The electrostatic repulsion between electron pairs such as lone pair – lone pair (l.p. – l.p.), bond pair – bond pair (b.p. – b.p.) and lone pair – bond pair (l.p. – b.p.) in a given valence shell dec

Question

The electrostatic repulsion between electron pairs such as lone pair – lone pair (l.p. – l.p.), bond pair – bond pair (b.p. – b.p.) and lone pair – bond pair (l.p. – b.p.) in a given valence shell decreases in the order of ________ .

UnlockIAS · Accepted Answer

Correct — option (2). The order of repulsion between electron pairs in a valence shell, from the strongest to the weakest, is lone pair against lone pair, then lone pair against bond pair, then bond pair against bond pair, and option (2) states exactly that with the inequality signs pointing the right way for a decreasing order. The reason behind the ranking is a matter of where the electron cloud sits. A bond pair is shared between two atoms and is therefore attracted by two nuclei at once, which pulls it out into the region between them and confines it fairly tightly along the line of the bond. A lone pair belongs to the central atom alone and is held by a single nucleus, so nothing pulls it away and its cloud spreads out, occupying more space around the central atom and lying closer to it. A more diffuse cloud that occupies more room repels its neighbours more strongly. It follows immediately that two lone pairs, each of them broad, repel each other most; that a lone pair and a bond pair, one broad and one narrow, repel each other less; and that two bond pairs, both narrow and both drawn away from the central atom, repel each other least. This is the central postulate of the theory of valence shell electron pair repulsion, and its use is to predict the shape of a molecule and, more delicately, to predict how the bond angles depart from the ideal values. The classic sequence of three molecules shows it working. In methane the central carbon has four bond pairs and no lone pair; the four repel one another equally and the molecule is a regular tetrahedron with bond angles of about 109 and a half degrees. In ammonia the central nitrogen has three bond pairs and one lone pair; the lone pair pushes the three bonds together more strongly than they push back, so the angle closes to about 107 degrees and the shape is described as trigonal pyramidal. In water the central oxygen has two bond pairs and two lone pairs; there are now two broad clouds squeezing the two bonds, the angle closes further to about 104 and a half degrees, and the molecule is bent. The steady contraction of the angle from methane through ammonia to water, as one bond pair after another is replaced by a lone pair, is the most direct evidence for the ranking that option (2) states. The same ranking explains why, in a molecule with five electron pairs around the central atom, lone pairs take the equatorial positions rather than the axial ones — the equatorial position has fewer close neighbours at ninety degrees, so placing the most repulsive cloud there costs the molecule least. Option (2) is the answer.

Why the other options are wrong — (a) This option contains the correct ranking with every inequality sign reversed: it is the increasing order, where the stem asks for the decreasing one. It is by far the most dangerous option in the set, and it is placed first so that a candidate who recognises the three terms in the familiar sequence may mark it before noticing which way the signs point. Note that options (1) and (2) are exact mirror images of one another, which is a signal in itself — when two options in a set state the same relation in opposite directions, the question is not testing whether you know the relation but whether you read the stem, and the word that decides it is the verb, here 'decreases'. The remedy costs a few seconds: underline the direction word in the stem before looking at the options, and then check the first inequality sign of the option you intend to mark. Read as a statement of physics rather than as a sequence, this option would mean that two bond pairs repel each other more strongly than two lone pairs do, which contradicts the observation that bond angles contract as lone pairs are added.  (c) This option correctly places bond pair against bond pair as the weakest repulsion, but it then puts the lone pair against bond pair interaction above the lone pair against lone pair interaction, which inverts the top two terms. That cannot be right on the reasoning that produces the ranking in the first place: the strength of a repulsion depends on how diffuse the two clouds involved are, and an interaction between two diffuse lone pairs must exceed one between a diffuse lone pair and a compact bond pair. The consequence of this option, if it were true, would be visible in the geometry of water, where the two lone pairs on the oxygen atom would repel each other less than they repel the bonds, and the observed bond angle would be larger rather than smaller than that in ammonia. The measured angles run the other way, about 107 degrees in ammonia and about 104 and a half in water, which settles the order. The option is also written as an increasing sequence, so it does not answer the stem's request for a decreasing one.  (d) This option is a decreasing sequence, so it at least matches the direction the stem asks for, but the terms are in the wrong order twice over: it makes the lone pair against bond pair interaction the strongest and the lone pair against lone pair interaction the weakest, which is the opposite of the truth on the most important term. Placing lone pair against lone pair at the bottom of the ranking would make lone pairs the least disruptive feature of a valence shell, and the whole predictive value of the theory rests on the opposite proposition — that lone pairs are the most disruptive, which is why molecules with lone pairs on the central atom have shapes and angles that depart from the regular geometries. Without that proposition there would be no way to explain why water is bent rather than linear, or why sulphur tetrafluoride adopts a see-saw shape instead of a regular arrangement.

Concept: The valence shell electron pair repulsion theory, proposed by Sidgwick and Powell and developed by Gillespie and Nyholm, predicts the shape of a molecule from the number of electron pairs in the valence shell of its central atom. The pairs, whether shared in bonds or held as lone pairs, repel one another and settle into the arrangement that keeps them as far apart as possible: two pairs give a linear arrangement, three trigonal planar, four tetrahedral, five trigonal bipyramidal and six octahedral. The shape of the molecule is then read from the positions of the bonded atoms alone, since lone pairs are not visible as atoms, which is why four pairs give a tetrahedral molecule when all four are bonds, a trigonal pyramid when one is a lone pair and a bent molecule when two are. The refinement that makes the theory useful is the ranking of the repulsions: lone pair against lone pair is the strongest, lone pair against bond pair intermediate, and bond pair against bond pair the weakest, because a lone pair is held by only one nucleus and its cloud spreads out more than that of a bond pair, which is drawn out between two nuclei. This ranking predicts the direction in which bond angles depart from their ideal values, and the series of methane at about 109 and a half degrees, ammonia at about 107 and water at about 104 and a half is its standard demonstration. A further application of the same ranking places lone pairs in the equatorial rather than the axial positions of a trigonal bipyramid, which is what gives sulphur tetrafluoride its see-saw shape, chlorine trifluoride its T shape and xenon difluoride its linear one.

State PCS 2022 Q53 — Science & Technology PYQ with Answer

State PCS 2022 Q53 — Science & Technology PYQ with Answer