Atomic Structure
A-Level Chemistry Topic 1 23:05 English narration · English + 中文 subtitles burned in
Chapters
Transcript
No one has ever seen inside an atom.
从来没有人看到过原子的内部。
It is far too small — a single atom is about a million times thinner than a human hair.
它实在太小了——一个原子的直径大约比头发丝细一百万倍。
And yet we know exactly how its electrons are arranged: two in the first shell, then eight, then one.
然而我们却确切地知道它的电子是怎样排布的：第一层两个，然后八个，然后一个。
How can chemists be so sure?
化学家为什么能这么肯定？
The answer is a staircase of numbers.
答案是一串阶梯状的数字。
By the end of this lesson, you will be able to read it.
等这节课结束，你就能读懂它了。
Welcome to atomic structure.
欢迎来到原子结构。
We will look at the particles inside the atom, at how the electrons are arranged, and at the energy needed to pull one out.
我们将认识原子内部的粒子，了解电子如何排布， 以及把一个电子拉出来需要多少能量。
Let's begin.
让我们开始吧。
Atoms are not just a textbook idea.
原子不只是课本上的概念。
A scanning tunnelling microscope can image individual atoms on a surface — the bright bumps you see here are real atoms.
扫描隧道显微镜能拍到表面上一个个单独的原子—— 你看到的那些亮点就是真实的原子。
Everything around you is made of atoms.
你周围的一切都由原子构成。
Each atom is mostly empty space, with a tiny heavy nucleus at the centre and electrons moving in shells around it.
每个原子大部分是空的，中心有一个微小而沉重的原子核，电子在周围的电子层里运动。
The picture is fuzzy because atoms are so small, but the pattern is clear: matter is built from discrete particles, not from continuous stuff.
图像有些模糊，因为原子实在太小，但图案很清楚：物质由分立的粒子构成，而不是连成一片的东西。
Start with the atom itself.
先从原子本身说起。
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，里面装着质子和中子。
Around it, in almost empty space, the electrons move in shells.
原子核周围几乎是空的，电子在电子层中运动。
Learn these values. A proton has a relative charge of plus one and a relative mass of one.
记住这些数值：质子的相对电荷是正一， 相对质量是一。
A neutron has no charge and a relative mass of one.
中子不带电，相对质量是一。
An electron has a charge of minus one, and almost no mass at all.
电子的电荷是负一，而质量几乎为零。
So the nucleus holds nearly all the mass, and the electrons take up nearly all the space.
所以原子核几乎占据了全部质量，而电子占据了几乎全部空间。
Look at this diagram again.
再看这张图。
The nucleus is drawn large so you can see it, but in reality it is tiny compared with the whole atom.
原子核画得很大，好让你看清，但实际上它比整个原子小得多。
Protons and neutrons sit packed inside that nucleus.
质子和中子挤在原子核里。
Electrons move in shells at set distances from the centre.
电子在离中心固定距离的电子层里运动。
The nucleus is very small but holds almost all the mass.
原子核很小，却几乎占了全部质量。
The electrons take up almost all the space and have almost no mass.
电子占了几乎全部空间，质量却几乎为零。
That split — mass in the middle, space outside — is the first fact every atomic-structure answer leans on.
这种分工——质量在中间、空间在外面——是原子结构答题里最先要用到的事实。
We compare the three particles using relative charge and relative mass.
我们用相对电荷和相对质量来比较这三种粒子。
These are simple numbers, not real units.
这些是简单的数字，不是真实的单位。
The proton is plus one and mass one.
质子是正一，质量为一。
The neutron is zero charge and mass one — it is neutral.
中子电荷为零，质量为一——它是中性的。
The electron is minus one, and its relative mass is one over one thousand eight hundred and thirty-six — about zero for most calculations.
电子是负一，相对质量是一千八百三十六分之一——多数计算里可以当成零。
A proton and a neutron have almost the same mass.
质子和中子质量几乎相同。
An electron is about one thousand eight hundred and thirty-six times lighter.
电子大约轻一千八百三十六倍。
Learn the table cold: exam questions quote it without warning.
这张表要背熟：考题会不经提醒直接引用。
Two numbers describe the nucleus.
