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cloud of one atom overlaps that of the other, they interact with each other electrically. The atoms then, remain together rather than to exist as separate neutral atoms. When the atoms are held together because of the above fact, they are said to form molecules. The molecule so formed is said to be stable, if energy is required to split it into its constituent atoms.
The minimum energy required to split a molecule into its constituent atoms is called dissociation energy. Atoms can combine in many ways to form molecules:
(i)
Atoms of the same element may combine to form stable molecules. For example, H2, N, O,, etc.
(ii)
Atoms of different elements may combine to form stable molecules. For example, NaCl, KCl etc. (iii) Atoms of more than two elements may combine to form complex molecules. Sucrose C2H22 01, is one such example. (iv) Thousands of atom may combine to form biological molecules like DNA and RNA. They exist in the form of chains.
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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.
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