Atoms & Bonding
IGCSE Chemistry Topic 2 16:42 English narration · English + 中文 subtitles burned in
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Look at these two things.
看看这两样东西。
A diamond — the hardest material you can buy.
钻石——你能买到的最硬的材料。
And the soft, grey lead in a pencil.
还有铅笔里柔软的灰色笔芯。
They could not seem more different.
它们看起来再不同不过了。
Yet both are made of the same element: pure carbon, and nothing else.
然而两者都由同一种元素构成：纯碳，别无其他。
So what makes them so different?
那么，是什么让它们如此不同？
The answer is bonding — how the atoms are joined together.
答案是化学键——原子是如何连接在一起的。
Today we look inside the atom, and at the bonds that build every substance around you.
今天我们要看看原子内部，以及构建你周围一切物质的化学键。
Welcome to atoms and bonding.
欢迎来到原子与化学键。
First the atom itself, then the three ways atoms join: ionic, covalent and metallic.
先认识原子本身，再学习原子结合的三种方式： 离子键、共价键和金属键。
Let's begin.
让我们开始吧。
Every substance is one of three kinds.
每种物质都属于三类之一。
An element is made of only one type of atom — like copper, or oxygen.
元素只由一种原子组成——比如铜，或氧气。
A compound is two or more elements chemically joined, or bonded, in a fixed ratio — like water.
化合物是两种或更多元素以固定比例化学结合、即键合在一起——比如水。
A mixture is just substances mixed together, not bonded — like air.
混合物只是把物质混在一起，并没有键合——比如空气。
The key idea: in a compound the atoms are bonded and hard to separate; in a mixture they are not bonded, and easy to separate.
关键在于： 化合物中的原子是键合的，很难分开；混合物中的原子没有键合，容易分开。
Look at the three boxes.
看这三个框。
The element box has only one type of particle.
元素框里只有一种粒子。
The compound box has two types joined in pairs — that fixed pairing is the bond.
化合物框里是两种粒子成对结合—— 那种固定的配对就是化学键。
The mixture box is a jumble: different particles side by side, but not joined.
混合物框是混杂的：不同粒子并排，但没有结合。
A compound has new properties of its own — water is nothing like hydrogen gas or oxygen gas.
化合物有自己全新的性质——水完全不像氢气或氧气。
In a mixture, each part keeps its own properties, so you can pull the parts apart with physical methods such as filtering, distilling, or chromatography.
在混合物里，各部分保留自己的性质， 所以你可以用过滤、蒸馏或色谱等物理方法把它们分开。
Compounds need a chemical reaction to split.
化合物则需要化学反应才能拆开。
Here is a pure element sample — a metal crystal made of only one kind of atom.
这是一个纯元素样品——一块只由一种原子组成的金属晶体。
All substances are built from about one hundred simple building blocks: the elements.
所有物质都由大约一百种 简单的积木构成：元素。
You cannot break an element into anything simpler by a chemical reaction.
你无法用化学反应把元素拆成更简单的东西。
Copper stays copper; oxygen stays oxygen.
铜还是铜； 氧还是氧。
When elements bond, they make compounds.
元素键合时形成化合物。
When they only mix, they make mixtures.
只是混合时形成混合物。
Sorting every substance into element, compound, or mixture is the first step in chemistry.
把每种物质归入元素、化合物或混合物，是化学的第一步。
Now, inside the atom.
现在看原子内部。
At the centre is a tiny, dense nucleus, holding protons and neutrons.
中心是一个微小而致密的原子核，里面有质子和中子。
Around it, electrons move in shells.
原子核周围，电子在电子层中运动。
Learn these values: a proton has a mass of one and a charge of plus one; a neutron has a mass of one and no charge; an electron has almost no mass and a charge of minus one.
记住这些数值：质子质量为一、电荷为正一； 中子质量为一、不带电；电子几乎没有质量、电荷为负一。
Two numbers describe the atom.
