Mixed Problems: Set 1

Transcript

0:00Introduction

Hello everyone! Okay, a small puzzle before we start. Fluorine is the most electronegative element of all, and it sits right at the top of group seventeen. So which element gives out the most energy when it gains an electron? Surely it is fluorine, right? Hmm, hold that thought. Today we solve eight quick questions, mixed from the whole unit, and this puzzle is one of them. Questions like these come up often in JEE Main, so let's begin.

0:33Round 1: size down a group

Round one is about size. On the right is a graph of atomic radius in picometres, written p m, against the position in the period, for periods two and three. Potassium and calcium, from period four, sit on top. Now the rule: going down a group, a new shell is added at each step, so the radius increases. Okay? So here is question one. Arrange lithium, sodium and potassium in increasing atomic radius. All three are in group one, so what is the order? Lithium, then sodium, then potassium, yes, exactly! Each less than sign means smaller than. On the graph, the first point of the bottom line is lithium, the first point of the middle line is sodium, and potassium is right on top. Clear?

1:19Round 2: count the electrons

Now, round two, and we count electrons. Species with the same number of electrons are called isoelectronic. And among isoelectronic species, a higher nuclear charge, capital Z, means a smaller radius. Okay? Now question three: name four species that are isoelectronic with argon. Think of the ions of argon's neighbours in the table. Which four are they? Let's count. Argon has atomic number, capital Z, equal to eighteen, so a neutral argon atom has eighteen electrons. So we want ions with eighteen electrons. Right? Sulphur has sixteen electrons; it gains two, and S two minus has eighteen. The two minus, written up top, is the charge, and the brackets show the count. Chlorine, seventeen plus one, so C l minus has eighteen. Potassium, nineteen minus one, so K plus has eighteen. Calcium, twenty minus two, so C a two plus has eighteen. These four ions are all isoelectronic with argon. Got it? Now question seven. Which is the largest: F minus, C l minus, B r minus, or I minus? You might think: iodine has the highest nuclear charge, so I minus is the smallest, right? Oh no, that is the trap, so be careful here! The rule, higher nuclear charge means smaller, is for isoelectronic species, and these four ions have different numbers of electrons. So we use the group trend: each step down group seventeen adds one more shell. So the largest is I minus, the iodide ion. With me?

2:58Round 3: an exception in ionisation enthalpy

Round three brings an exception. The graph on the right shows delta i H one, the first ionisation enthalpy, against the atomic number, for the first twenty elements; the little triangle is the Greek capital delta, and the small i and one written below mean ionisation of the first electron. Question two: which is higher, magnesium or aluminium? Aluminium is further right, and ionisation enthalpy generally increases across a period, so aluminium, right? Oh no, look at the graph, because it dips! Find atomic number twelve, magnesium, and thirteen, aluminium, and the line goes down. And why does it dip? Magnesium is three s two, a full three s subshell; the raised numbers count the electrons. Aluminium is three s two, three p one, with a lone electron in three p. Okay? Aluminium loses that three p electron. It is shielded by the three s pair, and it lies higher in energy than a penetrating three s electron, so it is easier to remove. Clear? So magnesium is higher: seven hundred and thirty eight, against five hundred and seventy seven kilojoules per mole; the minus one power on mol means per mole. Order questions often test exceptions like this one. Right?

4:15Round 4: a name and an oxide

Round four is a quick one. Question five: give the systematic name of element one hundred and six. Each digit gets a root, and the arrows mean becomes: one becomes un, zero becomes nil, and six becomes hex. Then we add the ending, i u m. So what is the name? Unnilhexium, that is correct! And the symbol is made of the first letters of the roots: U n h, so capital U, small n, small h. Got it? Question six: which is amphoteric, N a two O, Z n O, S O three, or C O? The small numbers written below count the atoms. Amphoteric means it reacts with acids and with bases. So which one is it? Zinc oxide, yes, exactly! Sodium oxide is basic. Sulphur trioxide is acidic, and carbon monoxide is neutral, neither acidic nor basic. Zinc oxide is one of the amphoteric oxides you simply have to remember. With me?

5:16Example 1: most negative electron gain enthalpy, Cl vs F

Now example one, and it is our puzzle from the start. Which element has the most negative electron gain enthalpy? Pause the video and try it first. Done? Okay, let's check. Delta e g H, with e g written small below, is the enthalpy change when a gas atom gains an electron. Negative means energy released, so most negative means most energy released. Now, it becomes less negative down a group, so the top of group seventeen, fluorine, right? Oh no, careful here, and look at the numbers on screen. Fluorine is minus three hundred and twenty eight, but chlorine is minus three hundred and forty nine kilojoules per mole. So chlorine is more negative than fluorine. Clear? So why is that? Fluorine is tiny, so its two p shell is crowded, and the incoming electron is repelled. The three p shell of chlorine is larger, so the new electron is repelled less. Okay? So the most negative electron gain enthalpy belongs to chlorine. And that solves our puzzle: fluorine is the most electronegative, but chlorine releases the most energy on gaining an electron. Got it?

6:29Example 2: block, period and group of Z = 29 (copper)

Now the harder one, example two. Locate capital Z equals twenty nine, the atomic number: give its block, its period and its group. On the right is the outline of the periodic table, with the s, p, d and f blocks. Pause the video and try it. Done? Okay, let's check. Element twenty nine is copper, C u, and copper is an exception: the argon core, A r in square brackets, then three d ten, four s one. Why not three d nine, four s two? Because, as the round bracket on screen says, a completely filled three d subshell has extra stability. Okay? Step one is the period. The highest principal quantum number, n, is four, so period four. Now, the configuration ends in four s one, a single s electron. So it goes in the s block, group one, with sodium and potassium, right? No, no, wait, that is exactly the trap. By the Aufbau order, the last electrons go into the three d subshell, so copper is a d block element. On the outline, that is the middle part of the table. Clear? And for the d block, the group is the number of n minus one d electrons, plus the n s electrons. Here that is ten plus one, so eleven. With me? So copper is in the d block, period four, group eleven. Remember, in the d block, always count the d electrons too. Right?

7:56Quick revision: size, Mg vs Al, Cl vs F, locating elements

So, quick revision. Size grows down a group, and among isoelectronic species, higher nuclear charge means smaller; for ions with different electron counts, like the halide ions, use the group trend. Exceptions: magnesium is above aluminium in first ionisation enthalpy, and chlorine is more negative than fluorine in electron gain enthalpy. To locate an element, the period is the highest n, and a d block group counts the n minus one d plus n s electrons. Now go and try the practice problems on this page. All the best!