Group 0: the noble gases

Hydrogen lifts more than helium, and it costs far less to make. So why do airships today use helium? Not because helium is lighter: hydrogen is the lightest gas there is. In nineteen thirty-seven the Hindenburg, an airship full of hydrogen, caught fire and burned out in about half a minute. What helium does not do turned out to be worth more than what it does.

This is the seventh video of nine in The periodic table. All you need behind you is The periodic table: groups, periods and position, where reading a shell is taught.

Group zero is the far right-hand column, and its elements are called the noble gases. All of them, from helium down to radon, are colourless gases at room temperature. All of this is for Foundation and Higher alike. Count the electrons into neon, two, eight. Two fill the first shell. Eight fill the second, and that shell can hold no more. The outer shell is full. With a full outer shell there is nothing to gain, nothing to lose and nothing to share, so the noble gases are unreactive. Helium needs saying plainly. It has only one shell, and that shell is full at two electrons, not eight. Full means as many as that shell can hold. Because they do not react, not even with each other, the noble gases exist as single atoms: H e, N e, A r. Most other gases you have met, like oxygen, come in pairs. Argon is two, eight, eight. Is its outer shell empty, half full, or full? Full, with eight electrons. Argon has nothing to gain, lose or share, which is why it stays unreactive. One examiner's report on a Higher paper discusses an answer about xenon: "The candidate has correctly identified that xenon does not need to gain or lose electrons but incorrectly states that this is due to an empty outer electron shell. This is a common misconception about the elements in group 0." Full, not empty. Make it a habit to say full whenever you explain a noble gas, and then say what full buys: nothing to gain, lose or share. Here is one to try, with the trap written into the question itself. Why does krypton not form compounds easily? Explain without the word empty. Krypton's outer shell is full. It has no need to gain, lose or share electrons, so it does not easily form compounds. Now, which forms compounds readily: Group seven or Group zero, and why? Group seven, the halogens. They are one electron short of a full shell, so they gain one readily, while the noble gases are already full. One examiner's report on a Higher paper notes: "Almost all candidates knew Group 7 were called the halogens; the noble gases were the most common incorrect answer." The habit that keeps them apart: Group seven, one short of full; Group zero, already full.

The uses follow from the properties. One examiner's report on a Foundation paper lists "Group 0 gases" among the topics that were "particularly well answered", and this is where that knowledge gets used. Helium fills balloons and airships for two reasons. It is less dense than air, so it lifts, and it is unreactive, so it cannot burn. Argon is blown around hot metal during welding. Being unreactive, it keeps oxygen away, so the hot metal does not react with the air. Neon fills advertising signs, where it glows when electricity passes through it. A lamp's filament glows white-hot. Why fill the bulb with argon rather than air? Air contains oxygen, which would react with the hot filament and burn it through. Argon is unreactive, so the filament lasts.

Now a second, separate idea. Going down Group zero, the boiling point and the density both increase. The atoms get bigger, and bigger atoms have stronger forces of attraction between them. More energy is needed to pull them apart, so the boiling point rises. Keep this reason apart from the first one. Why they do not react is about the outer shell. Why they boil where they do is about the forces between atoms. Their boiling points are all below zero. Neon boils at two hundred and forty-six degrees below zero, and argon at one hundred and eighty-six below. Which is higher: minus two hundred and forty-six, or minus one hundred and eighty-six? Minus one hundred and eighty-six. On a thermometer it sits nearer to zero, which is higher up. Here is a table of the kind a question gives you. Helium, minus two hundred and sixty-nine. Neon, minus two hundred and forty-six. Argon, minus one hundred and eighty-six. Krypton is missing. Is the krypton value higher or lower than the argon value, and why? Higher, so a smaller number below zero. Krypton sits below argon, and boiling points rise down the group. The reason is the forces between atoms. Krypton's atoms are bigger, so the attraction between them is stronger and more energy is needed to separate them. One examiner's report on a Higher paper, about a table of boiling points down a group, says "some were evidently confused by the negative values in the table and stated the opposite trend." It adds that "many students then confused boiling point with reactivity and tried to explain the trend using forces within the atom rather than forces between molecules." The way to avoid both is to place negative numbers on a thermometer before comparing them, and to explain a boiling point only with forces between particles, never with reactivity.

The word full answers more than one of these last questions. Xenon sits below krypton. Higher or lower boiling point, and why? Higher. Xenon's atoms are bigger, so the forces between them are stronger. Now, radon is at the bottom of Group zero. Will it form compounds easily? No. Its outer shell is full, so there is nothing to gain, lose or share. Here's another: helium has only two outer electrons. Why is its shell still full? It has only one shell, and the first shell is full at two. One more: what two properties make helium right for an airship? It is less dense than air, so it lifts. And it is unreactive, so it cannot catch fire. That is what the airship builders were paying extra for: the thing helium does not do.

Seventh of the nine in The periodic table is this one, Group 0: the noble gases. If this one's yours now, the thumbs up marks it done, so you skip it next time. Still shaky? Save it, and come back to it later in the week.

Next in the chapter: Predicting an element's reactions from its position.

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