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(i) Atoms of the same element may combine to form stable
molecules. For example, H,, N, O, etc. (ii) Atoms of different elements may combine to form stable
molecules. For example, NaCl, KCI etc. (iii) Atoms of more than two elements may combine to form
complex molecules. Sucrose CH220, is one such example. (iv) Thousands of atom may combine to form biological mol
ecules like DNA and RNA. They exist in the form of chains. Molecules possess extremely interesting properties. They can rotate like a rigid rotator and vibrate like a spring. These properties are not possessed by atoms. Just as discrete energy states exist in atoms due to orbital motion of electrons, the rotational and vibrational motion of molecule also correspond to discrete energy states. An excited molecule returns to the ground state by emitting photons of energy. This process in case of molecules gives rise to rotational and vibrational spectra. Bonding in Molecules When atoms combine to form a molecule, they are held together by the force of attraction called chemical bond. When atoms come close to each other, the energy of the system is reduced and it leads to the formation of bond. A chemical bond is defined as the attractive force, which holds the atoms together in molecule. Concept of Bond The outermost shell of an atom is called the valence shell. The Greek word valence means hook. According to old chemical theory, the atoms had hooks, which hold them together with other atoms. According to present day theory, the electrons in the outermost shell called valence electrons are impossible for holding the atoms together in atoms. The atoms combine with each other by the transfer or mutual sharing of electrons and in doing so, each combining atom attains the inert gas configuration. The outermost shell of atoms of the inert gases have 8 electrons
and are most stable. 89. (42 Gert frugt ziwll 3C (a)(b)(0)4429044 89.
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