Compound Structure and Properties
AP Chemistry Topic 2 8:45 English narration · English + 中文 subtitles burned in
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Three solids on a bench. Table salt. A diamond. A piece of copper. Hit the salt with a hammer and it shatters. The diamond scratches almost anything. The copper just bends, and it carries electricity. So why are they so different?
台面上有三种固体：食盐、一颗钻石、一块铜。 用锤子敲食盐，它会碎裂； 钻石几乎能划伤任何东西；而铜只是弯曲，还能导电。 它们为什么差别这么大？
The answer is the chemical bond.
答案就是化学键。
Six steps today: what the three kinds of bond really are; how bond length and bond energy live on one curve; ionic solids; metals and alloys; Lewis diagrams and resonance; and molecular shape.
今天分六步：三类化学键到底是什么； 键长和键能如何体现在同一条曲线上；离子固体；金属与合金； 路易斯结构与共振；以及分子的形状。
Electronegativity is how strongly an atom pulls the electrons in a bond.
电负性是指一个原子把键中电子拉向自己的强弱程度。
Take two atoms and look at the difference.
取两个原子，看它们的差值。
When the difference is small, they share almost equally.
当差值很小时，它们几乎是平均共用电子。
When the difference is bigger, the sharing is uneven, and one end goes slightly negative.
当差值变大时，共用就不均匀了，一端会略带负电。
When the difference is very large, the electron moves across and you get ions.
当差值非常大时， 电子干脆转移过去，于是形成离子。
There is no sharp line: it is one smooth scale.
这中间没有明确的分界线： 它是一条连续的刻度。
Start at the far end of that scale.
我们从这条刻度的最远端开始。
Sodium has one electron in its outer shell; chlorine has seven and needs one more.
钠的最外层只有一个电子； 氯有七个，还差一个。
So sodium hands its electron over.
于是钠把这个电子交了出去。
Both shells are now full, but neither atom is neutral. Sodium is a positive ion, chlorine a negative ion, and they attract.
现在两者的外层都填满了，但谁都不再是中性的： 钠成了正离子，氯成了负离子，它们相互吸引。
Now zoom into one covalent bond.
现在放大看一根共价键。
This graph plots potential energy against the distance between two atoms.
这张图画的是势能随两个原子间距离的变化。
Far apart, they barely feel each other. Closer, attraction pulls the energy down. Too close, the nuclei repel and it climbs steeply.
离得很远时，它们几乎感觉不到对方；靠近一些，吸引力把势能拉低； 靠得太近，原子核互相排斥，势能陡然升高。
The bottom of the dip is where the atoms sit.
势能最低处就是原子实际所处的位置。
Read across for the bond length. Read down for the bond energy.
横着读，得到键长；竖着读，得到键能。
Now change the number of shared pairs.
现在改变共用电子对的数目。
A single bond, one shared pair, sits in the long shallow well.
单键只有一对共用电子， 对应又长又浅的那口势阱。
A double bond pulls the atoms closer and digs deeper.
双键把两个原子拉得更近，势阱也挖得更深。
A triple bond is shortest and deepest of all.
三键则是最短、最深的一个。
So higher bond order means shorter and stronger, and the deeper the well, the more energy it takes to break.
所以键级越高，键就越短越强； 而势阱越深，把它打断所需的能量就越多。
An ionic bond is never just one pair.
离子键从来不只是一对离子。
Every positive ion pulls on every negative ion nearby, so they stack into a giant repeating three-dimensional lattice.
每一个正离子都在吸引周围每一个负离子， 于是它们堆叠成一个不断重复的三维巨大晶格。
Each ion is surrounded by ions of opposite charge, so the attractions are as large as possible.
每个离子周围都是带相反电荷的离子，这样吸引力就尽可能地大。
So ionic solids melt very high, shatter rather than bend, and conduct only when molten or dissolved.
所以离子固体熔点很高，受力时碎裂而不弯曲，而且只有熔化或溶解后才导电。
How strong is that lattice?
这样的晶格有多强？
Coulomb's law gives two rules.
库仑定律给出两条规则。
First, the charges: bigger charges pull harder.
第一是电荷：电荷越大，吸引越强。
Second, the distance: smaller ions sit closer, so they pull harder.
第二是距离：离子越小，靠得越近，吸引就越强。
Compare sodium chloride with magnesium oxide.