两个数字描述原子。
The proton number — how many protons — tells you which element it is.
质子数——有多少个质子——告诉你它是哪种元素。
The mass number is the protons plus the neutrons.
质量数是质子加中子。
So the number of neutrons is the mass number minus the proton number.
所以中子数等于质量数减去质子数。
This diagram shows a lithium atom.
这张图显示一个锂原子。
The orange centre is the nucleus — protons and neutrons packed tight.
橙色中心是原子核——质子和中子紧密堆积。
Around it, electrons sit in shells, also called energy levels.
周围， 电子在电子层中，也叫能级。
The first shell holds two electrons; the next shell holds the third.
第一层容纳两个电子；下一层容纳第三个。
An atom has no overall charge: the number of protons equals the number of electrons, so plus one and minus one cancel.
原子整体不带电：质子数等于电子数，正一与负一相互抵消。
If that balance changes, the particle becomes an ion — we meet ions when we bond later.
如果这个平衡改变，粒子就变成离子——后面讲化学键时会遇到。
Two names for each key number.
每个关键数字都有两个名字。
The proton number is also called the atomic number — it alone decides which element you have.
质子数也叫原子序数——单靠它就决定是哪种元素。
The mass number is also called the nucleon number — nucleons means protons and neutrons together.
质量数也叫核子数——核子是指质子和中子合在一起。
Neutrons equal mass number minus proton number.
中子数等于质量数减去质子数。
Take carbon twelve: six protons and six neutrons.
以碳十二为例：六个质子和六个中子。
Carbon fourteen still has six protons — same element — but eight neutrons, so a bigger mass.
碳十四仍然有六个质子——同一种元素—— 但有八个中子，所以质量更大。
Same chemistry later; different mass now.
以后化学性质相同；现在质量不同。
Electrons fill the shells from the inside out.
电子从内到外填充电子层。
The first shell holds up to two electrons; the next shells hold up to eight.
第一层最多容纳两个电子；后面的层最多容纳八个。
We write the electronic configuration shell by shell.
我们逐层写出电子排布。
Sodium, with eleven electrons, is two, eight, one.
钠有十一个电子，排布是二、八、一。
This links straight to the Periodic Table.
这直接对应周期表。
The number of electrons in the outer shell equals the group number — so sodium, with one outer electron, is in Group One.
外层的电子数等于族数——所以钠有一个外层电子，位于第一族。
The number of shells used equals the period number.
使用的电子层数等于周期数。
And the noble gases have a full outer shell, which makes them unreactive.
而稀有气体有满的外层，这使它们不活泼。
Sodium's picture makes the rule clear: two, eight, one.
钠的图把规则说清楚了：二、八、一。
One outer electron means Group One.
一个外层电子意味着第一族。
Three shells in use means Period Three.
三层在用意味着第三周期。
Try two more.
再试两个。
An atom with thirteen electrons is two, eight, three — Group Three, Period Three.
有十三个电子的原子是二、八、三——第三族、第三周期。
That is aluminium.
那是铝。
Calcium has twenty electrons: two, eight, eight, two — Group Two, Period Four.
钙有二十个电子：二、八、八、二——第二族、第四周期。
For the first twenty elements, outer shells hold up to eight.
对前二十种元素，外层最多八个。
The noble gases sit in Group Eight, or Group Zero: full outer shell, very unreactive.
稀有气体在第八族，或零族：外层满，非常不活泼。
Isotopes are atoms of the same element, with the same number of protons, but different numbers of neutrons.
同位素是同一种元素的原子，质子数相同，但中子数不同。
Same protons means same element; different neutrons means different mass.
质子相同意味着是同一种元素； 中子不同意味着质量不同。
Because they have the same electrons, isotopes react in exactly the same way — only their mass differs.
因为它们的电子相同，同位素的反应方式完全一样——只有质量不同。
Now, the relative atomic mass is the average mass of an element's atoms, weighted by how common each isotope is.
