Chemistry Chapter (3) Chemical Bonding

Points To Remember

•    A chemical bond is the force which holds together two or more atoms or ions to from a large variety of compounds.
•  According to modern theory of chemical bonding, atoms from bonds as it leads to a decrease in energy.
• Size of atoms are expressed in terms of atomic radii, ionic radii and covalent radii etc. depending upon the type of the compound used for its measurement.
• Atomic radii can be determined by measuring the distance between the centres of adjacent atoms with the help of x-rays or by spectroscopic measurements.
•  Ionic radius of a cation is smaller than the atomic radius of the elements from which it is derived while ionic radius of an anion is greater than the atomic radius of the corresponding atom.
• The interionic distance in a crystal lattice is equal to the sum of the cationic radius and the anionic radius.
• The covalent radius of an element is defined as half of the single bond length between two atoms covalently bonded in a molecule.
• Ionic bond is form by the complete transfer of electron (s) from an atom with low ionization energy to another atom with high electron affinity.
•  Covalent bond is form by the mutual sharing of electron between two atoms.
• A covalent bond may be non- polar or polar in character.
• In non-polar covalent bond, the bonding electron pairs are equally shared.
• In polar covalent bond, the bonding electron paires are not equally shared between the bonded atoms. The bonding pair of electrons will be displaced towards the more electronegative atom.
• A co-ordinate covalent or dative bond is form between two atoms when the shared pair of electrons  is donated by one of the bonded atoms.
• The minimum amount of energy required to remove an electron from an isolated atom in gaseous state is called ionization energy . It depends upon the atomic size, nuclear charge and shielding effect of electrons.
• The electron affinity of an atom is the energy given out when an electron is added to gaseous atom.
• The tendency of an atom to attract a shared pair of electron to itself is known as electronegativity.
• According to valence bond theory, the atomic orbitals overlap to rom bonds but the individual character of the atomic orbitals are retained. The greater the overlap, the stronger will be the bond formed.
• The geometrical shapes and bond angles are better explained by different hybridization schemes where different atomic orbitals are mixed to from hybrid orbitals.
• The VSEPR theory gives information about the general shapes and bond angles of molecules. It is based upon repulsion between bonding and lone pairs of electrons which tend to remain at maximum distance apart so that interaction between them is minmum.
•  According to molecular orbital theory, atomic orbitals overlap to from molecular orbitals. Half of them are bonding molecular orbitals and half anti-bonding molecular orbitals. The individual atomic orbital character is lost in order to from an entirely new orbital that belongs to the whole molecule.
• The number of bonds formed between  two atoms after the atomic orbitals overlap, is called the bond order and is taken as half of the difference between the number of bonding electrons anti-bonding electrons.
•  When a bond is formed between two atoms, energy is released.
• The same amount of energy is absorbed when the bond is broken to from neutral atoms.
•  Bond energy is determined experimentally by measuring the heat involved in a chemical reaction.
• Bond energy is also called bond enthalpy as it is a measure of enthalpy change at 298k.
• Bond energy is given in kJ mole^-1.
• Bond energies of multiple bonds are greater than those of single bonds.
•  Bond energy is a measure of the strength of a bond.
• The strength of a bond depends upon electronegativity, size, of the atoms and bond length.
• Stronger the dipole, greater is the bond energy.
• The bonds with higher bond energy values have shorter bond lengths.
• The dipole moment is a vector quantity which has magnitude as well as direction.
• Dipole moment is defined as the product of electric charge and the distance between the positive and negative centres.
• Dipole moment plays a major role in determining the percentage ionic character of a covalent bond and the shapes of molecules.
• Properties of substances are characterized by the type of bonds present in them.

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