Valence, Hydrides and Anomalies

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Carbon and lead sit in the same group, group fourteen. For carbon, the plus four state is the stable one. So lead should also prefer plus four, right? Well, it does not: for lead, the plus two compounds are the more stable ones! So why does this happen? Today we will find out, along with valence, hydrides, and the elements that break the rules. These trends come up often in JEE, so let's begin.

0:33Valence from the group number

So, let's start with valence, for the main-group elements. In groups one, two, thirteen and fourteen, the valence follows the group number, so it is one, two, three and four. For thirteen and fourteen, just drop the ten. Okay? Now, nitrogen sits in group fifteen. So tell me, is its valence five, or something else? It is three, not five. For groups fifteen to seventeen, the valence is eighteen minus the group number. So the valence is three, two and one for groups fifteen, sixteen and seventeen. Right? You can see this in the compounds with hydrogen. Carbon, valence four, gives C H four; the small numbers below are subscripts, counting the hydrogen atoms. Then N H three, H two O, and H F, for valences three, two and one. Clear?

1:30The inert pair effect

Now, the inert pair effect. On screen, n s two means the outer s orbital with two electrons: n is the shell number, and the small raised two counts the electrons, a pair. In the heavier elements of groups thirteen to fifteen, this pair resists bonding. Inert means inactive. With me? Let me ask you this: thallium is a heavy element of group thirteen, where the valence is three. If its s pair sits out, which charge becomes the stable one, plus three or plus one? Yes, exactly, it is plus one, because three minus the two of the pair leaves one. Down these groups, the lower oxidation state becomes more stable. So we get T l plus, with a small raised plus sign. In the same way, lead prefers P b two plus, and bismuth prefers B i three plus. Got it?

2:24Second-period elements are anomalous

Next, the anomalies. The second-period elements behave differently from the rest of their groups. Two reasons first: they are small in size, and they have high electronegativity. Okay? And a third reason, a big one: the second shell has no d orbitals. So these atoms can form at most four bonds; their maximum covalency is four. Clear? Now, here is a classic trap. Phosphorus, just below nitrogen, forms P C l five. So nitrogen can also form N C l five, right? No, no, careful, nitrogen cannot do that! It has no d orbitals in its second shell, so its covalency cannot go beyond four. That is why nitrogen forms only N C l three, while phosphorus forms both P C l three and P C l five. Okay? And one more feature: strong p pi, p pi multiple bonds. Pi is the Greek letter, and p pi means a pi bond made from p orbitals; the dash joins the p orbitals of the two atoms. Small second-period atoms make these double and triple bonds strongly. Got it?

3:33The diagonal relationship

Now, the diagonal relationship. Lithium resembles magnesium, which sits one step to the right and one step down. In the same way, beryllium resembles aluminium, and boron resembles silicon; the dash joins each pair. Okay? But why does this happen? The two members of each pair have a similar charge over size ratio; the slash on screen means over. This ratio decides the polarising power, so the pair behaves alike.

4:01Group 15 hydrides and the hydrogen halides

Right? Now the hydrides of group fifteen: ammonia, N H three, phosphine, P H three, then arsine, A s H three, and stibine, S b H three. Basicity falls down the group, so ammonia is the strongest base; the greater than signs show the order. Okay? Reducing power goes the other way, in the reverse order. So stibine, at the bottom, is the strongest reducing agent. With me? Next, the hydrogen halides. Fluorine is the most electronegative element of all. So hydrogen fluoride, H F, must be the strongest acid, right? Oh no, careful, that is the trap! H F is the weakest of the four. Acid strength rises from H F to H C l to H B r, and H I is the strongest; the less than signs show it. Clear? And one reason covers both orders. Capital E stands for the element, so E H is its bond with hydrogen. Down the group, this bond gets weaker and breaks more easily. That is why stibine is the best reducer, and H I the strongest acid. Got it?

5:13Reactivity down groups 1 and 17

Now the last trend, reactivity. In group one, the alkali metals, reactivity increases down the group. Okay? But in group seventeen, the halogens, it is the opposite: reactivity decreases down the group. Right? So tell me, which is the most reactive alkali metal towards water, and which is the most reactive halogen? Among the stable alkali metals, it is caesium, C s, near the bottom of group one. And the most reactive halogen is fluorine, right at the top of group seventeen. The two groups run in opposite directions, so do not mix them up. Clear?

5:52Example 1: formulae Al2S3 and Ca3N2 from valences

Now, let's use all this in example one. Predict the formulae of the compounds of aluminium with sulphur, and of calcium with nitrogen. Pause the video and try it first. Done? Okay, let's check. Aluminium is in group thirteen, so its valence is three. Sulphur is in group sixteen, so eighteen minus sixteen, which gives two. Right? Now balance them. Two aluminium atoms give two times three, which is six, and three sulphur atoms take three times two, also six. So the formula is A l two S three. The double arrow means therefore, and the cross means times. Okay? Next, calcium is in group two, so its valence is two. Nitrogen is in group fifteen, so eighteen minus fifteen, which gives three. With me? Three calcium atoms give three times two, which is six, and two nitrogen atoms take two times three, also six. So the formula is C a three N two. Once you know the valences, the formula writes itself.

6:57Example 2: why Pb2+ is more stable than Pb4+

Clear? Now, example two, and this one solves our opening puzzle. Explain why P b two plus compounds are more stable than P b four plus compounds, while for carbon, the plus four state is the stable one. Pause the video and try it first. Done? Okay, let's check. Lead is a heavy element of group fourteen, so this is the inert pair effect. Its outer pair, six s two, is held tightly. Clear? But wait a moment, the sixth shell is far from the nucleus. So why should this pair be held so tightly? The answer is poor shielding. Lead has filled four f and five d orbitals inside, and these electrons shield the nuclear charge poorly. So the six s pair feels a strong pull. Right? Now, to reach plus four, lead must use this pair in bonding. But the energy spent is not paid back by the bonds it would form. So lead keeps the pair, and P b two plus is the more stable state. With me? And what about carbon? It is light, at the top of group fourteen, so the inert pair effect does not hold it back; plus four is its stable state. So our puzzle is solved: same group, but lead is heavy, and its pair stays inert. Got it?

8:17Quick revision: valence, inert pair, diagonal, hydrides

So, quick revision. Valence follows the group number, or eighteen minus it for groups fifteen to seventeen. Heavy elements of groups thirteen to fifteen prefer the lower state. Period two has no d orbitals, so covalency is at most four, and there are three diagonal pairs. Ammonia is the most basic hydride, stibine the best reducer, and H I the strongest acid. And reactivity rises down group one, but falls down group seventeen. Now go and try the practice problems on this page. All the best!