Metallic Character and the Nature of Oxides

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Sodium oxide and sulphur trioxide are both just an element joined to oxygen, right? Yet sodium oxide is basic, and sulphur trioxide is acidic. And aluminium oxide, which sits between them, can behave both ways! Same oxygen every time, so what decides it? The answer is the element itself, and where it sits in the periodic table. Questions on this come up often in JEE, so let's begin.

0:32Metallic character

So, first the idea behind it all, metallic character. It simply means how easily an atom loses electrons; the easier it loses them, the more metallic the element. Now take any period and move from left to right. Tell me, does metallic character rise or fall? That is correct, it falls. Metallic character decreases across a period, so the elements on the left are the most metallic, and those on the right are the least. Okay? And down a group, it does the opposite: metallic character increases. Lower down a group, atoms lose electrons more easily. Right? And non-metallic character simply does the opposite of both. It increases across a period, and decreases down a group. Got it?

1:19Oxides across a period

Now let's see what this does to the oxides, walking across period three. We begin with sodium, a metal at the far left. So will its oxide be basic, or acidic? Yes, exactly, the oxide is basic! Sodium oxide, N A two O, is basic, and so is magnesium oxide, MGO, okay? The small two written below the symbol is a subscript; it counts the atoms, so two sodium atoms for one oxygen. Clear? Next comes aluminium, and aluminium oxide, A L two O three, is amphoteric. Amphoteric means it reacts with acids and with bases, both. With me? Then the non-metals on the right. Silicon dioxide, SIO two; then P four O ten, the oxide of phosphorus; then sulphur trioxide, S O three; and C L two O seven, the oxide of chlorine. All four are acidic, and increasingly so, with the chlorine oxide the most acidic. Right? So here is the whole period in one line: basic, then amphoteric, then acidic, from left to right; the arrows on screen mean changes to. As metallic character falls, the oxides turn acidic. Okay?

2:35Oxides down a group

Now let us go down a group instead. Remember, metallic character increases down a group. So what should happen to the oxides: do they become more basic, or more acidic? Let us check with group fifteen. At the top is nitrogen, a non-metal, and its oxide, N two O three, is acidic. Lower down is antimony, written S B, and S B two O three is amphoteric. And near the bottom is bismuth, written B I, and B I two O three is basic. With me? Yes, exactly, down a group the oxides become more basic. It is the same story as before: more metallic character, more basic oxide.

3:17Oxides to remember

Clear? Now, some oxides you simply have to remember. The common amphoteric ones: beryllium oxide, BEO; aluminium oxide again; gallium oxide, G A two O three; and zinc oxide, ZNO, okay? Then come the symbols S N for tin and P B for lead, so watch them carefully. Both oxides of tin, SNO and SNO two, and both oxides of lead, PBO and PBO two, are amphoteric. Right? And three more: arsenic oxide, A S two O three; the antimony oxide we just met; and chromium oxide, C R two O three. Got it? Now, a tricky one. Carbon is a non-metal, and we just saw that the oxides of non-metals are acidic. So carbon monoxide, C O, must be acidic too, right? No, no, careful, because carbon monoxide is a neutral oxide, neither acidic nor basic. Along with it, nitric oxide, N O; nitrous oxide, N two O; and water, H two O, are neutral too. Remember these four, because they are a common trap. Clear?

4:29One element, more than one oxide

Now, one last rule. Some elements form more than one oxide, okay? For one element, a higher oxidation state gives a more acidic oxide; the double arrow on screen means leads to. Take vanadium, written V, and its oxide V two O three. What is the oxidation state of vanadium here? Each oxygen is minus two, so three oxygens give minus six. Two vanadium atoms share plus six, so each one is plus three. And V two O three is basic. Right? Now V two O five: five oxygens give minus ten, so each vanadium is plus five. Higher state, so more acidic: it is amphoteric, but mainly acidic, as the brackets say. Got it?

5:15Example 1: classify Na2O, ZnO, CO, SO2, As2O3 and N2O

Now example one, and it is a nice easy one. Classify sodium oxide, zinc oxide, carbon monoxide, S O two, A S two O three and N two O as acidic, basic, amphoteric or neutral. Pause the video and try it first. Done? Okay, let's check. Sodium is a metal at the far left, so sodium oxide is basic. And S O two is an oxide of sulphur, a non-metal on the right, so it is acidic, just like S O three. Okay? Zinc oxide and A S two O three, the arsenic oxide, are both on our list of amphoteric oxides. With me? Two are left: carbon monoxide, and N two O, an oxide of nitrogen. Now, N two O three, another oxide of nitrogen, was acidic. So N two O must be acidic too, right? Careful, because that is exactly the trap! N two O is on the neutral list, along with carbon monoxide. So both of them are neutral, neither acidic nor basic. Clear? So, in the order the question asks: basic, amphoteric, neutral, acidic, amphoteric, neutral. Six oxides, and all you needed was the trend and two short lists. Got it?

6:36Example 2: rank CrO, Cr2O3 and CrO3 from basic to acidic

Now example two, a little harder, and it takes our puzzle one step further, because here the element is the same every time. Arrange CRO, C R two O three and CRO three from most basic to most acidic, and explain why. Pause the video and try it first. Done? Okay, let's check. Chromium, written C R, is a metal, and metals give basic oxides. So all three oxides must be basic, right? No, no, wait, that is the trap! For one element, the oxidation state decides. In CRO, one oxygen is minus two, so chromium is plus two. In C R two O three, it is plus three, as with vanadium. And in CRO three, three oxygens give minus six, so chromium is plus six. Okay? Now the reason. As the oxidation state rises, the chromium ion polarises the oxide ion more; it pulls the electron cloud of the oxide ion towards itself. With me? So the oxide becomes more covalent, and more acidic. That is the explanation the question asks for. Right? So the answer: CRO, basic, is greater than C R two O three, amphoteric, which is greater than CRO three, acidic; here the greater than sign means more basic than. Plus two is basic, plus three is amphoteric, and plus six is acidic. Got it?

8:09Quick revision: metallic character, oxide trends, neutrals

So, quick revision. Metallic character falls across a period and rises down a group. Oxides go from basic, through amphoteric, to acidic across a period, and become more basic down a group. C O, N O, N two O and water are neutral, and for one element, a higher oxidation state means a more acidic oxide. You have done really well today. Now go and try the practice problems on this page. All the best!