General Trends in Group 15

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle first. Phosphorus forms P C l five, phosphorus pentachloride. Nitrogen sits just above it, in the same group, with the same five valence electrons. So you would expect N C l five as well. But N C l five does not exist! Today we find out why. Group fifteen trends come up often in JEE, so let's begin.

0:28Group 15 at a glance

Group fifteen is the nitrogen family. Nitrogen and phosphorus, N and P, are non-metals; arsenic and antimony, A s and S b, are metalloids; bismuth, B i, is a metal. The valence shell is n s two, n p three, n being the outer shell and the raised numbers counting electrons: a stable, half-filled p subshell. Okay? Now size: the covalent radius, in picometres, the p m after the slash. Nitrogen seventy, phosphorus one hundred and ten, arsenic one hundred and twenty one, antimony one hundred and forty one, bismuth one hundred and forty eight. Where is the big jump? Yes, exactly, from nitrogen to phosphorus, forty picometres in one step. But from arsenic to bismuth the rise is only slight, because the filled d and f subshells shield the nucleus poorly. Right? Next, the first ionisation enthalpy, the Greek capital delta with a small i, then H with a small one, in kilojoules per mole; mol to the minus one means per mole. It falls from one thousand four hundred and two for nitrogen to seven hundred and three for bismuth, and for nitrogen, phosphorus and arsenic it is higher than for the group sixteen neighbour, thanks to the stable half-filled n p three. Careful: antimony and bismuth are below tellurium and polonium. Clear?

1:48Oxidation states and the inert pair effect

Next, oxidation states: from minus three up to plus five, and nitrogen shows every one of them. Now, going down the group, which becomes more stable, plus five or plus three? Yes, exactly, it is plus three. Down the group, plus five gets less stable and plus three more stable. This is the inert pair effect, where the n s two pair of heavy atoms is reluctant to bond. Got it? So bismuth five, the Roman five in brackets, is a strong oxidant, as in N a B i O three, sodium bismuthate; small low numbers count atoms. And the only well characterised bismuth five halide is B i F five. With me? Now nitrogen. In acid, its states plus one to plus four tend to disproportionate: part is oxidised, part reduced. The arrow means gives: three HNO two gives HNO three, plus two N O, plus H two O, water. Okay? And phosphorus three, the Roman three, does it too: four H three P O three gives three H three P O four, plus P H three. Clear?

2:54Why nitrogen is different

Now the heart of our puzzle. Nitrogen, the first member, is small, highly electronegative, with a high ionisation enthalpy, and it has no d orbitals. Okay? So nitrogen forms strong p pi, p pi multiple bonds, pi bonds between two p orbitals, with pi the Greek letter. N triple bond N, the three lines, has a bond enthalpy of nine hundred and forty one kilojoules per mole, so nitrogen is a diatomic gas. Can phosphorus do the same? No, it cannot, because phosphorus is too large for good p pi, p pi overlap. So it forms single bonds instead, and exists as P four tetrahedra. Clear? Now a trap. Nitrogen is smaller, so the N dash N single bond, the dash meaning a single bond, should be stronger than the P dash P single bond, right? Oh no, careful, it is actually weaker than the P P bond! The N N bond is so short that the lone pairs on the two small nitrogen atoms repel strongly. So nitrogen shows little catenation, that is, little chain forming.

4:01The hydrides EH₃

Got it? Next, the hydrides, E H three, E being any element of the group; the small circle means degrees. The bond angle is one hundred and seven point eight in N H three, then ninety three point six in P H three, then ninety one point eight in A s H three, then ninety one point three in S b H three, and ninety in B i H three. Why does it keep falling? As E grows larger and less electronegative, the bond pairs move away from it and repel less. The heavier hydrides use almost pure p orbitals. With me? From N H three to B i H three, thermal stability falls and reducing power rises, because the E H bond gets weaker; basicity falls too, as the lone pair sits in a larger, more diffuse orbital. Now boiling points. N H three is the lightest, so it boils the lowest, right? Oh no, careful, that is the trap! Hydrogen bonding lifts N H three above P H three and A s H three. So, with b p for boiling point and each pointed sign meaning less than, the order is P H three, A s H three, N H three, S b H three, B i H three. Okay?

5:15Halides and oxides

Next, the halides, with X for a halogen. All five form pyramidal trihalides, E X three, and all except nitrogen form trigonal bipyramidal pentahalides, E X five. So again, no N C l five. Clear? Now the oxides, E two O three and E two O five. For the same element, the plus five oxide is the more acidic one: higher oxidation state, more acidic oxide. Right? And acidity falls down the group: N two O three and P four O six are acidic, A s two O three and S b two O three are amphoteric, and B i two O three is basic. Got it?

5:51Example 1: why NCl5 does not exist although PCl5 does

Now example one, which settles our puzzle. Why does nitrogen not form N C l five, although phosphorus forms P C l five? Pause the video and try it first. Done? Okay, let's check. Nitrogen is in the second period. So how many valence orbitals does it have? It has just four: a single two s orbital and three two p orbitals. So nitrogen cannot expand its octet, and its maximum covalency is four. Right? Phosphorus can also use orbitals of its three d subshell, giving s p three d hybridisation, the three raised on the p, and it is large enough for five chlorine atoms. So P C l five exists, and our puzzle is solved.

6:38Example 2: NCl3 vs PCl3 hydrolysis, which atom water attacks

Clear? Now a harder one, example two. N C l three is hydrolysed by water to N H three and HOC l, but P C l three gives H three P O three and H C l. Pause the video, and try to explain why. Done? Okay, let's check. The oxygen of water attacks by donating its lone pair, so it needs an atom that can accept it. In N C l three, which atom can do that, nitrogen or chlorine? It is chlorine. Nitrogen is in the second period and has no vacant d orbitals, but chlorine has vacant three d orbitals. And nitrogen and chlorine have very close electronegativities, so the bond is nearly non-polar. Surprising, right? So water attacks chlorine, and nitrogen keeps the bond pair: N C l three plus three H two O gives N H three plus three HOC l. Okay? In P C l three, phosphorus is the positive end, and it has vacant d orbitals. With me? So water attacks phosphorus: P C l three plus three H two O gives H three P O three plus three H C l. Same formula type, but water attacks a different atom: chlorine in one, phosphorus in the other. Got it?

7:53Quick revision: inert pair, nitrogen anomaly, NH3 b.p.

So, quick revision. Down the group, plus five gets less stable and plus three more stable: the inert pair effect. Nitrogen has a p pi, p pi triple bond, a weak N N bond, and a maximum covalency of four. And hydrogen bonding lifts N H three above P H three and A s H three. Now try the practice problems on this page. All the best!