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 ________ .
- (1)l.p. – l.p. < l.p. – b.p. < b.p. – b.p.
- (2)l.p. – l.p. > l.p. – b.p. > b.p. – b.p.
- (3)b.p. – b.p. < l.p. – l.p. < l.p. – b.p.
- (4)l.p. – b.p. > b.p. – b.p. > l.p. – l.p.
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.
- (1)l.p. – l.p. < l.p. – b.p. < b.p. – b.p. — 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.
- (3)b.p. – b.p. < l.p. – l.p. < l.p. – b.p. — 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.
- (4)l.p. – b.p. > b.p. – b.p. > l.p. – l.p. — 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.
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.
MPSC's chemistry questions are drawn largely from the higher secondary syllabus, and molecular geometry is one of its most productive corners because a single principle generates many questions: the shape of a named molecule, the bond angle in a named molecule, why one angle is smaller than another, the arrangement of a given number of electron pairs, and the ranking asked here. The item is also a good example of a construction this paper uses more than once — two options stating the same relation in opposite directions. When that happens the question has stopped testing the chemistry and started testing whether the stem was read, and the direction word in the stem is the only thing that separates the two. It is worth forming the habit of marking that word before scanning the options, because in a paper of a hundred questions in two hours the temptation is to recognise a familiar sequence and move on. The underlying ranking should be understood rather than memorised, since the examiner may ask it indirectly through bond angles, through the shape of an unfamiliar molecule, or through the position a lone pair takes in a five-pair arrangement, and none of those can be answered from the sequence alone.
- The order of repulsion between electron pairs in a valence shell, from strongest to weakest, is lone pair against lone pair, then lone pair against bond pair, then bond pair against bond pair.
- A lone pair is attracted by only one nucleus and its electron cloud is therefore more diffuse and closer to the central atom than that of a bond pair, which is drawn out between two nuclei; the more diffuse cloud repels more strongly.
- The bond angle contracts as bond pairs are replaced by lone pairs: methane with four bond pairs has an angle of about 109 degrees 28 minutes, ammonia with three bond pairs and one lone pair about 107 degrees, and water with two of each about 104 and a half degrees.
- The valence shell electron pair repulsion theory predicts molecular shape from the total number of electron pairs around the central atom, giving linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral arrangements for two to six pairs.
- In a trigonal bipyramidal arrangement lone pairs occupy equatorial positions rather than axial ones, which produces the see-saw shape of sulphur tetrafluoride, the T shape of chlorine trifluoride and the linear shape of xenon difluoride.
The ranking is the refinement that makes the valence shell electron pair repulsion theory useful, because the number of electron pairs alone gives only the ideal arrangement — two linear, three trigonal planar, four tetrahedral, five trigonal bipyramidal, six octahedral — and it is the ranking that predicts which way the real angles depart from it. The same ranking sends lone pairs to the equatorial rather than the axial positions of a trigonal bipyramid, where each has fewer close neighbours at ninety degrees, and that is where the see-saw shape of sulphur tetrafluoride, the T shape of chlorine trifluoride and the linear shape of xenon difluoride come from.
- Marking the option that reverses every inequality sign, since two of the options here state the same ranking in opposite directions and only the stem's direction word separates them
- Placing lone pair against bond pair above lone pair against lone pair, which contradicts the observed contraction of bond angles as lone pairs are added
- Predicting the shape of a molecule from the total number of electron pairs while forgetting that only the bonded atoms are counted in naming the shape
- Assuming a lone pair takes an axial position in a trigonal bipyramidal arrangement, when the equatorial position has fewer close neighbours and is therefore preferred
Molecular geometry appears in MPSC papers as a shape question naming a molecule and asking what it looks like, as a bond angle question, as a comparison asking why one angle is smaller than another, and as the ranking of repulsions asked directly here. The Commission also sets the topic through hybridisation, requiring the hybrid state of a central atom to be identified, and the two lines of questioning meet, since the arrangement of electron pairs and the hybrid state describe the same thing in different vocabularies. The most economical preparation is a table of molecules against total electron pairs, bond pairs, lone pairs, arrangement, shape and bond angle, filled in for about a dozen standard examples from beryllium chloride to sulphur hexafluoride, since almost every question the Commission can set from this topic is answerable from such a table.
No directly related past PYQ was found.
- practice — not a real PYQ
The bond angle in a water molecule is smaller than that in an ammonia molecule. The reason is that
- (a)oxygen is more electronegative than nitrogen and shortens the bonds
- (b)the oxygen atom carries two lone pairs against one on the nitrogen atom, and lone pair repulsion is the strongest kind
- (c)water is a liquid while ammonia is a gas at room temperature
- (d)the water molecule has three bond pairs and the ammonia molecule has two
Answer(b) The oxygen atom carries two lone pairs against one on the nitrogen atom, and lone pair repulsion is the strongest kind — each lone pair occupies more space than a bond pair and pushes the bonded atoms closer together, so replacing a second bond pair with a second lone pair contracts the angle further. The sequence runs from about 109 and a half degrees in methane, which has no lone pair, to about 107 degrees in ammonia with one, and about 104 and a half degrees in water with two.
- practice — not a real PYQ
A central atom is surrounded by four electron pairs, of which two are bond pairs and two are lone pairs. The shape of the molecule will be
- (a)Tetrahedral
- (b)Trigonal pyramidal
- (c)Bent or angular
- (d)Linear
Answer(c) Bent or angular — four electron pairs arrange themselves tetrahedrally around the central atom, but the shape of a molecule is described by the positions of its atoms and not of its lone pairs, so with only two of the four positions occupied by bonded atoms the molecule is bent. Water is the standard example. If three of the four pairs were bonds the shape would be trigonal pyramidal, as in ammonia, and if all four were bonds it would be tetrahedral, as in methane.