相对原子质量是一种元素原子的平均质量，按每种同位素的常见程度加权。
Chlorine is seventy-five percent chlorine thirty-five, and twenty-five percent chlorine thirty-seven.
氯有百分之七十五是氯三十五，百分之二十五是氯三十七。
The weighted average comes to thirty-five point five — closer to thirty-five, because thirty-five is more common.
加权平均是三十五点五—— 更接近三十五，因为三十五更常见。
You write an atom with the mass number on top and the proton number below.
写原子时，质量数在上，质子数在下。
Carbon twelve is mass twelve over proton six.
碳十二是质量十二在上、质子六在下。
Chlorine thirty-five is mass thirty-five over proton seventeen.
氯三十五是质量三十五在上、质子十七在下。
For an ion, add the charge after the symbol — chlorine minus has gained one electron.
写离子时，在符号后加上电荷—— 氯负离子多得到一个电子。
Isotopes of one element share the same chemical properties because reactions only involve electrons, and isotopes have the same number of electrons and the same configuration.
同一种元素的同位素有相同的化学性质， 因为反应只涉及电子，而同位素电子数相同、排布相同。
Extra neutrons change mass only.
额外的中子只改变质量。
Relative atomic mass uses abundance — the percentage of each isotope.
相对原子质量用到丰度——每种同位素的百分比。
The formula is: sum of isotope mass times abundance, then divide by one hundred.
公式是：同位素质量乘丰度之和， 再除以一百。
For chlorine: thirty-five times seventy-five, plus thirty-seven times twenty-five, all over one hundred.
对氯：三十五乘七十五，加三十七乘二十五，全部除以一百。
That is two thousand six hundred twenty-five, plus nine hundred twenty-five, over one hundred — equal to thirty-five point five.
那是二千六百二十五，加九百二十五，除以一百——等于三十五点五。
The answer sits closer to the more common isotope.
答案更靠近更常见的同位素。
Always check that sense check before you move on.
继续之前，总要做这个合理性检查。
Worked example.
例题。
Boron has two isotopes, boron ten and boron eleven, and its relative atomic mass is ten point eight.
硼有两种同位素，硼十和硼十一，相对原子质量是十点八。
Find the percentage of each.
求各自的百分比。
Let the abundance of boron eleven be x percent, so boron ten is one hundred minus x.
设硼十一的丰度为百分之叉，则硼十是一百减叉。
Now set the average equal to ten point eight: eleven x plus ten times one hundred minus x, all over one hundred, equals ten point eight.
现在让平均值等于十点八： 十一叉加十乘一百减叉，全部除以一百，等于十点八。
That simplifies to x equals eighty.
化简得叉等于八十。
So boron is eighty percent boron eleven and twenty percent boron ten.
所以硼是百分之八十硼十一、百分之二十硼十。
Sense check: ten point eight is closer to eleven, so the heavier isotope must be the majority — if your answer puts the majority on the lighter one, you have them the wrong way round.
合理性检查：十点八更靠近十一， 所以较重的同位素必须占多数——如果你的答案把多数给了较轻的，那就搞反了。
The first way atoms bond: ionic bonding, between a metal and a non-metal.
原子结合的第一种方式：离子键，发生在金属和非金属之间。
An ion is an atom that has lost or gained electrons, so it now has a charge.
离子是失去或得到电子的原子， 因此带上电荷。
A metal atom loses electrons to become a positive ion, a cation.
金属原子失去电子，变成正离子，即阳离子。
A non-metal gains them to become a negative ion, an anion.
非金属得到电子，变成负离子， 即阴离子。
Watch sodium give its single outer electron to chlorine.
看钠把它唯一的外层电子交给氯。
Now both have full outer shells.
现在两者都有满的外层。
Sodium is positive, chlorine is negative, and the opposite charges pull together strongly.