有两个数字描述原子核。
The proton number is how many protons there are, and it alone decides which element the atom is.
质子数是质子的个数，只有它决定这个原子是哪种元素。
The nucleon number is the protons and the neutrons added together.
核子数是质子和中子加在一起的总数。
So the number of neutrons is the nucleon number minus the proton number.
所以中子数等于核子数减去质子数。
Try this aluminium ion. Nucleon number twenty-seven, charge three plus.
试试这个铝离子：核子数二十七，带三个正电荷。
Thirteen protons, fourteen neutrons — and only ten electrons, because it lost three.
它有十三个质子、十四个中子—— 而电子只有十个，因为它失去了三个。
Two other names appear on papers.
试卷上还会出现另外两个名字。
The proton number is also called the atomic number, written as Z.
质子数也叫原子序数，写成 Z。
The nucleon number is also called the mass number, written as A.
核子数也叫质量数，写成 A。
Protons and neutrons together are nucleons — that is why A counts both.
质子和中子合称核子——所以 A 把两者都算进去。
Neutrons equal A minus Z.
中子数等于 A 减 Z。
For a neutral atom, electrons equal protons equal Z.
对中性原子，电子数等于质子数，也等于 Z。
An ion has lost or gained electrons, so its charge is not zero: a positive ion has fewer electrons than protons, and a negative ion has more electrons than protons.
离子失去或得到了电子，所以电荷不为零： 正离子电子比质子少，负离子电子比质子多。
How are mass and charge spread out?
质量和电荷是怎样分布的？
Almost all the mass sits in the nucleus, because protons and neutrons are heavy and electrons are very light.
几乎全部质量都在原子核里，因为质子和中子重，电子很轻。
All the positive charge is in the nucleus — that is the protons.
全部正电荷都在原子核里——那就是质子。
The negative charge is spread out in the shells — that is the electrons.
负电荷分散在电子层里——那就是电子。
So the nucleus is dense, positive, and tiny.
所以原子核又密、又正、又小。
The shells are light, negative, and huge by comparison.
电子层又轻、又负，相比之下大得多。
Keep that picture when you explain beams, radius, or ionisation energy.
解释粒子束、半径或电离能时，都要记住这幅图。
Here is a neat way to tell the three particles apart.
有一个巧妙的方法可以把这三种粒子区分开。
Fire beams of them, at the same speed, between two charged plates.
让它们以相同的速度成束射入两块带电板之间。
The proton beam is positive, so it bends towards the negative plate.
质子束带正电，所以它偏向负极板。
The electron beam bends the opposite way, towards the positive plate — and it bends much further, because it is so much lighter.
电子束偏向相反的一侧，也就是正极板—— 而且偏得更远，因为它轻得多。
The neutron beam has no charge, so it passes straight through.
中子束不带电，所以它笔直地穿过去。
The same force on a smaller mass gives a bigger acceleration, so a bigger deflection.
同样的力作用在更小的质量上，加速度更大，所以偏转更大。
That is why the electron beam curves far more than the proton beam even though both feel equal-size charges.
这就是为什么电子束比质子束弯得远得多，尽管两者感受到的电荷大小相同。
The field sits between two plates: one positive, one negative.
电场在两块板之间：一块正极，一块负极。
Opposite charges attract, so each charged particle is pulled toward the plate of opposite sign.
异种电荷互相吸引， 所以每个带电粒子都被拉向符号相反的那块板。
Neutrons feel no electric force at all, so their path is a straight line.
中子完全不受电场力，所以路径是直线。
Same speed into the field is essential — then the only differences left are charge and mass.
进入电场时速度相同很关键——这样只剩下电荷和质量的差别。
Now, size.
再说大小。
Across a period, atoms get smaller.
沿着一个周期从左到右，原子变小。
Each step adds a proton, so the nuclear charge grows, but the new electron joins the same outer shell and the shielding hardly changes.
每往前一格就多一个质子，核电荷增大， 而新增的电子进入同一个外层，屏蔽几乎不变。
The stronger pull draws that shell inwards.
更强的吸引把外层拉得更近。
Down a group, atoms get larger, because each step adds a whole new shell.
