Note on Inter molecular bonds and molecules

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Metallic bonding and Molecules

In metallic conductor there are large number of valence electrons moving out of atoms forming metal ions (cation). These metals ion and electrons formed strong bonds due to opposite nature of charge and this types of bonding is known as the metallic bonding. During metallic bonding valence electron spread to form ‘Electron Sea’ that glues ion together.

By way of contradiction, ionically and covalently bonded materials are typically electrical and thermal insulators because of the absence of large numbers of free electrons. At room temperature, most of the metals and their alloys fail in a ductile manner; that is, fracture occurs after the materials have experienced significant degrees of permanent deformation. This behavior is explained in terms of deformation mechanism, which is directly related to the characteristics of the metallic bond. Conversely, at room temperature ionically bonded materials are intrinsically brittle as a consequence of the electrically charged nature of their component ions

  1. The metallic bonding is non directional that is bond cal be formed in any direction.
  2. The materials having metallic bonding cal experiences a significant degree of plastic deformation ( that can not regains original shaped).
  3. Metallic bonding may be weak or strong. There is weak metallic bonding in mercury i.e. 0.7 ev per atom and strong metallic bonding in tungsten 8.8 ev per atom.
  4. The compounds or materials having typical metallic bonding are almost good conductors of heat and electricity.
  5. At low temperatures most of the metals amd alloys may have fractures whereas high temperatures these materials are ductile in nature.
  6. The compounds having metallic bonding are insoluble in all solvents. For example: iron, steel, aluminum, silver, nickel, etc.

Secondary bonding or weak bonding:

Secondary bonding arise from atomic or molecular dipoles. During the formation of molecules, and electric dipole exist due to separation of positive and negative of an atom or molecule. The bonding that results from coulomb interaction between positive end of one dipole and negative end of another dipole each known as secondary bonding or hydrogen bonding as shown in figure:

fig: secondary bonds
fig: secondary bonds(www.wikiwand.com)

Mechanism for formation secondary bonding

  1. Fluctuating induced dipole bonds
  2. Polar molecule induced dipole bonds
  3. Permanent dipole bonds

In case of non polar molecules all atoms are experiences constant vibrational motion. That can create a small electric dipole and the electropositive force between two such dipoles ao more than two such dipoles form bond, such type of bonding is known as Vander Wal’s bonding. The characteristics of material having such type of bonding is,

  1. Low melting point
  2. Low boiling point

For example: Inert gas and Homonuclear molecules.(\(H_2, cl_2, o_2, etc\)).

Polar molecules induced dipole bonds:

fig:polar molecules
fig:polar molecules(www.school-for-champions.com)

When there is a mixture of polar and non-polar molecules the polar molecules and induced dipole moment on non-polar molecules resulting on et force between molecules, such types of bonding is known as polar molecule induced dipole bond. These molecules can also induce dipoles in adjacent non-polar molecules, and make a bond will form as a result of attractive forces between the two molecules. The magnitude of this bond will be greater than for fluctuating induced dipoles

Permanent dipole bonds:

fig: permanent dipole moment
fig: permanent dipole moment(2012books.lardbucket.org)

The bonds between already formed polar molecules are known as permanent dipole bonds. It generally occurs between molecules in which hydrogen is covalently bonded with fluorine, with oxygen, with nitrogen as in \(NH_3\) and \(H_2O\). Such type of bonding is also called as hydrogen bonding. It is the strongest secondary bonding. The melting and boiling point of materials having such type of bonding is higher than other having secondary bonding.

Most of those that have small molecules composed of a few atoms are gases at ordinary, or ambient, temperatures and pressures. Many of the modern polymers, being molecular materials composed of extremely large molecules, exist as solids. Some of their properties are dependent on the presence of van-der Waals and hydrogen secondary bonds. For hydrogen fluoride and water Melting and boiling temperatures are abnormally high in light of their low molecular weights, as a consequence of hydrogen bonding.

Molecules:

Most of the common molecules are composed of group of atoms bonded together by strong covalent bonds. Examples:\(H_2, cl_2, F_2, O_2, Co_2\) etc. And some of the compounds have multiple covalent bonds such as methane, ethane, \(HNO_3\)etc.

The bonds between two molecule is known as intermolecular bonds which determines the physical and chemical properties of water. So the nature of bonds must be known to determine all characteristics of material.

References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004.

1. Types of dipole moment

metallic bonding 

secondary bonding 

polar molecule induced dipole moment 

permanent dipole moment

 

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