钠带正电，氯带负电， 异性电荷强烈地相互吸引。
That attraction is the ionic bond.
这种吸引就是离子键。
Sodium starts as two, eight, one.
钠开始是二、八、一。
Chlorine starts as two, eight, seven.
氯开始是二、八、七。
Sodium gives its one outer electron to chlorine.
钠把它一个外层电子交给氯。
Now sodium is two, eight — a sodium plus ion.
现在钠是二、八——钠正离子。
Chlorine is two, eight, eight — a chloride minus ion.
氯是二、八、八——氯负离子。
Both have full outer shells, like a noble gas.
两者都有满的外层，像稀有气体。
The ionic bond is the strong electrostatic attraction between these opposite charges.
离子键就是这些异性电荷之间强烈的静电引力。
On a dot-and-cross diagram, one element's outer electrons are dots, the other's are crosses, and you show the electron moving from the metal to the non-metal.
在电子式图中，一种元素的外层电子用点， 另一种用叉，并画出电子从金属移向非金属。
Ionic compounds are not made of separate molecules.
离子化合物不是由分开的分子组成。
Instead, the ions pack into a giant lattice — a huge, regular pattern of alternating positive and negative ions.
相反，离子堆积成一个巨型晶格—— 一个由正负离子交替排列的巨大、规则图案。
This structure explains their properties.
这种结构解释了它们的性质。
The strong attraction between ions needs a lot of energy to break, so ionic compounds have high melting points.
离子之间强烈的吸引需要大量能量才能破坏，所以离子化合物熔点很高。
When solid, the ions are locked in place, so they cannot conduct electricity.
固态时，离子被锁在原位，无法导电。
But melt them, or dissolve them in water, and the ions are free to move — now they conduct.
但把它们熔化，或溶解在水中， 离子就能自由移动——这时就能导电了。
The lattice is a regular, repeating grid of alternating positive and negative ions — not little molecules.
晶格是正负离子交替的规则、重复网格——不是小分子。
High melting and boiling points come from the strong electrostatic attraction that holds every ion in place.
高熔点和高沸点来自把每个离子 固定在原位的强烈静电引力。
Solid ionic compounds do not conduct: the ions cannot move.
固态离子化合物不导电：离子不能移动。
When the solid is molten, or when it is aqueous — dissolved in water — the ions are free to carry charge, so the liquid conducts.
当固体熔融， 或呈水溶液——溶解在水中——时，离子能自由携带电荷，所以液体导电。
Remember three words for the exam: molten, aqueous, and free ions.
考试记住三个词：熔融、水溶液、自由离子。
The second way: covalent bonding, between non-metal atoms.
第二种方式：共价键，发生在非金属原子之间。
Here the atoms do not give and take electrons — they share them.
这里原子不是给予和获取电子——而是共用它们。
Each shared pair of electrons is one covalent bond, and by sharing, both atoms reach a full outer shell.
每一对共用电子就是一个共价键，通过共用，两个原子都达到满的外层。
Hydrogen shares one pair; water shares a pair with each hydrogen; methane shares four.
氢共用一对； 水与每个氢共用一对；甲烷共用四对。
These make small, separate molecules.
这些形成小的、分开的分子。
The bonds inside a molecule are strong, but the forces between molecules are weak — so molecular substances have low melting points, and cannot conduct electricity.
分子内部的键很强，但分子之间的力很弱——所以分子物质熔点低，也不能导电。
Here is a ball-and-stick model of water.
这是水的球棍模型。
The central oxygen shares one pair of electrons with each of two hydrogens — two single covalent bonds.
中心的氧与两个氢各共用一对电子——两个单共价键。
Every atom reaches a full outer shell by sharing, not by transfer.
每个原子通过共用而不是转移达到满的外层。
Covalent bonds form between non-metal atoms only.
共价键只在非金属原子之间形成。
The molecule is a small, separate unit.
