Chemical Bonding
A-Level Chemistry Topic 3 15:06 English narration · English + 中文 subtitles burned in
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Two molecules, almost the same size. Water, and hydrogen sulfide.
两种分子，大小几乎相同：水，和硫化氢。
Sulfur sits directly below oxygen in the Periodic Table, so you would expect the two to behave alike.
硫在周期表中就位于氧的正下方， 所以你会以为两者表现相似。
But water boils at one hundred degrees Celsius, while hydrogen sulfide boils at minus sixty. That is a gap of one hundred and sixty degrees.
但是水在一百摄氏度沸腾，而硫化氢在零下六十度就沸腾了， 相差一百六十度。
Something is holding water molecules together far more tightly than size alone can explain.
有某种东西把水分子拉得远比分子大小所能解释的更紧。
By the end of this lesson you will know exactly what.
等这节课结束，你就会确切地知道那是什么。
Welcome to chemical bonding.
欢迎来到化学键。
We will go from the pull of a single atom, through the three strong bonds, all the way to the weak forces between molecules.
我们将从单个原子的吸引力出发，经过三种强作用力， 一直讲到分子之间的弱作用力。
Let's begin.
让我们开始吧。
Start with one number.
先从一个数说起。
Pauling electronegativity is the power of an atom to attract the electrons in a bond towards itself. Three things decide it.
鲍林电负性是原子把成键电子吸引向自己的能力。
More protons pull the bonding pair harder. A smaller atomic radius holds that pair closer to the nucleus. And more inner shells shield the pull.
有三个因素决定它： 质子越多，对成键电子对的吸引越强；原子半径越小，电子对离原子核越近； 而内层电子越多，屏蔽作用越强。
Put those together and electronegativity rises across a period and falls down a group.
把这些放在一起，电负性沿周期递增，沿族递减。
Fluorine, at the top right, is the most electronegative element of all.
位于右上方的氟，是电负性最强的元素。
And here is why it matters: the difference between two atoms decides what kind of bond they make.
而这一点之所以重要，是因为： 两个原子之间电负性的差值，决定了它们形成哪种化学键。
A big difference means the electrons are taken. A small difference means they are shared.
差值大，电子被夺走； 差值小，电子被共用。
Read the trend off the table, because it is what decides which bond you get.
把趋势从周期表上读出来，因为正是它决定你得到哪一种键。
Electronegativity rises ACROSS a period: the nuclear charge grows while the shielding stays about the same, so the nucleus pulls the bonding pair harder.
电负性沿周期向右增大：核电荷增加，而屏蔽效应基本不变， 所以原子核把成键电子对拉得更紧。
It falls DOWN a group: the outer shell is further out and better shielded, so the pull is weaker.
它沿族向下减小：外层离核更远、屏蔽更强，所以吸引力更弱。
Fluorine sits at the top right and is the most electronegative element.
氟位于右上角，是电负性最强的元素。
Everything that follows — ionic versus covalent, polar versus non-polar — comes from the DIFFERENCE in electronegativity between the two atoms.
后面的一切——离子键还是共价键、极性还是非极性—— 都来自两个原子之间电负性的差值。
The first strong bond.
第一种强作用力。
When the difference is large — a metal meeting a non-metal — the electrons are transferred completely.
当电负性差值很大时——金属遇上非金属——电子被完全转移。
Watch sodium hand its single outer electron to chlorine.
看钠把它唯一的外层电子交给氯。
Now sodium is a positive ion, a cation, and chlorine is a negative ion, an anion.
现在钠成了正离子，即阳离子； 氯成了负离子，即阴离子。
Both have a full outer shell.
两者都有了满的外层。
The ionic bond is the electrostatic attraction between those opposite charges.
离子键就是这两种相反电荷之间的静电引力。
And it does not stop at one pair: the ions stack into a giant lattice, where every ion is pulled by all of its neighbours at once.
而且它并不止于一对： 离子堆积成一个巨型晶格，每一个离子都同时被它周围所有的邻居吸引。
In an ionic dot-and-cross diagram the electrons TRANSFER.
在离子键的点叉图里，电子发生的是转移。
Draw square brackets round each ion with its charge outside, and show the metal's outer shell now empty while the non-metal's is full.
给每个离子画上方括号，把电荷标在括号外面， 并画出金属的外层现在已经空了，而非金属的外层已经填满。
The dots and crosses matter: they show which atom each electron CAME from, which is the whole point of the convention.
