The Periodic Table: chemical periodicity
A-Level Chemistry Topic 9 8:43 English narration · English + 中文 subtitles burned in
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Sodium and argon sit in the same row of the Periodic Table, with only six elements between them.
钠和氩在元素周期表的同一行里，中间只隔着六种元素。
Drop sodium into cold water and it melts into a shining ball and fizzes across the surface.
把钠丢进冷水，它会熔成一个亮亮的小球， 在水面上嘶嘶地窜动。
Argon does nothing at all — it reacts with nothing.
氩却什么也不做——它和任何物质都不反应。
Yet the change between them is not random. It follows the same regular pattern in every row of the table.
但两者之间的变化并不是随机的：每一行都遵循同样的规律。
That pattern is periodicity, and reading it lets you predict new chemistry.
这个规律叫周期性；读懂了它，你就能预测全新的化学。
This is Topic Nine: chemical periodicity.
这是第九章：化学周期性。
We will cross Period Three, from sodium to argon, twice.
我们要从钠走到氩，把第三周期走两遍。
First the physical properties — size, melting point and conductivity.
第一遍看物理性质——原子大小、熔点和导电性。
Then the chemistry — oxides, chlorides and water.
第二遍看化学——氧化物、氯化物，还有水。
Let's begin.
让我们开始吧。
Start with atomic radius.
先看大小。
Going across Period Three, the atoms get smaller, even though we keep adding electrons.
沿着第三周期走，原子越来越小，尽管我们一直在加电子。
Every new electron joins the same outer shell, so the shielding hardly changes, while the nuclear charge climbs from plus eleven to plus seventeen.
每个新电子都进入同一个外层，屏蔽几乎不变，而核电荷却从正十一升到正十七。
A stronger pull on the same shell gives a smaller atom.
对同一层的吸引更强，原子就更小。
Ions are different. The metals lose their whole outer shell, so their positive ions are small; the non-metals gain electrons, so their negative ions are much bigger.
离子则不同：金属失去整个外层，所以正离子很小； 非金属则得到电子，所以负离子大得多。
Why does that matter?
这有什么用？
Look at the metal end of the row.
看这一行金属的那一端。
In sodium, magnesium and aluminium the outer electrons leave their atoms and roam through the whole structure — a lattice of positive ions in a sea of free electrons.
在钠、镁、铝里，外层电子离开各自的原子， 在整个结构中自由游走——形成正离子的晶格，浸在自由电子的"海"里。
That sea of electrons explains two trends at once.
这片电子海一次解释了两个趋势。
Sodium, magnesium and aluminium form giant metallic structures with high electrical conductivity, and the sea holds the lattice together, so across the metals the melting point rises: each atom gives more electrons to the sea, and the ions get smaller, so the bonding is stronger.
它能载运电荷，所以钠、镁、铝导电都很好； 它又把晶格拉在一起，所以在金属这一段熔点上升：每个原子交出更多电子， 离子也更小，键就更强。
Then silicon — a giant covalent network of strong bonds, so it has the highest melting point of the period and barely conducts.
接着是硅——一整张由强共价键连成的巨网， 所以它的熔点是整个周期里最高的，而且几乎不导电。
After silicon the melting point falls off a cliff.
硅之后，熔点断崖式下跌。
Why the cliff?
为什么会断崖？
Phosphorus, sulfur and chlorine are simple molecular — separate small molecules — so when you melt them the only things you break are the weak van der Waals forces between those molecules.
磷、硫、氯是分子晶体——一个个分开的小分子； 所以熔化时你打断的，只有分子之间那些弱的范德华力。
The strong covalent bonds inside each molecule are never touched.
每个分子内部的强共价键， 根本没有被碰到。
Write that, and the mark is yours.
写出这一点，分就是你的。
The sizes still follow the molecule: sulfur, a ring of eight atoms, holds more electrons than phosphorus with four, so it melts higher.
大小仍然跟着分子走：硫是八个原子的环，电子比四个原子的磷更多，所以熔点更高。
Argon, a single atom, is lowest.
氩是单个原子，最低。
Now the chemistry.
现在讲化学。
These elements burn in oxygen and react with chlorine, and the products are worth learning.
