Formation of Substances
The substances that exist in the universe are made up of atoms (consists charge particles electron and proton)/molecules (made up of atom s)/ions. Chemical bonding is the binding force of the constituents’ atoms of the molecule to maintain a mutual atomic order and a definite but specific geometric shape. There are three types of chemical bonding – Electrovalent or Ionic, Covalent bonding and Co-ordinate covalent bonding.
Types of chemical bonding
Electrovalent Bond: They are formed by transfer of electrons and have high melting and boiling points. These are soluble in water and conduct electricity in molten state or in aqueous solution. These bonds are held together by the strong electrostatic attractions between the positive and negative charges.
| Examples of ionic or electrovalent Bond |
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| LiF - Lithium Fluoride LiCl - Lithium Chloride LiBr - Lithium Bromide LiI - Lithium Iodide NaF - Sodium Fluoride NaCl - Sodium Chloride NaBr - Sodium Bromide NaI - Sodium Iodide KF - Potassium Fluoride KCl - Potassium Chloride KBr - Potassium Bromide KI - Potassium Iodide CsF - Cesium Fluoride CsCl - Cesium Chloride CsBr - Cesium Bromide CsI - Cesium Iodide BeO - Beryllium Oxide BeS - Beryllium Sulphide BeSe - Beryllium Selenide MgO - Magnesium Oxide MgS - Magnesium Sulphide MgSe - Magnesium Selenide CaO - Calcium Oxide CaS - Calcium Sulphide CaSe - Calcium Selenide BaO - Barium Oxide BaS - Barium Sulphide BaSe - Barium Selenide |
CuI - Copper(I) Iodide CuO - Copper(II) Oxide CuS - Copper(II) Sulphide CuSe - Copper(II) Selenide FeO - Iron(II) Oxide FeS - Iron(II) Sulphide FeSe - Iron(II) Selenide CoO - Cobalt(II) Oxide CoS - Cobalt(II) Sulphide CoSe - Cobalt(II) Selenide NiO - Nickel(II) Oxide NIS - Nickel(II) Sulphide NiSe - Nickel(II) Selenide PbO - Lead(II) Oxide PbS - Lead(II) Sulphide PbSe - Lead(II) Selenide SnO - Tin(II) Oxide SnS - Tin(II) Sulphide SnSe - Tin(II) Selenide Li2O - Lithium Oxide Li2S - Lithium Sulphide Li2Se - Lithium Selenide Na2S - Sodium Oxide Na2S - Sodium Sulphide Na2Se - Sodium Selenide K2O - Potassium Oxide K2S - Potassium Sulphide K2Se - Potassium Selenide |
Covalent Bond: They are formed by sharing of electrons and have low melting and boiling point. These are soluble in organic solvents and are non-conductor of electricity. However graphite and diamond are covalent compounds but they have very high melting point because of their giant structure. The properties of covalent bonds are given below:
- They are mostly gases and liquids.
- These compounds have low melting point and boiling point because intermolecular forces among the atoms are weaker as compare to the electrovalent compounds.
- Most of the covalent bonds exist in molecular forms and these compounds takes part very slowly in the chemical reactions with another covalent compound.
Co-ordinate covalent bonding (Dative Covalent bond): The pair of electrons in this bonding is obtain by only through single atom and in this bonding, the atom which supplies electrons pair is called donar and the atom which takes such pair of electrons is called accepter. The electrons pair donated by the donor atom is called singleton pair. It is represented generally an arrow ( ). Here a convention is followed in which a +ve charge (S+) is given to the donor atom and a –ve charge (S-) on the acceptor atom.
For example- NH3 (g) +HCl (g) →NH4Cl(s) NH3 (g) +HCl (g) →NH4Cl(s)
Ammonium ions, NH4+, are formed by the transfer of a hydrogen ion (a proton) from the hydrogen chloride molecule to the lone pair of electrons on the ammonia molecule.
Courtesy: www.chemwiki.ucdavis.edu
Covalent Bond: They are formed by sharing of electrons and have low melting point and boiling points. These are soluble in organic solvents and are non-conductor of electricity. However graphite and diamond are covalent compounds but they have very high melting point because of their giant structure. The properties of covalent bonds are given below:
- They are mostly gases and liquids.
- They tend to be hard and brittle, and incapable of appreciable bending. These facts are understandable in terms of the underlying atomic forces. Since the bonds have well defined directions in space attempts to alter them are strongly restricted by the crystals.
- The melting and boiling points are usually low as compared to those of ionic crystals because the covalent bond is not so strong as ionic bond and also because the atoms are less powerfully attracted towards each other, the force that attracts them towards each other is called van der waals forces, dipoles etc.
- Most of the covalent substances do not conduct electricity because of the non-availability of free electrons or charged ions to carry the current. However certain substances like HCl which exhibit polarity in aqueous solutions behave like ionic substances and allow the passage of electricity through them accompanied by their own decomposition thereby acting as electrolytes.
- Covalent substances are insoluble in polar solvents like water. However they are soluble in non-polar solvents like benzene, carbon disulphide etc. This is because of the covalent nature of the solvent. However the giant molecules are not soluble in any solvent because of the large size of the molecules.
- A very interesting property of covalent crystals is the apparent lack of sensitivity of their physical properties of their bonding type. For example, carbons in the diamond structure are the hardest substance and have a very high melting point 3280K. The hardness and melting point decreases as we proceed to other elements. For example, Tin is the very soft element and has very low melting point. Depending on the number of electrons shared, the bond length and bond energy vary. When the number of electrons shared is more, the bond length between the atoms is decreased and bond energy is increased. Diamond, silicon, germanium, silicon carbide, tin and rutile are some examples of covalent crystals.