The element which has the strongest metallic bond among \({ }_{11} \mathrm{~A}^{23},{ }_{12} \mathrm{~B}^{24},{ }_{13} \mathrm{C}^{27}\) and \({ }_{19} \mathrm{D}^{39}\) is (a) \(\mathrm{A}\) (b) \(\mathrm{B}\) (c) C (d) D

The effective nuclear charge (ENC) is a concept that helps us understand the strength of the attraction between the nucleus of an atom and its electrons. When we speak about the ENC, we are referring to the net positive charge experienced by electrons in the valence shell.

Within the periodic table, as we move from left to right across a period, the ENC generally increases. This happens because each successive element in a period has an additional proton in its nucleus, which increases the positive charge, attracting the valence electrons more strongly. However, not every electron feels the full charge of the nucleus due to shielding by inner-shell electrons.

Electrons in the same shell do not shield each other from the attractive force of the nucleus significantly, but electrons in inner shells repel the valence electrons, effectively reducing the net positive charge that those valence electrons experience. Higher ENC correlates with a tighter grip on valence electrons, which can affect properties like atomic size and the strength of metallic bonds.

Regarding valence electrons, these are the electrons found in the outermost shell of an atom and play a crucial role in chemical bonding and reactions. In the context of metallic bonds, the valence electrons are particularly important because they become delocalized, meaning they are not bound to any specific atom and can move freely throughout the metal.

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