拿氯化钠和氧化镁比一比。
Two plus and two minus is four times the charge, and both ions are smaller.
二正和二负，电荷的乘积是四倍，而且两个离子都更小。
Look at the melting points: eight hundred degrees against two thousand eight hundred.
看看熔点：一个大约八百度，另一个是两千八百度。
A metallic bond does something different.
金属键的做法不一样。
Each metal atom gives up its outer electrons.
每个金属原子交出最外层的电子。
Those electrons are delocalized: free to move through the whole metal.
这些电子是离域的：可以在整块金属中自由移动。
What is left is a lattice of positive ions in a sea of electrons.
剩下的是一群正离子排成的晶格，浸在电子的海里。
The moving electrons carry current and heat, and the sea holds sliding layers together, so a metal bends instead of breaking.
移动的电子既传导电流也传导热量；而电子海把滑动的层连在一起， 所以金属会弯曲而不是断裂。
Now mix in a second element.
现在掺进第二种元素。
In the pure metal on the left, every atom is the same size, so whole layers slide easily and pure metals are soft.
左边是纯金属，每个原子大小相同， 整层可以轻易滑过去，所以纯金属较软。
On the right, atoms of a different size sit in the lattice.
右边，大小不同的原子占据了晶格中的位置。
The rows are not smooth, the layers catch, and the alloy is harder.
排列不再整齐，层与层卡住，合金因此更硬。
Similar-sized atoms swapping places is substitutional, like brass; small atoms in the gaps is interstitial, like steel.
大小相近的原子互相替换，叫做置换型，比如黄铜； 小原子挤进空隙里，叫做间隙型，比如钢。
Non-metals do not transfer electrons; they share them, and Lewis structures — Lewis diagrams — show how.
非金属不转移电子，而是共用电子，路易斯结构正好把这一点画了出来。
Look at water on the left. Oxygen shares one pair with each hydrogen: two bonding pairs. It keeps two more pairs to itself, and those are lone pairs.
看左边的水：氧和每个氢各共用一对电子，这就是两对成键电子对； 它自己还留着两对，这就是孤对电子。
Now nitrogen on the right. Each atom needs three electrons, so they share three pairs at once: a triple bond.
再看右边的氮： 每个原子还差三个电子，于是一次共用三对，形成三键。
The aim is eight electrons around each atom.
目标是让每个原子周围都有八个电子。
Let's build one properly: carbon dioxide.
我们来认真画一个：二氧化碳。
Step one, count every valence electron.
第一步，数清所有价电子。
Carbon brings four, each oxygen six, so sixteen.
碳提供四个，每个氧提供六个，一共十六个。
Step two, put the least electronegative atom in the centre, carbon, with one bond to each oxygen.
第二步， 把电负性最小的原子放在中心，也就是碳，和每个氧各连一根单键。
That uses four; twelve are left.
这用掉四个，还剩十二个。
Step three, give the outer atoms a full octet.
第三步，让外围原子满足八隅体。
The twelve go on the oxygens, but carbon has only four.
这十二个都放到两个氧上，可是碳身边只有四个。
Step four, pull one lone pair from each oxygen in, making two double bonds.
第四步，从每个氧上各拉一对孤对电子过来，形成两根双键。
Check the answer: every atom has eight, and all sixteen are used.
检验一下：每个原子都是八个电子，十六个也刚好用完。
Some ions need more than one diagram.
有些离子没法只用一张图画出来。
Take the nitrate ion.
就拿硝酸根来说。
Formal charge tells you whether a drawing is good: valence electrons, minus lone-pair electrons, minus half the bonding electrons.
形式电荷可以判断一张图画得好不好：价电子数，减去孤对电子数， 再减去成键电子数的一半。
Start with the nitrogen: five, minus zero, minus four gives plus one.
先看氮：五，减零，减四，得到正一。
Now the oxygens: the double-bonded one gives zero, each single-bonded one gives minus one.
再看氧：双键那个得到零，每个单键氧得到负一。
Add the total: minus one, exactly the ion's charge.
加起来是负一，恰好等于这个离子的电荷。
And since the three oxygens are identical, the double bond could sit on any of them.
而既然三个氧完全一样，那根双键放在哪一个上都可以。
That is resonance.
这就是共振。
We can draw the electrons.
电子我们已经会画了。
Now, what shape does the molecule take?
那么，分子会呈现什么形状？
VSEPR is the rule: groups of electrons around the central atom all repel, so they spread as far apart as they can.
