Chemistry MCQs for NEET — Practice Questions with Answers

The NCERT states, '…the new electron enters a d orbital each time the nuclear charge increases by unity. It may be recalled that the shielding effect of a d electron is not that effective, hence the net electrostatic attraction between the nuclear charge and the outermost electron increases and the ionic radius decreases.'

The NCERT mentions, 'However, the shielding of one 4f electron by another is less than one d electron by another with the increase in nuclear charge along the series. There is fairly regular decrease in the sizes with increasing atomic number.' Less effective shielding by 4f electrons means the increased nuclear charge is more effective in pulling the electrons closer, leading to a decrease in size.

The NCERT clearly states, 'The cumulative effect of the contraction of the lanthanoid series, known as lanthanoid contraction, causes the radii of the members of the third transition series to be very similar to those of the corresponding members of the second series.' And '...the imperfect shielding of one electron by another in the same sub-shell. However, the shielding of one 4 f electron by another is less than one d electron by another...'. Therefore, option o3 is incorrect because it is due to imperfect (less effective) shielding, not perfect shielding.

Within a given transition series, atomic radii generally decrease with increasing atomic number due to increasing effective nuclear charge and relatively poor shielding by d-electrons. Therefore, V (Z=23) would have a smaller atomic radius than Ti (Z=22), which would have a smaller radius than Sc (Z=21). The order of increasing atomic radii would be V < Ti < Sc.

The NCERT defines, 'One practical approach to estimate the size of an atom of a non-metallic element is to measure the distance between two atoms when they are bound together by a single bond in a covalent molecule and from this value, the “Covalent radius” of the element can be calculated... For metals, we define the term 'metallic radius' which is taken as half the internuclear distance separating the metal cores in the metallic crystal.'

The NCERT states, 'The same trend is observed in the atomic radii of a given series. However, the variation within a series is quite small.'

The NCERT states, 'Within a family or vertical column of the periodic table, the atomic radius increases regularly with atomic number... This happens because the inner energy levels are filled with electrons, which serve to shield the outer electrons from the pull of the nucleus. Consequently the size of the atom increases as reflected in the atomic radii.'

According to the provided text, 'The entropy of any pure crystalline substance approaches zero as the temperature approaches absolute zero. This is called third law of thermodynamics.' The other options describe the Second Law, First Law, and a consequence of the Second Law, respectively.

The context states, 'This is so because there is perfect order in a crystal at absolute zero.' While molecular motion significantly decreases, the fundamental reason for zero entropy in a perfect crystal at 0 K is the perfect order, meaning only one microstate is accessible.

The NCERT text explicitly states, 'The statement is confined to pure crystalline solids because theoretical arguments and practical evidences have shown that entropy of solutions and super cooled liquids is not zero at 0 K.'