沿着一个族往下，原子变大，因为每往下一格就多一整个电子层。
Ions follow from that. A positive ion is smaller than its atom — it has lost a shell. A negative ion is larger, because the extra electrons repel each other.
离子也是同样的道理： 正离子比它的原子小——它失去了一个电子层；负离子比它的原子大，因为多出的电子互相排斥。
Atomic radius is the size of an atom.
原子半径是原子的大小。
Ionic radius is the size of an ion.
离子半径是离子的大小。
Across a period left to right, radius shrinks: nuclear charge rises while shielding by inner shells stays about the same, so the outer shell is pulled in.
沿周期从左到右，半径缩小： 核电荷增大，而内层屏蔽大致不变，外层被拉近。
Down a group top to bottom, radius grows: each step adds a new shell, so outer electrons sit further out and feel more shielding.
沿族从上到下，半径增大： 每往下一格就多一层壳，外层电子更远，屏蔽也更强。
Name nuclear charge, distance, and shielding in every radius explanation — examiners look for those three words.
解释半径时一定要写出核电荷、距离和屏蔽——阅卷人就盯着这三个词。
A positive ion is a cation.
正离子叫阳离子。
It is smaller than its atom because it has lost its outer shell, and the electrons that remain each feel a stronger pull from the same number of protons.
它比对应的原子小，因为失去了外层，剩下的电子在相同质子数下 各自感受到更强的吸引。
A negative ion is an anion.
负离子叫阴离子。
It is larger than its atom because it has gained electrons, so there is more repulsion between the electrons in the outer shell.
它比对应的原子大，因为得到了电子， 外层电子之间的排斥更强。
Among ions that have the same number of electrons — we call them isoelectronic — the one with more protons is smaller.
电子数相同的离子——我们叫它们等电子体—— 质子更多的那个更小。
More nuclear charge pulls the same electron cloud in tighter.
更强的核电荷把同一团电子云拉得更紧。
Isotopes are atoms of the same element, with the same number of protons but a different number 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。
Chlorine has two common ones: chlorine thirty-five and chlorine thirty-seven.
氯有两种常见的同位素： 氯三十五和氯三十七。
Both have seventeen protons.
两者都有十七个质子。
Chemistry depends on the electrons, and isotopes have identical electrons, so they react in exactly the same way.
化学性质取决于电子， 而同位素的电子完全相同，所以它们的反应方式一模一样。
Only properties that depend on mass differ — the heavier isotope is denser.
只有依赖质量的性质才会不同——较重的同位素密度更大。
Here are chlorine thirty-five and chlorine thirty-seven side by side.
这是氯三十五和氯三十七并排比较。
Both have seventeen protons, so both are chlorine.
两者都有十七个质子，所以都是氯。
One has eighteen neutrons, the other twenty.
一个有十八个中子，另一个有二十个。
So they share the same proton number Z but differ in nucleon number A.
所以它们质子数相同，核子数不同。
Chemical properties depend on the electrons, especially the outer ones.
化学性质取决于电子，尤其是外层电子。
Isotopes of one element have the same number of electrons arranged the same way, so they have the same chemical properties.
同一元素的同位素电子数相同、排布相同， 所以化学性质相同。
Some physical properties depend on mass — density is the classic example.
有些物理性质依赖质量——密度是经典例子。
The heavier isotope is denser.
较重的同位素密度更大。
The syllabus limits that difference to mass and density.
考纲把这种差别限制在质量和密度上。
We write an isotope as A on top, Z below, then the element symbol. A is the nucleon number on top; Z is the proton number below.
我们写同位素时，上面是核子数 A，下面是质子数 Z，然后是元素符号。
For chlorine, one isotope is thirty-five over seventeen C L, and the other is thirty-seven over seventeen C L.
对氯来说，一种是三十五在上、十七在下的氯，另一种是三十七在上、十七在下的氯。
Always check that Z matches the element: seventeen is chlorine, not anything else.
一定要核对 Z 是否对应正确的元素：十七是氯，不是别的。
If the numbers disagree with the Periodic Table, the symbol is wrong.