分子是小的、分开的单元。
When you melt or boil water, you are not breaking those strong bonds inside the molecule — you are only overcoming the weak forces between one molecule and the next.
当你融化或煮沸水时，并不是在破坏分子内部的强键—— 你只是在克服一个分子与下一个分子之间的弱力。
Learn these molecules.
记住这些分子。
Hydrogen gas shares one pair — a single bond.
氢气共用一对——单键。
Water shares two pairs, one with each hydrogen, and oxygen keeps lone pairs that are not shared.
水共用两对，与每个氢各一对，氧还保留不共用的孤对。
Methane has four shared pairs around carbon.
甲烷在碳周围有四对共用电子。
Ammonia has three shared pairs around nitrogen.
氨在氮周围有三对。
Oxygen gas and carbon dioxide use double bonds — two shared pairs.
氧气和二氧化碳用双键——两对共用。
Nitrogen gas uses a triple bond — three shared pairs.
氮气用三键——三对共用。
On a dot-and-cross diagram, draw the outer electrons and show which pairs sit in the overlap between atoms.
在电子式图中，画出外层电子，并标出哪些对在原子重叠处。
Simple molecular substances are made of small, separate molecules.
简单分子物质由小的、分开的分子组成。
Inside each molecule the covalent bonds are strong.
每个分子内部共价键很强。
Between molecules the intermolecular forces are weak, so little energy is needed to separate them — low melting and boiling points.
分子之间的分子间作用力很弱， 所以分开它们只需很少能量——熔点沸点都低。
They are poor conductors of electricity: the molecules have no overall charge, and there are no free electrons or free ions to carry charge.
它们是电的不良导体：分子整体不带电， 也没有自由电子或自由离子来携带电荷。
Contrast that with ionic lattices and metals later — structure decides properties every time.
后面与离子晶格和金属对比——结构每次都决定性质。
Remember the diamond and the pencil?
还记得钻石和铅笔吗？
Now we can explain them.
现在我们能解释它们了。
Both are giant covalent structures — millions of carbon atoms bonded together.
两者都是巨型共价结构—— 数百万个碳原子键合在一起。
In diamond, every carbon is bonded to four others, in a rigid three-dimensional network.
在钻石中，每个碳都与另外四个碳键合，形成刚性的三维网络。
That makes diamond extremely hard, perfect for cutting tools.
这让钻石极其坚硬，非常适合做切割工具。
In graphite, each carbon bonds to only three others, forming flat layers.
在石墨中，每个碳只与另外三个键合，形成平坦的层。
Weak forces between the layers let them slide, so graphite is slippery — a good lubricant.
层与层之间的弱力让它们能滑动，所以石墨很滑——是很好的润滑剂。
And graphite's spare electrons are free to move, so, unlike diamond, graphite conducts electricity.
而石墨多余的电子能自由移动，所以与钻石不同，石墨能导电。
Side by side: diamond's three-dimensional network versus graphite's flat hexagonal layers.
并排对比：钻石的三维网络与石墨平坦的六边形层。
In diamond, all four outer electrons of each carbon are used in bonds — no free electrons, so diamond does not conduct, and the network is very hard with a very high melting point.
在钻石中，每个碳的四个外层电子 都用于成键——没有自由电子，所以钻石不导电，网络很硬、熔点很高。
In graphite, three electrons bond into layers; the fourth is delocalised and free to move, so graphite conducts and is used as an electrode.
在石墨中， 三个电子在层内成键；第四个是离域的、能自由移动，所以石墨导电，可用作电极。
Weak forces between layers make graphite a lubricant and the grey lead in a pencil.
层间弱力使石墨成为润滑剂，也是铅笔里的灰色笔芯。
One more giant covalent substance you must know: silicon four oxide, silicon dioxide, also called silica.
还有一种你必须知道的巨型共价物质：二氧化硅，也叫硅石。
Its formula is silicon, one; oxygen, two.
