NEET Practice Questions (MCQs) with Answers & Solutions

Mycorrhizae, a form of mutualism, involves fungi helping plants absorb nutrients and plants providing carbohydrates to the fungi, fitting the description exactly.

As per the NCERT text, 'In general, ions of the same charge in a given series show progressive decrease in radius with increasing atomic number. This is because 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 same trend is observed in the atomic radii of a given series.' This means atomic radii generally decrease across a period due to increasing effective nuclear charge, as the shielding effect of d electrons is not very effective.

The NCERT states, 'The radii of the third (5d) series are virtually the same as those of the corresponding members of the second series. This phenomenon is associated with the intervention of the 4f orbitals which must be filled before the 5d series of elements begin. The filling of 4f before 5d orbital results in a regular decrease in atomic radii called Lanthanoid contraction'. This contraction essentially compensates for the expected increase in atomic size with increasing atomic number.

The NCERT text explicitly mentions, 'The almost identical radii of Zr (160 pm) and Hf (159 pm), a consequence of the lanthanoid contraction, account for their occurrence together in nature and for the difficulty faced in their separation.'

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.'