如果数字和元素周期表对不上，符号就错了。
Neutrons are A minus Z — eighteen for chlorine thirty-five, twenty for chlorine thirty-seven.
中子数等于 A 减 Z—— 氯三十五是十八个，氯三十七是二十个。
Before we unpack shells further, look at this evidence.
在进一步拆解电子层之前，先看这条证据。
Hydrogen emits light only at discrete, characteristic wavelengths — its line emission spectrum shows four bright lines on black in the visible region.
氢只在分立的、特征的波长上发光—— 它的线状发射光谱在可见区是黑底上四条亮线。
Continuous rainbow light would mean any energy is allowed.
如果是连续的彩虹光， 就意味着任何能量都允许。
Sharp lines mean only certain energy jumps are allowed.
尖锐的谱线意味着只有某些能量跃迁是允许的。
Electrons sit at set energy levels, not at any energy they like.
电子待在确定的能级上，而不是任意能量。
That is why we talk about shells and sub-shells with fixed energies, not a blur of continuous possibilities.
所以我们谈的是能量固定的电子层和亚层， 而不是一片连续的可能。
Each shell is labelled by the principal quantum number n.
每个电子层用主量子数 n 来标记。
n equals one, two, three, and so on.
n 等于一、二、三，依此类推。
A larger n means a shell that is further from the nucleus and higher in energy.
n 越大，电子层离核越远，能量越高。
Shell one is closest and lowest.
第一层最近、最低。
Shell two is further out.
第二层更远。
Shell three is higher still.
第三层更高。
Electrons fill lower shells first when they can.
电子能填低层时就先填低层。
Remember: shells split into sub-shells, and sub-shells are built from orbitals.
记住：电子层再分成亚层，亚层由轨道组成。
We need all three layers — shell, sub-shell, orbital — to write configurations correctly.
写电子排布时，壳层、亚层、轨道这三层都要清楚。
Shells are not as simple as they look.
电子层并不像看上去那么简单。
Each shell splits into sub-shells, named s, p and d.
每个电子层又分成亚层，分别叫 s、p 和 d。
Every sub-shell is built from orbitals, and one orbital holds at most two electrons.
每个亚层由轨道组成，一个轨道最多容纳两个电子。
An s sub-shell has one orbital, so it holds two electrons. A p sub-shell has three orbitals, so six. A d sub-shell has five, so ten.
s 亚层有一个轨道，所以容纳两个电子； p 亚层有三个轨道，所以是六个；d 亚层有五个，所以是十个。
The shapes matter too: an s orbital is a sphere, and a p orbital is a dumbbell, with three of them pointing at right angles to each other.
形状也很重要：s 轨道是一个球，p 轨道是一个哑铃，三个 p 轨道互相垂直。
An s orbital is a sphere centred on the nucleus.
s 轨道是以原子核为中心的球体。
A p orbital has two lobes, like a dumbbell, pointing along one axis.
p 轨道有两个瓣，像哑铃，沿一个轴指向。
The three p orbitals point along three directions at right angles — the x, y and z axes.
三个 p 轨道沿三个互相垂直的方向——x、y 和 z 轴。
You do not need fancy drawings in the exam, but you must know sphere versus dumbbell, and that the three p orbitals are mutually perpendicular.
考试里不需要精美的图， 但你必须知道球和哑铃的区别，以及三个 p 轨道彼此垂直。
One orbital still holds at most two electrons, whatever the shape.
无论什么形状，一个轨道最多仍只容纳两个电子。
Shape is about where the electron is likely to be found; capacity is always two.
形状说的是电子可能在哪里被找到； 容量始终是二。
Electrons always fill the lowest-energy sub-shell first.
电子总是先填能量最低的亚层。
Here is the order for the first four shells. One s, two s, two p, three s, three p — and then a surprise. Four s comes before three d.
这是前四个电子层的填充顺序： 一 s、二 s、二 p、三 s、三 p——然后是一个意外：四 s 排在三 d 前面。
Four s sits slightly lower in energy, so it fills first.
四 s 的能量略低，所以它先被填满。
That one crossing causes most of the mistakes students make in this topic.
