(σ) bond, two pi (π) bonds and two delta(δ) bonds.

Learning Objectives

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

  • Explain the formation of different types of bonds such as ionic bond, covalent bond and metallic bond.
  • Distinguish electrovalency and covalency and predict the valency of bonded atoms.
  • Name the ionic compounds and describe the characteristics of ionic compounds.
  • Calculate the lattice energy of ionic crystals by using Born‐Haber cycle.
  • Explain the octet rule and its limitations, draw Lewis structures of simple molecules and discuss the resonance phenomenon.
  • Explain the valence bond theory of formation of covalent molecules.
  • Predict the geometry of simple molecules based on VSEPR theory.
  • Explain the reason for origin of polarity(dipole moment) in molecules.
  • Discuss the effects of dipole moment on properties of molecules.
Molecular shapes and chemical bonding Molecular shapes and chemical bonding The three dimensional shape of the molecules and the existence of different things around us is because of chemical bonding.

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Introduction to chemical bonding

The atoms in molecules, crystals, metals, diatomic gases and most of the physical environment around us are held together by chemical bonds. Molecular shapes, the three‐dimensional arrangements of the atoms that constitute a molecule are determined by the nature of bonds between atoms in a molecule.

Molecular geometry determines several properties of a substance including its reactivity, polarity, phase of matter, color, magnetism, and biological activity and hence there is no topic more fundamental to chemistry than the nature of the chemical bond.

The molecular shapes define all the life processes right from the way a biological "cell" works, the way nerve impulses are communicated and the way immune system works. Genes function when certain nucleic acid molecules fit into specific regions of other nucleic acid.

Hydrogen bonding for example, plays an important role in determining the three‐dimensional structures adopted by proteins and nucleic bases. In these macro molecules, bonding between parts of the same macro molecule causes it to fold into a specific shape, which helps to determine the molecule's physiological or biochemical role.

The double helical structure of DNA, for example, is largely due to hydrogen bonding between the base pairs, which link one complementary strand to the other and enable replication.

Chemical bonding examples in every day life Chemical bonding examples in every day life Diamond is made up of covalently bonded carbons. Water, an essential liquid, is a result of bonding between Hydrogen and Oxygen. Pure Gold is weak to make ornaments. So, it is bonded with copper to get the desired shape and hardness. Currency note is a special type of paper, a compound of cellulose. Cellulose, a major component of paper, has Carbons bonded with Hydrogens and Oxygens.
Chemical bonding of atoms - Valence electrons

Chemical bond is the attraction between two atoms or ions that holds them together. Formation of bonds in the primordial soup is the fundamental step in the evolution of life on the planet earth. We cannot imagine a life without the bonds.

Atoms bond with other atoms to form molecules, the smallest chemically stable units of a compound. Formation of a molecule has a lot to do with the structure of atoms, the electronic configurations and valence. The main principle behind the chemical bonding can be explained using the electronic configurations of the noble gases (non reactive elements). All the noble gases, except helium, have valence shells in which the outermost 's' and 'p' sub-levels are completely filled (ns2, np6 ). This configuration gives stability to the gases. Stability is also observed with atoms that can lose, gain or share electrons to attain the same configuration.

It is necessary to understand the formation and nature of chemical bonds to arrive at the structures of molecules and their chemical activity. The types of bond depend on the attractive and repulsive forces between the charged species, the ions. These forces also affect the bond length. Van der Waals forces are simple attractive forces in which there is no reorganization of electrons observed between bonded atoms. The force is not classified under chemical bond but placed under the inter-molecular forces.