Chemical bond: The attractive force which holds together the constituent atoms of chemical compounds is called chemical bond.
Combination of atoms: It. is universal law of nature that systems with lower energy are more stable. So atoms combine to form a molecule because it has a· lower energy i.e., more stable.
Modern concept of chemical bonding: The formation -of a chemical bond between two atoms depends upon the net result. of forces of attraction and repulsion between two atoms when they are brought closer. If the potential energy of the system decreases at a certain distance (called bond length) the potential energy is minimum a chemical bond results. If the potential energy increases, no bonding is possible. This is due to the mutual repulsion of the interpenetrating electronic shell.
Stable electronic configuration: The electronic arrangement in which 8 electrons are present in the valence shell is called’ stable configuration. If the valence shell is the (first as in He) stable configuration is 2.
Hydrogen molecule is more stable than two hydrogen atoms: In an isolated Hydrogen atom is electron is attracted by only a single nucleus, but when two H atoms combine to for H2 molecule each of the two electrons experiences the attractive pull of two nuclei. The energy decreases i.e., H2 molecule is more stable.
Lewis and Kossel Theory of covalent bond: A covalent bond is formed by the. mutual contribution and sharing of electrons between the bonding atoms. If the two atoms share one electron pair, bond formed is called a single bond (-). If the two atoms share two electron pairs bond formed is called a double bond (=). If the two atoms share three electron pairs, bond formed is called as triple bond (≡).
Orbital concept of covalent bond formation: A covalent bond is formed as a result of the partial merger (overlap) of the half-filled orbitals with opposite spins of electrons and represent in the valence shell of the bonding atoms. Due to the overlap, the two orbitals actually merge together to form a new orbital, known as molecular orbital.
Condition for the formation of covalent bond:
(i) Two atoms should have half-filled valence orbitals.
(ii) Two sharing electrons must have opposite spins (Pauli’s exclusion principle).
Co-ordinate covalent bond: is a covalent bond in which both. the shared electrons are contributed by one atom only and not H one by each atom as a covalent bond and is generally referred as co-ordinate bond.
Characteristic of covalent compounds:
(a) These have low melting points and boiling points.
(b) They exist mostly as gases or liquids.
(c) They are poor conductors of electricity in the solution -or in the fused state.
(d) They are insoluble in water but soluble in organic solvents.
He2 molecule does not exist. In a helium atom, the 1s orbital is full (has 2 electrons) and so 1s is not able to accommodate more electrons (Peuli’s exclusion principle). Thus 1s orbital of two He atoms cannot overlap to form He2 molecule.
Ionic bond: Ionic bond is formed by the transference of one or more electrons from ·one atom to the other in order to achieve stable configuration. For example when sodium reacts with chlorine, Na atom loses one electron to form Na+ ion and Cl— atom gains one· electron to. form Cl— ion. These ion an held together in the form of NaCl.
Condition for the formation of ionic bonds :
(i) Smaller the value of ionization energy the easier it is for the natural atom to form cation.
(ii) The other atom must possess high value of electron affinity so that anion may be formed easily. Thus one atom should have a low ionization energy and the other should have a high value of electron affinity.
Characteristic of Ionic compounds :
(i) They are hard crystalline solid.
(ii) They are soluble in water (and polar compounds) but insoluble in organic compounds.
(iii) They are poor conductors of electricity and heat.
(iv) They have high melting and boiling points.
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