Formation of Ionic Bonds

Transcript

0:00Introduction

Hello everyone! Let's start with a small puzzle. Sodium and chlorine combine to give common salt, sodium chloride, and overall, the process gives out energy. In this process, sodium hands one electron to chlorine, right? But here is the strange part. Moving that electron, on its own, does not give out energy. It costs energy! So if the transfer costs energy, why does salt form at all? Puzzling, isn't it? Today we will find the hidden energy that pays this bill. And in JEE, questions on how ionic bonds form, and why, come up often. So, let's begin.

0:41Ionic bond: transfer, then attraction

So, what exactly is an ionic bond? Take sodium, a metal, and chlorine, a non-metal. One of them gives away an electron, and the other one takes it. Who gives? The metal, sodium, correct! So, an ionic bond, or electrovalent bond as the brackets say, forms by complete transfer of electrons from a metal to a non-metal. The arrow on screen shows that direction. Complete transfer, okay? Not sharing. Let's write it. Sodium, N a, becomes N a plus, plus e minus. The arrow means becomes, the small plus on top of N a means one electron lost, and e minus is that electron. Then chlorine, C l, takes it: C l plus e minus becomes C l minus. Right? Now we have a positive ion and a negative ion, and opposite charges attract. This electrostatic attraction holds the ions together in a crystal lattice. So, two stages: transfer first, then attraction. With me?

1:46When does an ionic bond form easily?

Alright, when does an ionic bond form easily? Start with the metal. It has to lose an electron. Which metal does that happily: one that needs a lot of energy to lose it, or one that needs very little? Very little, yes, exactly! So, condition one: a low ionisation enthalpy of the metal. On screen, that is delta i H: the Greek capital delta, then H, with a small i written below. Clear? Condition two is about the non-metal. When it takes the electron, we want plenty of energy to come out. So its electron gain enthalpy, delta e g H, with a small e g below, should be large and negative. Okay? And condition three, the most important one: a large lattice enthalpy. This is the energy released when the gaseous ions come together to form the solid. Got it? Now underline this line. This last term, the lattice enthalpy, makes the overall process exothermic: it gives out heat. The answer to our puzzle is hiding right here. Right?

2:52Sodium chloride: the real numbers

Back to our puzzle, sodium chloride, now with real numbers. Delta i H of sodium, with sodium in brackets: plus four hundred and ninety six kilojoules per mole, written k J mol to the minus one. Plus means this energy must be put in. With me? And delta e g H of chlorine: minus three hundred and forty nine kilojoules per mole. Minus means energy comes out. So the transfer gives out energy overall, right? No, no, careful! This is the trap. Sodium needs four ninety six, and chlorine gives back only three forty nine. Four ninety six minus three forty nine is one hundred and forty seven. So the transfer alone still needs one hundred and forty seven kilojoules per mole. Then who pays this bill? The lattice enthalpy, yes! When the gaseous ions form the solid crystal, seven hundred and eighty eight kilojoules per mole are released. That more than pays for the one hundred and forty seven. So, puzzle solved: the ionic bond forms only because the lattice enthalpy pays the bill. Clear?

3:59Properties of ionic compounds

Alright, now the properties. Picture a crystal of common salt. Ionic compounds are hard, brittle, crystalline solids, with high melting points. Okay? But wait. Hard, and yet brittle? Why does such a hard crystal split so easily? See, in the lattice, the ions sit plus, minus, plus, minus. Now shift one layer just a little. Suddenly plus sits beside plus, and minus beside minus. Like charges repel, and the crystal splits along that plane. Got it? Next, electricity. Ionic compounds conduct electricity when molten or dissolved, but not as solids. But the solid is full of ions! So why no current? Because in the solid, the ions are held in fixed lattice positions. They cannot move. Melt it, or dissolve it, and the ions are free to move, so they carry the current. With me? And last, ionic compounds usually dissolve in polar solvents. Notice that word, usually. Not always, okay?

5:04Example 1: formula and bond of X (3s2) with Y (2p5)

Now, our first example. Element X has the configuration on screen: N e in square brackets, which means the neon core, then three s two, with the small two on top counting the electrons in that subshell. Element Y has the helium core, then two s two, two p five. Give the formula of their compound, and the type of bond. Pause the video and try it. Done? Okay, let's check. X has the neon core, ten electrons, plus two more: twelve electrons. That is magnesium, M g. Magnesium is a metal, so it loses those two outer electrons. The double arrow means so: X becomes X two plus. Okay? Now Y: the helium core, two electrons, plus seven more: nine. That is fluorine, F, a non-metal. Its outer shell, two s two, two p five, has seven electrons, just one short of an octet. So Y gains one electron and becomes Y minus. With me? So we have X two plus and Y minus. The formula is simply X Y, right? No, no, wait! The charges must balance. X two plus carries two positive charges, and each Y minus carries just one negative charge. So one X two plus needs two Y minus ions. Clear? So, the formula is X Y two, with a small two written below the Y as a subscript. And the bond? Magnesium, a metal, transfers its electrons completely to fluorine, a non-metal. So the bond is ionic. Got it?

6:43Example 2: why MgO is Mg2+ O2- and not Mg+ O-

Now a harder one, JEE level. The first two ionisation enthalpies of magnesium are seven hundred and thirty eight, and one thousand four hundred and fifty one kilojoules per mole. Then why does magnesium oxide, M g O, form as M g two plus, O two minus, as written in brackets, and not as M g plus, O minus? Pause the video and think it through. Done? Okay, let's check. The second electron costs fourteen fifty one, against seven thirty eight for the first. Much more! So M g plus, O minus is the cheaper choice, right? Oh no, careful! You counted only the cost. In our salt puzzle, the lattice enthalpy paid the bill, remember? And lattice enthalpy depends on the product of the ionic charges. Okay? So, multiply the charges. Two plus with two minus, and one plus with one minus. What do you get in each case? Two times two is four, and one times one is one. So the lattice enthalpy of the two plus, two minus solid is about four times as large, for ions of similar size, as the brackets say. Got it? And that extra lattice enthalpy more than repays the extra costs. Not only the second ionisation of magnesium, but also the second electron pushed onto oxygen. O minus already repels an incoming electron, so that step costs energy too. Just like in sodium chloride, the lattice pays the bill. Right? So, the answer: magnesium oxide forms as M g two plus, O two minus, because its lattice enthalpy is much larger. See, the costlier ions win, because the lattice pays back far more. Beautiful, isn't it?

8:29Quick revision: transfer, conditions, lattice enthalpy

So, quick revision. One: an ionic bond is complete electron transfer from a metal to a non-metal, then electrostatic attraction. Two: it is favoured by a low ionisation enthalpy, a large negative electron gain enthalpy, and a large lattice enthalpy. Three: the transfer alone costs energy, and the lattice enthalpy pays the bill. Now go and try the practice problems on this page. All the best!