就是这一处交叉， 造成了学生在这个专题里绝大多数的错误。
Read the ladder from bottom to top: one s, two s, two p, three s, three p, then four s just below three d, then three d, then four p.
从下往上读这把梯子：一 s、二 s、二 p、三 s、三 p，然后四 s 刚好在三 d 下面， 再是三 d，再是四 p。
Electrons fill lowest energy first.
电子先填最低能量。
The surprise is four s slightly below three d, so four s fills first.
意外之处是四 s 略低于三 d，所以四 s 先填。
Say the full order out loud until it is automatic: one s less than two s less than two p less than three s less than three p less than four s less than three d less than four p.
把完整顺序大声背到自动化：一 s 小于二 s 小于二 p 小于三 s 小于三 p 小于四 s 小于三 d 小于四 p。
That order is the ground-state filling rule for the atoms you meet at this level.
这个顺序就是本阶段你遇到的原子的基态填充规则。
An electronic configuration lists how many electrons sit in each sub-shell.
电子排布式列出每个亚层里有多少个电子。
Take iron, which has twenty-six electrons.
以铁为例，它有二十六个电子。
Fill them in energy order, and you get this.
按能量顺序填进去，就得到这个式子。
You can shorten it using the nearest noble gas in square brackets: argon, then three d six, four s two.
你可以用最近的稀有气体加方括号来简写： 氩，然后是三 d 六、四 s 二。
Now the trap. When a transition metal forms a positive ion, it loses its four s electrons first, before any three d electron.
现在说那个陷阱：当过渡金属形成正离子时， 它先失去四 s 电子，然后才轮到三 d 电子。
Iron atom is one s two, two s two, two p six, three s two, three p six, three d six, four s two — or shorthand argon, three d six, four s two.
铁原子是一 s 二、二 s 二、二 p 六、三 s 二、三 p 六、三 d 六、四 s 二—— 或简写为氩、三 d 六、四 s 二。
That lowest-energy arrangement is the ground state.
这种最低能量排布就是基态。
For ions, add or remove electrons.
对离子，要加上或去掉电子。
Iron three plus has lost three electrons.
铁的正三价失去了三个电子。
Remove the two four s electrons first, then one three d electron.
先去掉两个四 s 电子， 再去掉一个三 d 电子。
So iron three plus is argon, three d five — not argon, three d three, four s two.
所以铁正三价是氩、三 d 五——不是氩、三 d 三、四 s 二。
The filling order and the removal order are different.
填充顺序和失去顺序不同。
That is the trap examiners love.
这就是阅卷人最爱的陷阱。
We can also draw each orbital as a box, and each electron as an arrow.
我们也可以把每个轨道画成一个方框，把每个电子画成一支箭。
Two rules.
有两条规则。
First, two electrons in the same box must point opposite ways — they have opposite spin.
第一，同一个方框里的两个电子必须方向相反——它们的自旋相反。
Second, within a sub-shell, electrons go into empty orbitals one at a time, with parallel arrows, before any orbital takes a second electron.
第二，在同一个亚层里，电子先一个一个地进入空轨道，箭头方向相同， 然后才会有轨道容纳第二个电子。
Spreading out keeps the negative electrons apart.
分散开可以让带负电的电子彼此远离。
Nitrogen shows both rules at once.
氮同时体现了这两条规则。
Here is nitrogen: one s two, two s two, two p three.
这是氮：一 s 二、二 s 二、二 p 三。
The one s and two s boxes each hold a pair of opposite arrows — opposite spin in a shared orbital.
一 s 和二 s 方框里各有一对方向相反的箭—— 共用轨道里自旋相反。
The three two p boxes each hold one upward arrow.
三个二 p 方框里各有一支向上的箭。
No pairing in two p yet, because there are three orbitals and only three electrons.
二 p 里还没有成对， 因为有三个轨道、只有三个电子。
Electrons fill from low energy to high energy because that gives the most stable, lowest-energy atom.
电子从低能量填到高能量，因为这样得到最稳定、 能量最低的原子。
Within a sub-shell they spread out singly first to reduce repulsion between negative electrons.