点和叉是有意义的：它们表明每个电子原来属于哪个原子， 而这正是这套画法的全部意义所在。
Notice the metal ion is drawn smaller than its atom — it has lost a whole shell — and the non-metal ion larger, because the extra electron increases repulsion in the outer shell.
注意金属离子画得比它的原子小——它整整少了一层电子壳—— 而非金属离子画得更大，因为多出来的那个电子增大了外层内部的排斥。
An ionic compound is not made of molecules.
离子化合物不是由分子构成的。
It is a giant lattice — every ion surrounded by ions of the opposite charge, repeating in three dimensions, and this cubic salt crystal is that lattice made big enough to see.
它是一个巨型晶格—— 每个离子周围都是带相反电荷的离子，在三维空间中不断重复， 而这块立方的食盐晶体正是这个晶格放大到肉眼可见的样子。
That structure explains the properties directly.
这个结构直接解释了它的性质。
Melting points are high, because you must overcome strong electrostatic attraction throughout the whole lattice.
熔点很高， 因为你必须克服贯穿整个晶格的强静电吸引力。
It conducts when molten or dissolved but NOT when solid, because only then are the ions free to move.
它在熔融或溶解时导电，固态时却不导电， 因为只有那时离子才能自由移动。
And it is brittle: shift one layer and like charges line up, so the crystal splits along a clean plane.
而且它很脆：让一层错开，同种电荷就会正对， 于是晶体沿着一个平整的面裂开。
The second strong bond.
第二种强作用力。
In a metal, every atom lets go of its outer electrons.
在金属中，每个原子都放开了自己的外层电子。
What is left is a regular array of positive metal ions, sitting in a sea of delocalised electrons that can move anywhere in the whole piece of metal.
剩下的是排列规则的金属正离子，浸在一片离域电子的海洋里， 这些电子可以在整块金属中任意移动。
Metallic bonding is the attraction between those positive ions and that electron sea.
金属键就是这些正离子与这片电子海之间的吸引。
Two properties follow at once. Because the electrons are free to move, metals conduct electricity. And because the layers of ions can slide without breaking the bond, metals bend instead of shattering.
由此立刻得到两个性质：因为电子可以自由移动，金属能导电； 又因为离子层可以滑动而不破坏化学键，金属会弯曲而不是碎裂。
The third strong bond.
第三种强作用力。
When two non-metals meet, neither atom can win the electrons outright, so they share instead.
当两个非金属相遇时，谁都无法把电子完全夺走，于是它们共用。
A covalent bond is the attraction between the two nuclei and a shared pair of electrons.
共价键就是两个原子核与一对共用电子之间的吸引。
Share one pair and you have a single bond. Share two pairs and it is a double bond. Share three, as nitrogen does, and it is a triple bond.
共用一对，就是单键； 共用两对，就是双键；共用三对——比如氮气——就是三键。
One more thing: from Period three downwards an atom can expand the octet — it can hold more than eight electrons in its outer shell.
还有一点： 从第三周期往下，原子可以扩展八隅体——外层可以容纳超过八个电子。
Sulfur hexafluoride holds twelve.
六氟化硫就容纳了十二个。
There is a fourth case, and examiners love it.
还有第四种情形，而且考官很爱考。
In a coordinate bond — also called a dative covalent bond — both of the shared electrons come from the same atom.
在配位键中——也叫配位共价键—— 共用的两个电子都来自同一个原子。
Ammonia has a lone pair sitting on its nitrogen.
氨的氮上有一对孤对电子。
Bring in a hydrogen ion, which has no electrons at all, and that lone pair forms the fourth bond by itself. The result is the ammonium ion.
引入一个完全没有电子的氢离子，这对孤对电子就独自形成了第四个键， 于是生成铵离子。
Once made, the new bond is identical to the other three.
一旦形成，这个新键与另外三个完全相同。
Draw the arrow from the lone pair, to show where both electrons came from.
要从孤对电子画出箭头，表明两个电子都来自哪里。
Now look closer at how a covalent bond actually forms: two orbitals overlap.
现在更仔细地看共价键是怎样形成的：两个轨道发生重叠。
A sigma bond comes from direct, head-on overlap, straight along the line between the two nuclei.
西格玛键来自沿两核连线方向的正面、头碰头重叠。
A pi bond comes from two p orbitals overlapping sideways, above and below that line.
派键来自两个 p 轨道在这条连线上下方的侧面重叠。
So a single bond is one sigma bond. A double bond is one sigma plus one pi. A triple bond is one sigma plus two pi.
