CuCl is covalent and NaCl is ionic
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The correct statement is (a) CuCl is covalent and NaCl is ionic.

Explanation:
CuCl is covalent because copper (Cu) is a transition metal and chlorine (Cl) is a nonmetal. Transition metals tend to form covalent compounds because they have multiple oxidation states and can share electrons with nonmetals. In CuCl, the copper atom shares one electron with the chlorine atom to form a covalent bond.

On the other hand, NaCl is ionic because sodium (Na) is a metal and chlorine (Cl) is a nonmetal. Metals tend to lose electrons to form cations, while nonmetals tend to gain electrons to form anions. In NaCl, sodium loses one electron to chlorine, resulting in the formation of Na+ cation and Cl- anion. The electrostatic attraction between the oppositely charged ions leads to the formation of an ionic bond.

Inner electrons have poor shielding effect on the nucleus and thus, electronegativity of the 18-electrons shell is increased
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The statement (b) "Inner electrons have poor shielding effect on the nucleus and thus, electronegativity of the 18-electrons shell is increased" is incorrect.

Explanation:
Inner electrons do not have a poor shielding effect on the nucleus. In fact, inner electrons provide better shielding to the outer electrons from the positively charged nucleus. This is because inner electrons are closer to the nucleus and their negative charge repels the outer electrons, reducing the attractive force between the outer electrons and the nucleus. As a result, the electronegativity of the 18-electron shell is decreased, not increased. Electronegativity generally increases across a period from left to right, as the effective nuclear charge increases, but it decreases down a group due to increased shielding by inner electron shells.

Cations with 18-electron shells have greater polarising power than the cations with 8-electron shells
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The statement (c) "Cations with 18-electron shells have greater polarising power than the cations with 8-electron shells" is incorrect.

Explanation:
Cations with 8-electron shells have greater polarising power than cations with 18-electron shells. Polarising power refers to the ability of a cation to distort the electron cloud of an anion in an ionic compound. Cations with smaller size and higher charge have greater polarising power because they can more effectively pull the electrons of the anion towards themselves.

Cations with 8-electron shells, such as Al3+ or Fe3+, have high charge density and smaller size compared to cations with 18-electron shells, such as Na+ or K+. Therefore, cations with 8-electron shells have greater polarising power as they can distort the electron cloud of the anion more effectively, leading to stronger ionic bonds.