Shapes with Lone Pairs

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle to start. B F three: three fluorine atoms round boron, and the molecule is a flat triangle. C l F three: again, three fluorine atoms round the central atom. So again a flat triangle, right? No! It is shaped like the letter T! Same three fluorines, a completely different shape. Why? The lone pairs. Today, the shapes of molecules with lone pairs. Shapes come up often in JEE, so let's begin.

0:36Three or four pairs, with lone pairs

So, first a short code for shapes. Capital A is the central atom, capital X an attached atom, and capital E a lone pair on the central atom. On the right, our five basic arrangements of electron pairs, each with its angle and an example molecule. The thick wedge comes towards you, the dashed line goes away, the small circle means degrees, and the little dash means minutes. Okay? Now three electron pairs, one of them a lone pair. The pairs still spread out in a flat triangle, the second drawing. So what shape do the atoms make? Bent, exactly! One corner holds the lone pair, so the atoms make a bent shape. The code is A X two E: the small two below the X, the subscript, means two atoms. In the brackets, examples: S O two, ozone O three, S n C l two, and N O two minus; the minus at the top right is the charge. Clear? Next, four pairs with one lone pair: the code is A X three E, okay? The pairs point to the corners of a tetrahedron, the third drawing, but one corner is a lone pair. So we get a trigonal pyramid: N H three, P C l three. Right? Now four pairs, with two lone pairs. Two corners of the tetrahedron are lone pairs, and only two atoms are left. Shape? Bent again, correct! That is A X two E two, with two lone pairs. Water, H two O, is the famous one; also O F two, and S C l two. So, two kinds of bent shape: from three pairs, and from four pairs. Got it?

2:20Five pairs: lone pairs go equatorial

Now five pairs: the fourth drawing, the trigonal bipyramid. Two positions are axial, straight up and straight down. Three are equatorial, round the middle. Now a lone pair: where does it go, axial or equatorial? Equatorial, always! Lone pairs want as few neighbours at ninety degrees as possible. An axial position has three neighbours at ninety degrees. An equatorial position has only two, the two axial ones. So the lone pair goes equatorial. Clear? So, one lone pair, and the code is A X four E, okay? The lone pair takes one equatorial place, and the four atoms make a see-saw shape. Examples: S F four, and T e C l four. Got it? Now our puzzle, C l F three: five pairs, three fluorines and two lone pairs. Put the lone pairs axial, top and bottom, and the fluorines make a flat triangle, like B F three in the second drawing. Trigonal planar, right? No, no, careful! This is the trap. Lone pairs go equatorial, so both sit round the middle. That leaves two fluorines axial and one equatorial: a T-shape! So C l F three and B r F three are T-shaped, A X three E two. Puzzle solved. With me? And three lone pairs? They fill all three equatorial places, so the two atoms left are axial, at one hundred and eighty degrees. So X e F two is linear, A X two E three, even with three lone pairs! Same for the I three minus ion, and I C l two minus. Okay?

4:05Six pairs: two lone pairs sit opposite

Now six pairs: the octahedron, the last drawing. One lone pair takes one corner, and the five atoms make a square pyramid. That is A X five E: B r F five and I F five. Right? Now X e F four: six pairs, and two of them are lone pairs. Where do the two lone pairs go? Side by side, at ninety degrees, or opposite each other? Opposite each other, yes! Lone pair with lone pair is the strongest repulsion, so they get as far apart as possible: one above, one below. We call this trans, and the code is A X four E two. Clear? And the four fluorines sit in one flat square round the xenon. Square planar! Same for I C l four minus, and B r F four minus. So, four atoms, but X e F four is not tetrahedral. Got it?

5:03Seven pairs: two xenon cases

Finally, seven pairs: two cases to know. X e F five minus has five fluorines and two lone pairs, A X five E two. The lone pairs sit above and below, and the five fluorines lie flat in a pentagon: pentagonal planar. Okay? And X e F six has six fluorines and one lone pair, A X six E, seven pairs in all. The lone pair pushes the fluorines out of place: a distorted octahedron. Right?

5:31Example 1: shapes of NH3, H2O, SO2 and SnCl2

Now let's use all this. Example one. Give the shapes of N H three, H two O, S O two, and S n C l two. Pause the video and try it. Hint: count the pairs first. Done? Okay, let's check. N H three: nitrogen has five valence electrons, plus three hydrogens, so half of, five plus three, four pairs. Three atoms, so one lone pair; l p means lone pair. The code is A X three E: pyramidal. Okay? Next, H two O, water. Oxygen has six, plus two hydrogens: half of, six plus two, four pairs. Two atoms, so two lone pairs, and the code is A X two E two: bent. Clear? Now S O two. Two oxygen atoms on sulphur, just like C O two, which is linear. So S O two is linear too, right? Oh no, careful! Count first. Sulphur has six, and the oxygens add nothing: half of six, three pairs. Two atoms, so one lone pair, and it bends the molecule. The code is A X two E: bent, not linear. Got it? Last, S n C l two. Tin has four, plus two chlorines: half of, four plus two, three pairs. Two atoms, one lone pair: bent again. So: pyramidal, bent, bent, bent. With me?

7:08Example 2: shapes of XeO3, XeO2F2 and XeOF4

Now a harder one. Example two, three xenon compounds. Give the shapes of X e O three, X e O two F two, and X e O F four. Pause and try. Hint: oxygen adds nothing to the count. Done? Let's check. X e O three: xenon has eight valence electrons, and the oxygens add nothing, so half of eight, four pairs. Three O means three oxygen atoms, so one lone pair. The code is A X three E: pyramidal, like ammonia. Okay? Next, X e O two F two. Four atoms on xenon: two oxygens and two fluorines. Four atoms, so tetrahedral, right? No, no, wait! Count the pairs. Only the two fluorines are monovalent: half of, eight plus two, five pairs. Four atoms, so one lone pair, and it goes equatorial. The code is A X four E: see-saw, like S F four. Clear? And X e O F four: half of, eight plus four, six pairs. Five atoms, so one lone pair: A X five E, square pyramidal. So: pyramidal, see-saw and square pyramidal. One lone pair each time, three different shapes! The count decides everything. Okay?

8:34Quick revision: bent, see-saw, T-shaped, square planar

So, quick revision. Four pairs: A X three E is pyramidal, and A X two E two is bent. Five pairs: lone pairs go equatorial, giving see-saw, T-shape and linear. Six pairs: two lone pairs sit opposite, so X e F four is square planar. Now try the practice problems on this page. Count first, then place the lone pairs. All the best!