所以单键是一个西格玛键；双键是一个西格玛键加一个派键； 三键是一个西格玛键加两个派键。
More bonds means a shorter bond, and more energy is needed to break it — so a triple bond is the shortest and the strongest of the three.
键越多，键长越短，断开它所需的能量越大—— 所以三键在这三者中最短也最强。
Look at where the overlap happens.
看清重叠发生在什么位置。
A sigma bond overlaps END-ON, directly along the line joining the two nuclei, so the electron density sits between them and the bond is strong.
西格玛键是头碰头重叠的， 正好沿着连接两个原子核的直线，所以电子云密度集中在两核之间，键很强。
A pi bond overlaps SIDEWAYS, above and below that line, so its density is further from the nuclei and the bond is weaker.
派键是肩并肩重叠的，位于那条连线的上方和下方， 所以它的电子云离原子核更远，键更弱。
Two consequences worth stating.
有两个值得说出来的推论。
A single bond is one sigma.
单键是一个西格玛键。
A double bond is one sigma plus one pi, and a triple is one sigma plus two pi.
双键是一个西格玛键加一个派键，三键是一个西格玛键加两个派键。
And because a pi bond locks the p orbitals parallel, a double bond cannot ROTATE, while a single sigma bond can.
而且由于派键把 p 轨道锁定成平行，双键无法旋转， 而单独的西格玛键可以。
Hybridisation mixes orbitals in the same shell to make new, EQUAL orbitals for bonding — which is how carbon manages four identical bonds when its s and p orbitals are not identical.
杂化把同一电子层内的轨道混合起来，形成新的、完全等同的成键轨道—— 碳的 s 轨道和 p 轨道本来并不相同，它却能形成四条完全相同的键，靠的就是这个。
Three cases to know.
要掌握三种情形。
s p gives two equal orbitals, used in a linear molecule.
s p 杂化给出两条等同轨道，用于直线形分子。
s p two gives three equal orbitals, used in a flat molecule like ethene, C two H four.
s p 二杂化给出三条等同轨道，用于乙烯 C 二 H 四这样的平面分子。
And s p three gives four equal orbitals, used in methane.
而 s p 三杂化给出四条等同轨道，用于甲烷。
Count the equal orbitals you need and the name follows: two means s p, three means s p two, four means s p three.
数一数你需要几条等同轨道，名称就跟着出来了： 两条是 s p，三条是 s p 二，四条是 s p 三。
Two measurable properties, and both definitions must be precise.
两个可测量的性质，而两个定义都必须说准确。
Bond energy is the energy needed to break one mole of a particular covalent bond IN THE GAS STATE — that last phrase is part of the definition, because breaking bonds in a liquid would also involve intermolecular forces.
键能是在气态下断裂一摩尔某种共价键所需要的能量—— 最后这个短语是定义的一部分， 因为在液态中断键还会牵涉到分子间作用力。
Bond length is the distance between the CENTRES of the two bonded atoms.
键长是两个成键原子中心之间的距离。
And the two are linked: shorter is usually stronger.
而这两者是相关的：越短通常越强。
So a triple bond is shorter and stronger than a double, which is shorter and stronger than a single.
所以三键比双键更短更强，双键又比单键更短更强。
Stronger bonds make a molecule harder to react — which is exactly why nitrogen gas, with its triple bond, is so unreactive.
键越强，分子就越难反应—— 这正是带着三键的氮气如此不活泼的原因。
Here are all seven, and it is worth learning them as a table.
这就是全部七种，值得当作一张表来记。
Carbon dioxide is linear at one hundred and eighty degrees.
二氧化碳是直线形，一百八十度。
Boron trifluoride is trigonal planar at one hundred and twenty.
三氟化硼是平面三角形，一百二十度。
Methane is tetrahedral at one hundred and nine point five.
甲烷是四面体形，一百零九点五度。
Ammonia is pyramidal at one hundred and seven.
氨是三角锥形，一百零七度。
Water is bent at one hundred and four point five.
水是角形，一百零四点五度。
Phosphorus pentafluoride is trigonal bipyramidal, with both one hundred and twenty and ninety degree angles.
五氟化磷是三角双锥形， 同时有一百二十度和九十度两种键角。
And sulfur hexafluoride is octahedral at ninety.
而六氟化硫是八面体形，九十度。
Notice ammonia and water sit below methane's angle — same four pairs, but one and two lone pairs squeezing them.
注意氨和水的角度都低于甲烷——同样是四对电子， 只是有一对和两对孤对电子在挤压它们。
To predict a shape you need only one idea: the electron pairs around the central atom all repel each other, so they spread as far apart as they possibly can.