这些元素能在氧气中燃烧，也能与氯气反应，产物值得背下来。
Sodium, magnesium and aluminium each give their oxide; phosphorus gives phosphorus oxide and sulfur gives sulfur dioxide.
钠、镁、铝各自生成自己的氧化物；磷生成十氧化四磷，硫生成二氧化硫。
With chlorine you get sodium chloride, magnesium chloride, aluminium chloride, silicon tetrachloride and phosphorus pentachloride.
与氯气反应，你得到氯化钠、氯化镁、氯化铝、四氯化硅和五氯化磷。
Water is the fussy one: only sodium and magnesium react.
水最挑剔：只有钠和镁会反应。
Sodium reacts fast; magnesium only very slowly.
钠反应很快；镁只在冷水中非常缓慢地反应。
Look at those formulas again and count.
再看一遍这些化学式，数一数。
In the oxides the oxidation number rises steadily across the row: plus one, plus two, plus three, plus four for silicon dioxide, plus five for phosphorus, plus six for sulfur trioxide.
在氧化物里，第三周期元素的氧化数沿着这一行稳步上升： 正一、正二、正三，二氧化硅是正四，磷是正五，三氧化硫是正六。
The chlorides do the same, from plus one up to plus five.
氯化物也一样，从正一一直到正五。
That is no coincidence. The oxidation number is simply the number of valence shell electrons — outer-shell electrons — the atom uses in bonding, and each step across the period gives it one more.
这不是巧合：氧化数就是该原子用于成键的外层电子数目， 而每往右一格，原子就多一个外层电子。
Now drop those oxides into water.
现在把这些氧化物放进水里。
On the left, sodium oxide and magnesium oxide are basic: sodium oxide dissolves to give sodium hydroxide, pH about thirteen, and magnesium oxide is far less soluble, so about nine.
左边，氧化钠和氧化镁是碱性的： 氧化钠溶解生成氢氧化钠，pH 大约十三；氧化镁溶解度小得多，大约九。
In the middle, aluminium oxide and silicon dioxide are insoluble, so the water stays near seven.
中间，氧化铝和二氧化硅都不溶，所以水基本停在七。
On the right, the non-metal oxides are acidic: phosphorus oxide gives phosphoric acid, and sulfur trioxide gives sulfuric acid, so the pH drops to about one.
右边，非金属氧化物是酸性的：十氧化四磷生成磷酸，三氧化硫生成硫酸， pH 降到大约一。
Aluminium oxide sits right at the changeover, and it is amphoteric — it reacts with acids and with alkalis.
氧化铝正好坐在转折点上，它是两性的——既与酸反应，也与碱反应。
With hydrochloric acid it behaves as a base and gives aluminium chloride and water. With sodium hydroxide it behaves as an acid and gives a soluble aluminate.
遇到盐酸，它表现得像碱，生成氯化铝和水；遇到氢氧化钠，它表现得像酸， 生成可溶的铝酸盐。
Its hydroxide behaves in exactly the same way.
它的氢氧化物也是两性的，表现完全一样。
Now the trap: amphoteric does not mean neutral.
现在说陷阱：两性并不等于中性。
Aluminium oxide simply does not react with water, and that is why its pH is seven.
氧化铝只是根本不与水反应，这才是它 pH 为七的原因。
The chlorides split the same way, and the bonding is the reason.
氯化物也分成同样的两类，原因就在于化学键。
When sodium meets chlorine, the sodium atom hands its one outer electron across.
当钠遇到氯， 钠原子把它唯一的外层电子交了过去。
Both atoms reach a full outer shell, and the two ions that are left attract each other strongly.
两个原子都达到满外层， 剩下的两个离子彼此强烈吸引。
Sodium chloride is ionic.
氯化钠是离子化合物。
Sodium chloride and magnesium chloride are ionic.
氯化钠和氯化镁是离子化合物。
In water nothing dramatic happens — they simply dissolve, and the solution is close to neutral.
把它们放进水里，没什么戏剧性的事发生—— 它们只是溶解，溶液接近中性。
Silicon tetrachloride and phosphorus pentachloride are covalent, and behave completely differently. They undergo hydrolysis: they react with the water, fume with hydrogen chloride gas, and leave a strongly acidic solution, pH one or two.