在亚层内先单个铺开，以减少负电子之间的排斥。
That is why the configuration takes this shape.
这就是排布呈现这种形状的原因。
A free radical is a species with one or more unpaired electrons.
自由基是带有一个或多个未成对电子的微粒。
Look at those three single arrows in nitrogen's two p boxes — each is an unpaired electron.
看氮的二 p 方框里那三支单独的箭—— 每一支都是一个未成对电子。
Free radicals are very reactive, because an unpaired electron is hungry to pair up.
自由基非常活泼，因为未成对电子渴望成对。
You will meet free radicals again in organic mechanisms, but the definition starts here: count unpaired electrons in the box diagram.
你在有机反应机理里还会再遇到自由基，但定义从这里开始：在方框图里数未成对电子。
One or more unpaired means a free radical.
有一个或多个未成对，就是自由基。
Paired arrows in every box means no free-radical character from those orbitals.
每个方框里都是成对的箭， 就没有来自这些轨道的自由基特征。
Now, ionisation energy.
现在讲电离能。
The first ionisation energy is the energy needed to remove one electron from each atom in one mole of gaseous atoms, making one mole of gaseous plus one ions.
第一电离能是指：把一摩尔气态原子中每个原子的一个电子移走， 生成一摩尔气态正一价离子所需要的能量。
Say gaseous, or you lose the mark: we want free atoms, with no forces between them.
一定要说"气态"，否则会失分： 我们要的是自由原子，粒子之间没有作用力。
You can keep going.
还可以继续往下电离。
The second ionisation energy takes an electron off the plus one ion, and every step is harder, because the ion is more positive each time.
第二电离能是从正一价离子上再拿走一个电子，而且一级比一级难， 因为离子每一次都带更多的正电荷。
Write the first ionisation energy as a chemical equation.
把第一电离能写成化学方程式。
Element X as a gas goes to X plus as a gas, plus one electron.
气态元素 X 变成气态的 X 正离子，加上一个电子。
The state symbol g in brackets shows each species is a gas.
括号里的气态符号表示每一种微粒都是气体。
We use gaseous atoms so there are no forces between the particles.
我们用气态原子，是为了粒子之间没有作用力。
The unit is kilojoules per mole.
单位是千焦每摩尔。
The second ionisation energy starts from the plus one ion as a gas and makes the plus two ion as a gas, plus another electron.
第二电离能从气态正一价离子出发，生成气态正二价离子，再加一个电子。
These are successive ionisation energies.
这些就是逐级电离能。
Each is larger than the one before, because every electron is pulled away from a more positive ion.
每一级都比前一级大，因为每一个电子都是从更正的离子上被拉走的。
Every ionisation-energy question comes back to the same three factors.
每一道电离能的题目，最后都回到同样的三个因素。
Nuclear charge: more protons pull harder, so the energy is higher.
核电荷：质子越多，吸引越强， 电离能越高。
Distance: the further the outer electron sits from the nucleus, the weaker the pull, so the energy is lower.
距离：外层电子离原子核越远，吸引越弱，电离能越低。
And shielding: inner shells block part of the pull, so more inner shells means a lower energy.
屏蔽：内层电子挡住了一部分吸引，内层越多，电离能越低。
Name all three, and explain each one.
三个因素都要写出来，而且每一个都要解释清楚。
There is a smaller fourth effect too: spin-pair repulsion.
还有一个较小的第四个效应：自旋成对排斥。
When two electrons share one orbital, they push each other a little, so one is easier to remove.
当两个电子共用一个轨道时， 它们会稍稍互相推开，所以其中一个更容易被移走。
That is why some ionisation energies dip even when nuclear charge is rising.
这就是为什么有些电离能 在核电荷仍在增大时还会下凹。
You already saw the idea in the box diagram — paired arrows in one box mean two electrons in the same orbital.
你在方框图里已经见过这个想法—— 同一个方框里成对的箭，表示两个电子在同一轨道。
Name the three big factors first, then add spin-pair repulsion when a dip or a Group fifteen to Group sixteen comparison needs it.
先写三个大因素， 当需要解释下凹或第十五族到第十六族的比较时，再加上自旋成对排斥。
Across a period, first ionisation energy generally rises: nuclear charge grows while shielding stays about the same, so outer electrons are held more tightly.