要预测分子形状，你只需要一个思想：中心原子周围的电子对彼此排斥， 所以它们会尽可能张开到最远。
That is VSEPR theory. Count every pair.
这就是价层电子对互斥理论。
Two pairs give a linear shape, at one hundred and eighty degrees. Three give trigonal planar, at one hundred and twenty. Four give tetrahedral, at one hundred and nine point five.
把每一对都数进去： 两对给出直线形，键角一百八十度；三对给出平面三角形，一百二十度； 四对给出四面体形，一百零九点五度；五对给出三角双锥形； 六对给出八面体形，九十度。
Five give trigonal bipyramidal, and six give octahedral, at ninety degrees. But watch out — a lone pair pushes harder than a bonding pair, and squeezes the angle by about two and a half degrees for each lone pair.
但要注意——孤对电子的排斥比成键电子对更强， 每有一对孤对电子，键角就被压缩大约二点五度。
Let's do the two the exam asks for most.
我们来做考试最常考的两个。
Ammonia: nitrogen has five outer electrons and makes three bonds, so it has three bonding pairs and one lone pair. That is four pairs in total, so start from tetrahedral.
氨：氮有五个外层电子，形成三个键， 所以有三对成键电子和一对孤对电子，总共四对，因此从四面体形出发。
One lone pair squeezes the angle once, by about two and a half degrees, so ammonia is pyramidal at about one hundred and seven degrees.
一对孤对电子把键角压缩一次，大约二点五度，所以氨是三角锥形，约一百零七度。
Now water. Oxygen makes only two bonds and keeps two lone pairs.
再看水：氧只形成两个键，保留两对孤对电子。
Still four pairs, but now two squeezes, so water is bent at about one hundred and four point five degrees.
仍然是四对，但被压缩两次， 所以水是角形，约一百零四点五度。
Now to the weak forces between molecules — but first, polarity.
现在讲分子间的弱作用力——但先说极性。
When two atoms of different electronegativity share a bond, the electrons sit closer to the greedier atom.
当电负性不同的两个原子共用一个键时， 电子会更靠近吸引力强的那个原子。
One end of the bond becomes slightly negative, the other slightly positive.
键的一端略带负电，另一端略带正电。
That separation of charge is called a dipole.
这种电荷的分离叫做偶极。
If the dipoles do not cancel, the molecule has a dipole moment and is polar.
如果偶极不能抵消，整个分子就有偶极矩，就是极性分子。
Whether the whole molecule is polar then depends on the shape.
而整个分子是否有极性，取决于它的形状。
In carbon dioxide the two dipoles point in opposite directions and cancel exactly, so the molecule is non-polar overall.
在二氧化碳中，两个偶极方向相反、完全抵消，所以整个分子没有极性。
Water is bent, so its dipoles do not cancel — water is polar.
水是角形的，它的偶极不会抵消——所以水是极性分子。
A dot-and-cross diagram shows the OUTER electrons only, using dots for one atom and crosses for the other.
点叉图只画最外层电子，一个原子用点，另一个原子用叉。
The convention exists for one reason: it makes clear where each bonding electron came from.
这套约定的存在只有一个理由：它让每个成键电子的来源一目了然。
Look at water — two bonding pairs, each with one dot and one cross, plus two lone pairs on the oxygen that belong to oxygen alone.
看水——两对成键电子，每一对都是一个点加一个叉， 另外氧上还有两对孤对电子，那是只属于氧的。
Then nitrogen: three shared pairs, a triple bond, plus one lone pair on each atom.
再看氮：三对共用电子，也就是一个三键，另外每个原子上各有一对孤对电子。
You can draw these for ionic, covalent and coordinate bonding, including molecules with an expanded octet or an odd number of electrons.
离子键、共价键和配位键都可以这样画， 也包括扩展八隅体的分子和含奇数电子的分子。
Follow the sequence.
跟着这个过程走一遍。
Electrons in a molecule are constantly moving, so at any instant they can be unevenly spread — that is an INSTANTANEOUS dipole.
分子里的电子一直在运动， 所以在任何一个瞬间它们都可能分布不均——这就是瞬时偶极。
That temporary dipole then repels the electrons in a neighbouring molecule, INDUCING a dipole in it, and the two attract.
这个暂时的偶极随后排斥相邻分子中的电子，在它里面诱导出一个偶极， 于是两者相互吸引。
The attraction is weak and it vanishes and reforms constantly.
