Periods, Groups and Blocks

Transcript

0:00Introduction

Hello everyone! Okay, one small puzzle before we start. Helium has just two electrons, both in the one s subshell. So by its electrons, helium belongs in the s block, on the left side of the table, doesn't it? But in every periodic table, helium sits on the far right, in group eighteen. So why is it there? That is strange, isn't it? Today we will read the address of any element, its period, group and block, straight from its electron configuration. This comes up often in JEE, so let's begin.

0:36The long form: 7 periods, 18 groups

So, here is the outline of the long form of the periodic table. It has seven periods, the rows, numbered down the left side, and eighteen groups, the columns, with some of the group numbers, like one, two, thirteen and eighteen, marked along the top. The coloured regions are the blocks. Okay? Now, the number of elements in each period: two, eight, eight, eighteen, eighteen, thirty two, and thirty two. Strange numbers, right? So tell me, why does period one hold only two elements? Because period one fills only the one s subshell, which holds just two electrons. Each new element adds one electron, so two electrons means two elements. Period two fills two s and two p, and two plus six gives eight. Period three fills three s and three p, again eight. With me? Now period four. Here the d subshell joins in: four s, then three d, then four p. Two plus ten plus six gives eighteen. The three d part adds ten elements, and these form the first row of the d block, the wide middle region of the picture. Clear? And period six fills four subshells: six s, four f, five d and six p. Two plus fourteen plus ten plus six gives thirty two. So a period holds as many elements as its subshells hold electrons. Got it?

2:11Period and block from the configuration

Now, the address of an element. Rule number one: the period equals the highest principal quantum number, n, in its electron configuration. Here n is the number written before each subshell letter in the configuration. One exception to remember: palladium, the krypton core then four d ten, has no five s electrons at all, yet it sits in period five. Okay? Rule number two: the block is the subshell that receives the last electron. Last electron in s, the s block; in p, the p block; and the same for d and f. Right? Let's try chlorine, C l, whose atomic number, capital Z, is seventeen. Its configuration is one s two, two s two, two p six, three s two, three p five; the small raised numbers count the electrons in each subshell. So, which period, and which block? The highest n is three, so chlorine is in period three. And the last electron went into three p, so it is in the p block. Two rules, and two parts of the address are already done.

3:20The group number, block by block

Now the third part, the group. Here the rule depends on the block. For the s block, the group is simply the number of outer s electrons, one or two. Okay? Take barium, B a, with Z equal to fifty six. The X e in square brackets is the xenon core, all of xenon's electrons written in short. After it comes only six s two. So barium is in period six, and with two s electrons, in group two. Now the p block. Our chlorine has seven outer electrons, two in three s and five in three p. But it sits in group seventeen, not group seven. So where does the extra ten come from? Look at the picture. The d block takes ten columns, groups three to twelve, so the p block starts only at group thirteen. So for the p block, the group is ten, plus the n s and n p electrons. Okay? So for chlorine, three s two, three p five: two plus five is seven, and ten plus seven gives seventeen. Group seventeen, and that is exactly where chlorine sits. Next, the d block. The group is the number of n minus one d electrons, plus the n s electrons. The n minus one in brackets means the shell just below the outer one, like three d with four s. With me? And the f block, the lanthanoids and actinoids in the strip at the bottom. All of them are placed in group three, even though they are drawn below the table. Got it?

4:53Trap: where does helium go?

Now, back to our puzzle. Helium, H e, is one s two. The highest n is one, so period one, and the last electron went into one s, so by configuration it is in the s block. And by our s block rule, two s electrons means group two, right? Oh no, careful, that is exactly the trap! Helium is placed in group eighteen, with the noble gases. See the small box at the top right of the picture, under the eighteen? That is where helium sits. And the reason? Its properties are those of a noble gas. So here the properties decide the place, not the electron count. That solves our puzzle from the start. Remember this exception, okay?

5:38Example 1: period, group and block of Z = 26 (iron)

Now, example one. Locate the element with atomic number twenty six. Locate means find all three parts of its address: the period, the group and the block. Pause the video and try it first. Done? Okay, let's check. Twenty six electrons: the argon core, A r in square brackets, then three d six, four s two. Okay? Now the period. The d subshell being filled here is three d. So the element is in period three, right? Careful, because that is the trap. The period is the highest n, and here four s is present. So the highest n is four, and the period is four. Now the block. Four s is written last, but the filling order is four s, then three d. So the last electron went into three d: a d block element, the middle of our picture. Clear? Finally the group, with the d block rule: three d electrons plus four s electrons. Six plus two gives eight. So period four, group eight, d block, and that element is iron. Nice and quick, right?

6:52Example 2: locate [Kr]4d10 5s2 5p3 (antimony)

Now a slightly harder one. An element has the configuration K r in square brackets, the krypton core, then four d ten, five s two, five p three. Identify its period, group and block, and the element. Pause the video and try it. Hint: find the last electron. Done? Okay, let's check. Look at that four d ten. Ten d electrons, so this must be a d block element, right? No, wait, that is the trap here. The four d is already full, and the last electron went into five p. So this is a p block element. Now the period. The highest n is five, from five s and five p, so period five. Not four, even though four d is written first. With me? Now the group. Two electrons in five s, and three in five p, so five outer electrons. So, group five, right? Careful again, because group five lies in the d block. For the p block, we add ten: ten plus two plus three gives fifteen. So period five, group fifteen, p block, and the element is antimony. Two traps, and we beat both of them!

8:13Quick revision: period, block and group rules, helium

So, quick revision. The period is the highest n, and the block is the subshell of the last electron. For the group: s block, the n s electrons; p block, ten plus n s plus n p; d block, n minus one d plus n s; and the f block, group three. And helium, one s two, is placed in group eighteen, with the noble gases. Now go and try the practice problems on this page. All the best!