沿一个周期，第一电离能总体上升：核电荷增大而屏蔽大致不变，外层电子被抓得更紧。
Down a group, first ionisation energy falls.
沿一个族往下，第一电离能下降。
Lower elements have more shells, so more shielding and a larger radius, and the outer electron is easier to remove.
下面的元素电子层更多，所以屏蔽更强、半径更大， 外层电子更容易被移走。
Always say which factor wins.
一定要说明哪个因素占上风。
Down a group, distance and shielding beat the extra protons.
沿族往下，距离和屏蔽压过了多出的质子。
Across a period, nuclear charge usually wins — until a sub-shell effect causes a small dip.
沿周期，核电荷通常占上风——直到亚层效应造成小的下凹。
Plot the first ionisation energy across Period Three.
把第三周期各元素的第一电离能画成图。
Overall it rises, because the nuclear charge grows while the shielding stays about the same.
总体上它是上升的， 因为核电荷增大而屏蔽基本不变。
But the rise is not smooth.
但上升并不平滑。
It dips at aluminium, because aluminium's outer electron comes from a higher three p sub-shell instead of a full three s.
在铝这里出现下凹， 因为铝失去的外层电子来自能量更高的三 p 亚层，而不是填满的三 s。
And it dips again at sulfur, where two electrons now share one three p orbital and push each other apart.
在硫这里又一次下凹，因为此时有两个电子共用一个三 p 轨道，互相排斥。
Those two dips are the evidence that sub-shells are real.
这两处下凹，正是亚层真实存在的证据。
Read the Period Three graph carefully.
仔细读第三周期的图。
From sodium to magnesium the value rises.
从钠到镁数值上升。
Then it dips at aluminium — that is Group two to Group thirteen.
然后在铝处下凹——那是第二族到第十三族。
The electron removed from aluminium comes from a three p sub-shell, higher in energy than the full three s sub-shell in magnesium, so it is easier to remove.
铝移走的电子来自三 p 亚层，能量高于镁中填满的三 s 亚层，所以更容易移走。
The climb continues through silicon and phosphorus, then dips again at sulfur — Group fifteen to Group sixteen.
经硅和磷继续上升，然后在硫处再次下凹——第十五族到第十六族。
In sulfur, one three p orbital now holds a pair, and spin-pair repulsion makes one electron easier to remove.
在硫里，一个三 p 轨道现在成对，自旋成对排斥让其中一个电子更容易移走。
These dips are evidence that sub-shells exist.
这些下凹是亚层真实存在的证据。
Learn the same idea for Period Two.
第二周期也要学会同样的想法。
The first ionisation energy of oxygen is below that of nitrogen.
氧的第一电离能低于氮。
Nitrogen has three unpaired two p electrons — one in each two p orbital.
氮有三个未成对的二 p 电子—— 每个二 p 轨道各一个。
Oxygen has four two p electrons, so one orbital must hold a pair.
氧有四个二 p 电子，所以有一个轨道必须成对。
Those paired two p electrons repel, so one is easier to remove.
那对成对的二 p 电子互相排斥，所以其中一个更容易被移走。
That is the exact reason examiners want: oxygen's paired two p electrons repel.
这就是阅卷人要的准确理由：氧的成对二 p 电子互相排斥。
Do not just say oxygen is more reactive.
不要只说氧更活泼。
Say the sub-shell and the spin-pair repulsion.
要说出亚层和自旋成对排斥。
And now the answer to our opening question.
现在来回答开头的问题。
Take sodium, and pull off its eleven electrons one at a time. Plot every value.
以钠为例，把它的十一个电子一个一个地拿走， 把每一级的数值都画出来。
The values rise, but twice they jump — enormously.
数值一路上升，但有两处出现了巨大的跳跃。
A big jump happens when the next electron must come from a shell closer to the nucleus.
每一次大跳跃，都是因为下一个电子必须来自更靠近原子核的电子层。
Count the points between the jumps: one, then eight, then two.