这种吸引很弱，而且不断消失又不断重新形成。
It gets stronger with more electrons, which is why boiling points rise down a homologous series, and it is stronger for long straight molecules than for branched ones, because they can pack closer together.
电子越多它就越强，这正是同系物沿着序列沸点升高的原因； 而且它对细长的直链分子比对支链分子更强， 因为直链分子能堆积得更紧密。
Look at what an exam wants in this diagram, because drawing it earns the marks.
看看考试在这幅图里想要什么，因为把它画对就能拿分。
A hydrogen bond forms when hydrogen is attached to nitrogen, oxygen or fluorine and is attracted to a LONE PAIR on the N, O or F of another molecule.
当氢连接在氮、氧或氟上，并被另一个分子中氮、氧或氟上的孤对电子所吸引时， 就形成氢键。
So show the lone pair — a hydrogen bond points AT a lone pair, not vaguely at the atom.
所以要把孤对电子画出来—— 氢键指向的是一对孤对电子，而不是笼统地指向那个原子。
Show it as a dashed line, not a solid one, because it is an intermolecular force and not a covalent bond.
要画成虚线，不能画成实线，因为它是分子间作用力，不是共价键。
And show it straight: the N-H to O arrangement is linear, at about one hundred and eighty degrees.
还要画成直的：N 到 H 再到 O 的排列是直线形，大约一百八十度。
Miss the lone pair or draw it bent and you lose the mark even though you named the right force.
漏掉孤对电子，或者把它画弯，即使你说对了是哪种作用力，这一分也丢了。
Van der Waals forces is the umbrella name for the forces between molecules.
范德华力是分子间作用力的统称。 它有两类。
There are two kinds. The first is the instantaneous dipole–induced dipole force, also called London dispersion.
第一类是瞬时偶极—诱导偶极作用，也叫伦敦色散力。
Electrons are always moving, so for an instant one side of a molecule holds more of them — an instantaneous dipole. That induces a matching dipole in the neighbour, and the two attract.
电子一直在运动， 所以某一瞬间分子的一侧电子会多一些，形成瞬时偶极； 它又在邻近分子中诱导出方向匹配的偶极，两者相互吸引。
These act between all molecules, and get stronger as the number of electrons grows.
这种力存在于所有分子之间，电子数越多就越强。
The second kind is the permanent dipole force, which acts between molecules that are polar all the time.
第二类是永久偶极作用，发生在始终具有极性的分子之间。
And now the answer to our opening question.
现在来回答开头的问题。
Hydrogen bonding is the strongest of all these forces.
氢键是所有这些作用力中最强的。
It needs two things: a hydrogen atom bonded to nitrogen, oxygen or fluorine, and a lone pair on a nitrogen, oxygen or fluorine atom nearby.
它需要两个条件： 一个与氮、氧或氟相连的氢原子，以及附近氮、氧或氟原子上的一对孤对电子。
Water has both, so every molecule is gripped by its neighbours.
水两个条件都满足，所以每个水分子都被邻居牢牢拉住。
That is why water boils at one hundred degrees, while hydrogen sulfide, which has no hydrogen bonding, boils at minus sixty.
这就是为什么水在一百度沸腾，而没有氢键的硫化氢在零下六十度就沸腾。
The same force gives water its high surface tension.
同一种作用力也给了水很高的表面张力。
It also explains why ice floats: the hydrogen bonds hold the molecules in an open, spread-out structure, so ice is less dense than liquid water.
它也解释了冰为什么会浮起来：氢键把分子固定成一种张开的、间隙很大的结构， 所以冰的密度比液态水小。
Three marks students throw away.
三个学生常丢的分。
First, for a shape: count the bonding pairs and the lone pairs, name the shape, then give the exact angle — ammonia is pyramidal at one hundred and seven degrees.
第一，判断形状时：数清成键电子对和孤对电子， 说出形状名称，再给出确切的键角——氨是三角锥形，一百零七度。
Second, name the force precisely.
第二，作用力的名称要准确。
Hydrogen bonding needs hydrogen on nitrogen, oxygen or fluorine; otherwise call it permanent dipole or induced dipole.
氢键需要氢连在氮、氧或氟上； 否则就叫永久偶极作用或诱导偶极作用。
And never call van der Waals forces bonds.
而且永远不要把范德华力叫做"键"。
Third, when you explain a physical property, say which forces are broken — not just that the bonds are strong.
第三，解释物理性质时，要说清楚被破坏的是哪一种作用力， 而不只是说"键很强"。