化学式是硅一氧二。
The structure is like diamond — a huge three-dimensional network of covalent bonds — so silicon dioxide is very hard and has a very high melting point.
结构像钻石——巨大的共价键三维网络——所以二氧化硅很硬，熔点很高。
Do not confuse it with simple molecules of carbon dioxide.
不要把它和二氧化碳的简单分子混淆。
Same formula pattern in words, totally different structure: carbon dioxide is small molecules; silicon dioxide is a giant lattice.
文字上式子模式相同，结构完全不同： 二氧化碳是小分子；二氧化硅是巨型晶格。
The third way: metallic bonding.
第三种方式：金属键。
In a metal, the atoms lose their outer electrons, leaving positive ions packed in a lattice.
在金属中，原子失去它们的外层电子，留下正离子堆积在晶格中。
Those electrons no longer belong to any one atom — they form a sea of delocalised electrons, free to move through the whole metal.
那些电子不再属于任何一个原子——它们形成一片离域电子的海洋，能在整块金属中自由移动。
Metallic bonding is the attraction between the positive ions and this electron sea.
金属键就是正离子与这片电子海之间的吸引。
It explains two things.
它解释了两件事。
The free electrons carry charge, so metals conduct electricity.
自由电子携带电荷， 所以金属能导电。
And the layers of ions can slide without breaking the bond, so metals bend and stretch instead of shattering.
而离子层能滑动却不破坏化学键，所以金属会弯曲、拉伸，而不是碎裂。
The picture is a lattice of positive metal ions with free electrons in the gaps — the sea of delocalised electrons.
图中是正金属离子的晶格，空隙里有自由电子——离域电子的海洋。
Metallic bonding is the strong electrostatic attraction between those ions and that sea.
金属键是这些离子 与这片电子海之间的强烈静电引力。
Good electrical conductivity: the delocalised electrons move and carry charge.
良好的导电性：离域电子移动并携带电荷。
Malleability means you can hammer the metal into sheets.
展性是指你可以把金属锤成薄片。
Ductility means you can pull it into wires.
延性是指你可以把它拉成丝。
Both work because layers of positive ions slide over each other without breaking the metallic bond, so the metal changes shape instead of shattering.
两者都成立， 因为正离子层可以相互滑动而不破坏金属键，所以金属改变形状而不是碎裂。
Lock the structure-to-properties map.
锁住结构到性质的对应图。
Ionic: giant lattice, high melting point, conducts only when molten or aqueous.
离子型：巨型晶格，高熔点，只在熔融或水溶液时导电。
Simple molecular: weak forces between molecules, low melting point, no conduction.
简单分子型：分子间弱力，低熔点，不导电。
Giant covalent: very hard, very high melting point; graphite is the special case that conducts and is slippery.
巨型共价型：非常硬、熔点非常高； 石墨是能导电又滑的特例。
Metallic: delocalised electron sea — conducts and bends.
金属型：离域电子海——导电又能弯曲。
When an exam question gives a property, name the structure that causes it.
当考题给出一种性质时，说出造成它的结构。
Three marks to lock in.
锁住三个分。
First: the number of neutrons is the mass number minus the proton number — and the proton number alone decides which element it is.
第一：中子数等于质量数减去质子数——而只有质子数决定它是哪种元素。
Second, match structure to properties: ionic means a giant lattice, high melting point, conducting only when molten or dissolved; simple molecular means weak forces between molecules, so low melting points and no conduction; giant covalent means very hard and very high melting.
第二，把结构和性质对应起来：离子型是巨型晶格，熔点高，只有熔化或溶解时才导电； 简单分子型是分子之间的弱力，所以熔点低、不导电；巨型共价型非常硬、熔点非常高。
Third: graphite conducts and is slippery, diamond does neither — because of how many bonds each carbon makes.
第三：石墨导电且滑，钻石两者都不是——因为每个碳形成的键数不同。
Master these, and this topic is yours.
掌握这些，这个专题就是你的了。