Position and Electronic Configuration

Transcript

0:00Introduction

Hello everyone! Okay, here is a small puzzle to start. Zinc sits right inside the d-block of the periodic table, and yet zinc is not counted as a transition element. And copper, whose atom has a completely filled three d subshell, is counted as a transition element. So how can that be? By the end of this video, you will see exactly why. This comes up often in JEE, so let's begin.

0:27Where the d-block sits

So, first, where does the d-block sit? It fills groups three to twelve of the periodic table, right between the s-block and the p-block. Okay? It has four series. The three d series runs from scandium to zinc, and the four d series runs from yttrium to cadmium. With me? Then the five d series: lanthanum, and then hafnium to mercury. And the six d series: actinium, and then rutherfordium to copernicium. Right? Now, the general configuration: n minus one in brackets, d one to ten, then n s one to two. Here n is the outermost shell, so n minus one is the shell just inside it. The small raised numbers count electrons: one to ten in d, and one or two in s. Clear? And remember the odd one out: palladium is four d ten, five s zero. Ten electrons in four d, and not a single electron in five s. Got it?

1:27What makes an element a transition element?

Now, the key definition. So, a transition element has an incompletely filled d subshell, either in its ground state, that is the atom itself, or in one of its common oxidation states. Two chances, okay? Now, back to our puzzle. Zinc is three d ten, four s two, and its common ion, zinc two plus, is three d ten. The raised two plus is the charge on the ion. So tell me, is zinc a transition element? That is right, zinc is not a transition element. Zinc, cadmium and mercury are d ten, both in the atom and in the plus two state. The double arrow on screen means therefore: no incomplete d subshell, so not transition elements. Right? Now the second half of the puzzle. Copper is three d ten, four s one, a completely filled three d subshell in the atom. Yet copper, silver and gold are all transition elements. So, why is that? Think about the definition. Yes, exactly, look at their common ions! Copper two plus is three d nine. Silver two plus, in A g F two, where the small two below means two fluorine atoms, is four d nine. And gold three plus is five d eight. These are incomplete d subshells, so all three are transition elements, and our puzzle is solved.

2:55The 3d series and its two exceptions

Okay? Alright, now let us write the whole three d series. Scandium is three d one, four s two. Titanium, three d two, four s two. Vanadium, three d three, four s two. Each element adds one more three d electron. See the pattern? So, next comes chromium. Following this pattern, what should chromium be? Did you say three d four, four s two? That is what the pattern says, but careful, it is exactly the trap. Chromium is actually three d five, four s one, not three d four, four s two. That is a half-filled d subshell, with five d electrons. Okay? Then the pattern comes back. Manganese, three d five, four s two. Iron, three d six, four s two. Cobalt, three d seven, four s two. And nickel, three d eight, four s two. With me? And at copper it breaks again. The pattern says three d nine, four s two, but copper is three d ten, four s one, a completely filled d subshell. Then zinc is three d ten, four s two, and the series is complete. Clear? But why do chromium and copper break the pattern? First, the three d and four s energies are very close. Second, a half-filled d five and a completely filled d ten subshell are extra stable, because of exchange energy. So moving one electron is worth it. Got it? And the heavier series have more exceptions. Molybdenum is four d five, five s one. Silver, four d ten, five s one. Platinum, five d nine, six s one. And gold, five d ten, six s one. Just know them, and keep chromium and copper at your fingertips. Okay?

4:49Ions: the outer s electrons leave first

Now, let us form some ions. Iron is three d six, four s two. When iron forms iron two plus, it loses two electrons. Which two electrons leave, three d or four s? The rule is simple: the n s electrons are removed first. For iron, n s means four s, the outer s subshell. Okay? So iron loses both four s electrons, and iron two plus is three d six. The arrow means the atom forms the ion. Writing three d four, four s two, taking electrons from three d first, is the classic trap. Right? The same rule gives the rest. Iron three plus is three d five, manganese two plus is three d five, and copper one plus is three d ten. With me?

5:40Example 1: configurations of Cr, Cu, Cr3+ and Cu2+

Alright, time for example one, an easy one. Write the configurations of chromium, copper, chromium three plus and copper two plus. Pause the video and try it first. Done? Okay, let's check. Chromium is argon, three d five, four s one, and copper is argon, three d ten, four s one. Argon in square brackets stands for the argon core of eighteen electrons. Both atoms have just one four s electron, our two exceptions. Okay? Now chromium three plus. It loses three electrons. So, from where do they come? The four s electron goes first, and then two electrons from three d. Five d electrons minus two leaves three, so chromium three plus is argon, three d three. Right? Copper two plus loses two electrons: the four s electron, and then one from three d. Ten minus one leaves nine, so copper two plus is argon, three d nine. And see, this incomplete three d nine is exactly why copper is a transition element, our puzzle from the start!

6:50Example 2: same configuration among Fe3+, Mn2+, Cr+, Co3+

Now a slightly harder one. Which of these ions have the same configuration: iron three plus, manganese two plus, chromium one plus and cobalt three plus? Pause the video and try it. Here is a hint: remove the four s electrons first. Done? Okay, let's check. Iron, three d six, four s two, loses both four s electrons and one three d electron, giving three d five. Manganese, three d five, four s two, loses its two four s electrons, giving three d five again. Clear? Now chromium one plus, and here is the trap. Remember, chromium is three d five, four s one, the exception. So what is chromium one plus? Yes, chromium one plus is three d five! It loses its single four s electron and keeps all five d electrons. Starting from three d four, four s two would have given you a wrong answer. Okay? Last, cobalt is three d seven, four s two. It loses two four s electrons and one three d electron, so cobalt three plus is three d six, the odd one out. Right? So, the answer is iron three plus, manganese two plus and chromium one plus, all three d five. Three different elements, and the same configuration. Nice, isn't it?

8:17Quick revision: definition, Cr and Cu exceptions, ion rule

So, a quick revision. First, a transition element has an incomplete d subshell in the atom or in a common oxidation state. Second, the exceptions: chromium is three d five, four s one, half-filled, and copper is three d ten, four s one, completely filled. Third, ions lose their n s electrons first. Now go and try the practice problems on this page. All the best!