Q.5 Explain why: Why hydrogen molecule is more stable than hydrogen atom?, Why Helium exists in the form of He and not He2 ?, Why hydrogen chloride is a covalent compound but sodium chloride is an ionic compound?, Why is NaCl a bad conductor of an electricity in the solid crystalline form but a good conductor in the molten state?,In the formation of H2 molecule, why is the octet rule not obeyed?, Why metals are good conductors of electricity?

• Why hydrogen molecule is more stable than hydrogen atom?
• Every system in this universe tends to attain stability by lowering it's energy. similarly hydrogen also tends to gain stability. so two moles hydrogen atoms combine by releasing 435 KJ enegy to form one mole hydrogen molecule. This means that hydrogen molecule is more stable than hydrogen atoms as it has less energy than hydrogen atoms.
• Why Helium exists in the form of He and not He2 ? 
• Helium atom has only one shell i-e K-shell which can accommodate a maximum of two electrons which it already has. therefore Helium atom does not need any more electrons to become stable as it is already stable by duplet rule. Thus helium atom does not combine with another helium atom and hence exists as He and not He2.
• Why hydrogen chloride is a covalent compound but sodium chloride is an ionic compound?
• Covalent bonds are formed when the electronegativity difference between the two atoms is less than 1.7. Ionic bond is formed between two atoms having electronegativity difference of 1.7 or above. The electronegativity of hydrogen is 2.1 and that of chlorine is 3.0. So in HCl the electronegativity difference is 3.0-2.1=0.9 which is less than 1.7. Therefore they will form a covalent compound. Electronegativity of Sodium is 0.9 and that of chlorine is 3.0. So the electronegativity difference is 3.0-0.9=2.1 that is greater than 1.7 and will form an ionic bond and hence it will be an ionic bond.
• Why is NaCl a bad conductor of an electricity in the solid crystalline form but a good conductor in the molten state?
• In solid crystalline form the sodium ions and the chloride ions are firmly held together due to electrostatic forces of attraction and are unable to move. Therefore solid sodium chloride is a bad conductor of electricity. On the other hand in molten form the ions are free to move and become responsible for conducting electricity.
• In the formation of H2 molecule, why is the octet rule not obeyed?
• hydrogen atoms have only one shell( K-shell) which can accommodate only two electrons. Therefore each hydrogen atom in hydrogen molecule contributes one electron forming a pair which is shared between the two atoms. Thus in hydrogen molecule duplet rule is followed and not the octet rule.
•  Why metals are good conductors of electricity?
• According to electron sea theory the valence electrons of a metal atom are not firmly held by the nucleus due to large size and low ionization of metal atoms. These electrons form a sea of electrons and can move easily from one atom to the other. Therefore metals conduct heat are known as good conductors of electricity.