四氯化硅和五氯化磷是共价化合物，表现完全不同： 它们水解——与水反应、冒出氯化氢的白雾，留下强酸性的溶液，pH 只有一到二。
That difference is a test — dissolves means ionic, fumes means covalent.
这个差别本身就是一种检验：溶解就是离子键，冒烟就是共价键。
Step back, and one idea explains all of it: electronegativity.
退一步看，有一个概念可以解释全部：电负性。
On the left the elements are metals with low electronegativity, so the bonding in their oxides and chlorides is ionic and those compounds are basic.
左边的元素是金属，电负性低， 所以它们的氧化物和氯化物是离子键，这些化合物呈碱性。
But on the right they are non-metals with high electronegativity, so the bonding is covalent and the compounds are acidic.
而右边是非金属，电负性高，所以是共价键，化合物呈酸性。
And you can run the argument backwards: a melting point, a conductivity and an effect on water together tell you the bonding.
而且你可以反过来推：熔点、导电性和它对水的作用合在一起，就能告诉你是哪种键。
Let's use it.
我们来用一用。
Predict the pH when sodium oxide, aluminium oxide and sulfur trioxide are each added to water.
预测把氧化钠、氧化铝和三氧化硫分别加入水中时的 pH。
Pause here and try it.
先暂停，自己试一试。
Sodium oxide is a metal oxide, ionic and basic; it gives sodium hydroxide, so the pH is about thirteen.
氧化钠是金属氧化物，离子键，碱性；它溶解生成氢氧化钠， 所以 pH 大约十三。
Sulfur trioxide is a non-metal oxide, covalent and acidic; it gives sulfuric acid, so the pH is about one.
三氧化硫是非金属氧化物，共价键，酸性；它生成硫酸， 所以 pH 大约一。
And aluminium oxide sits at the changeover: amphoteric and insoluble, so the pH stays near seven — not because amphoteric means neutral, but because it does not react with water.
而氧化铝正处在转折点：两性，而且不溶，所以 pH 停在七左右—— 不是因为两性等于中性，而是因为它根本不与水反应。
The same thinking works anywhere in the table, and the exam runs it both ways.
同样的思路在周期表的任何地方都适用，而考试会双向考你。
Forwards: give me a position, and I predict the properties — a Group One element is a soft reactive metal forming a one plus ion and a basic oxide.
正着来：给我位置，我来预测性质——第一族元素是活泼的软金属， 形成正一价离子，氧化物呈碱性。
Backwards: give me the properties, and I deduce the position — low melting point, no conductivity, acidic oxide, and you have a non-metal on the right.
反着来：给我性质，我来找位置—— 熔点低、不导电、氧化物呈酸性，那就是某个周期右边的非金属。
Three marks students throw away.
三个学生常丢的分。
First, never give a trend without its reason: the radius falls because the nuclear charge rises while the shielding stays about the same.
第一，写趋势一定要带原因：半径变小，是因为核电荷增大， 而屏蔽几乎不变。
Second, when the melting point collapses after silicon, name the van der Waals forces between the molecules — saying the covalent bonds are weak throws the mark away.
第二，硅之后熔点骤降时，要点名分子之间的范德华力—— 写成"共价键很弱"，这一分就没了。
Third, for an oxide or a chloride in water, give the products and the resulting pH, not just the word acidic.
第三，氧化物或氯化物与水的题， 要写出产物和 pH，而不是只写一个"酸性"。
Five leftover facts.
还剩五件事实。
First, periodicity means properties repeat in a regular pattern across each period.
第一，周期性是指性质在每个周期上按规则重复。
Second, electrical conductivity is almost zero from silicon onwards.
第二，从硅开始，导电性几乎为零。
Third, S eight melts higher than P four because it is bigger, with more electrons and stronger van der Waals forces.
第三，八硫比四磷熔点高，因为它更大、电子更多、范德华力更强。
Fourth, argon, a single atom, has the lowest melting point of all.
第四，氩是单原子，熔点最低。
Fifth, silicon dioxide is insoluble and only weakly acidic, so the pH stays about seven.
第五，二氧化硅不溶、只弱酸性，所以 pH 大约是七。