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Formation of Substances

24-FEB-2016 15:01

    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.

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    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

    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.

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    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.

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