规则很简单： 中心原子周围的各组电子彼此排斥，于是尽可能地散得最开。
Two groups point opposite ways, a straight line. Three make a flat triangle. Four lift into three dimensions.
两组指向相反的方向，成一条直线；三组构成一个平面三角形； 四组则撑到三维空间里。
Here are the shapes you must know, with their angles.
这些就是你必须记住的形状，以及它们的键角。
But there is a catch, the most common mistake in this unit.
但这里有个陷阱，是本单元最常见的错误。
Count the electron groups, then count how many are lone pairs.
先数电子组的数目， 再数其中有几组是孤对电子。
Methane has four groups and no lone pairs: a tetrahedron near one hundred and nine degrees.
甲烷有四组，没有孤对电子： 是四面体，键角接近一百零九度。
Ammonia also has four groups, but one is a lone pair.
氨也有四组，但其中一组是孤对电子。
We name only the atoms we can see, so it is trigonal pyramidal, and the angle drops to one hundred and seven.
我们只给看得见的原子命名，所以它是三角锥形，键角降到一百零七度。
Hybridization is the same geometry, seen from the orbitals.
杂化就是同一套几何形状，只不过换成从轨道的角度来看。
Four single bonds mix into four identical orbitals pointing at a tetrahedron's corners: that is s p three.
四根单键会混成四个完全相同的轨道，指向四面体的四个顶点， 这就是 s p 三。
Three groups give s p two, a flat triangle. Two give s p, a straight line.
三组给出 s p 二，是一个平面三角形； 两组给出 s p，是一条直线。
So count the groups and the hybridization follows.
所以数清电子组，杂化方式也就定了。
But look at the middle molecule: that double bond is not two of the same.
不过看看中间那个分子：那根双键并不是同一种键的两份。
Orbitals can overlap in two ways.
轨道重叠有两种方式。
Head-on, along the line joining the atoms: that is a sigma bond, and every single bond is one.
沿着两个原子连线正面相对地重叠，这就是西格玛键，每一根单键都是它。
Sideways, meeting above and below that line: that is a pi bond.
在这条连线的上方和下方侧面相对地重叠，这就是派键。
So count them like this. A single bond is one sigma; a double is one sigma plus one pi; a triple is one sigma plus two pi.
所以可以这样数：单键是一根西格玛键；双键是一根西格玛键加一根派键； 三键是一根西格玛键加两根派键。
And a pi bond locks the two atoms so they cannot rotate about the bond.
而派键还有一个本事： 派键会把两个原子锁住，不能绕键旋转。
That is why a double bond between two carbons has fixed cis and trans forms — geometric isomers.
所以两个碳之间的双键有固定的顺式和反式——几何异构体。
One last question: is the whole molecule polar?
最后一个问题：整个分子是极性的吗？
Carbon dioxide has two polar bonds, because oxygen pulls the electrons harder than carbon.
二氧化碳有两根极性键， 因为氧把电子拉得比碳更强。
But the molecule is straight, so the two pulls are equal and exactly opposite.
但这个分子是直的， 所以两个拉力大小相等、方向正好相反。
Carbon dioxide is non-polar.
二氧化碳是非极性的。
Water has the same two polar bonds, but water is bent.
水有同样的两根极性键，但水是弯的。
The two pulls do not cancel; they add up to one net pull towards the oxygen, so water is polar.
两个拉力不会抵消， 而是合成一个指向氧的净拉力，所以水是极性的。
That is the rule: the shape decides, and polar bonds only make a polar molecule when the shape stops them cancelling.
这就是规则：形状说了算——只有当形状让极性无法抵消时， 极性键才会造出极性分子。
Before you go, four marks students throw away.
结束之前，说四个学生常丢的分。
First, count every valence electron before you draw anything, and for an ion add one for every minus charge.
第一，动笔之前先数清所有价电子； 如果是离子，每带一个负电荷就加一个电子。
Second, lone pairs do not show in the name of the shape, but they still bend it.
第二，孤对电子不会出现在形状的名称里，但它照样把形状压弯。
Third, a molecule with polar bonds can still be non-polar overall if the shape makes them cancel.
第三，含有极性键的分子，如果形状让极性相互抵消，整体仍然可以是非极性的。
Fourth, higher bond order means shorter and stronger, every time.
第四，键级越高，键就越短越强，每次都是这样。