数一数两次跳跃之间有几个点：一个、八个、两个。
That is sodium's shell structure, read straight off a graph, without ever seeing an atom.
这就是钠的电子层结构——直接从图上读出来，而从未真正看见过原子。
This log-scale graph shows all eleven successive ionisation energies of sodium.
这张对数刻度图显示了钠的全部十一级逐级电离能。
The big jumps split the points into groups of one, then eight, then two — the two, eight, one shell structure written from the outside in as you remove electrons.
大跳跃把点分成一组一个、 然后八个、然后两个——从外向内移走电子时读到的二、八、一壳层结构。
Count how many electrons come off easily before the first big jump — that is the number of electrons in the outer shell, which tells you the group.
数一数第一次大跳跃之前轻易移走了几个电子——那就是外层电子数，也告诉你族数。
You can also use the whole pattern to work out the electronic configuration and the position of the element in the Periodic Table.
你还可以用整个图案推出电子排布和元素在周期表中的位置。
Memorise the mapping.
记住对应关系。
If one electron is removed before the first big jump, the element is in Group one.
如果第一次大跳跃之前只移走一个电子，元素在第一族。
If two electrons come off easily before the jump, it is Group two.
如果跳跃前轻易移走两个，是第二族。
If three come off before the jump, it is Group thirteen — not Group three, because the modern numbering jumps after the alkaline earths into the boron group.
如果跳跃前移走三个，是第十三族—— 不是第三族，因为现代编号在碱土金属之后跳到硼族。
Always count electrons removed before the jump, and explain the jump properly: the next electron is pulled from a shell closer to the nucleus, not from a different element.
一定要数跳跃之前移走的电子，并正确解释跳跃：下一个电子是从更靠近原子核的壳层拉出的， 而不是来自另一种元素。
Your turn.
轮到你了。
The first five ionisation energies of an element are five hundred and ninety, one thousand one hundred and fifty, four thousand nine hundred and forty, six thousand four hundred and eighty, and eight thousand one hundred and twenty.
某元素的前五级电离能分别是五百九十、一千一百五十、四千九百四十、 六千四百八十、八千一百二十。
Which group is it in?
它属于第几族？
Pause and try it yourself.
先暂停，自己试一试。
Look for the jump, not the biggest number.
要找的是跳跃，而不是最大的数值。
From the second value to the third, it more than quadruples.
从第二级到第三级，数值增大了四倍多。
Two electrons came off easily, so the outer shell holds two — Group Two.
有两个电子很容易被移走，所以外层有两个电子——第二族。
Three marks students throw away.
三个学生常丢的分。
First, define isotopes in full — same protons, different neutrons. The mark scheme wants both halves.
第一，同位素的定义要写完整——质子数相同，中子数不同， 评分标准两半都要。
Second, four s fills before three d, but four s electrons are removed first, so an iron three plus ion ends in three d five.
第二，四 s 先于三 d 被填充，但形成离子时先失去四 s 电子， 所以铁的正三价离子最后是三 d 五。
Third, in successive ionisation energies, count the electrons removed before the big jump, and say the next electron comes from a shell closer to the nucleus.
第三，在逐级电离能中， 要数出大跳跃之前移走了几个电子，并说明下一个电子来自更靠近原子核的电子层。
Get these right, and this topic is yours.
把这些做对，这个专题就是你的了。
One last habit.
最后一个习惯。
Explain every ionisation-energy trend with the same three factors: nuclear charge, distance, and shielding.
解释每一道电离能趋势题，都用同样的三个因素：核电荷、距离和屏蔽。
Add sub-shell effects for the small dips — three p higher than three s, or spin-pair repulsion in a shared orbital.
小的下凹加上亚层效应——三 p 高于三 s，或共用轨道里的自旋成对排斥。
Add the big-jump idea for successive ionisation energies.
逐级电离能再加上大跳跃的想法。
If your answer names the factors and links each one to higher or lower energy, you will pick up the explanation marks even when the graph looks messy.
如果你的答案点出因素，并把每一个与更高或更低的能量联系起来， 即使图看起来乱，你也能拿到解释分。
Practise saying the three factors out loud until they come automatically.
练习大声说出这三个因素，直到脱口而出。