Atomic Structure and Properties
AP Chemistry Topic 1 9:19 English narration · English + 中文 subtitles burned in
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Transcript
A chemical equation counts particles. Two of this react with one of that.
化学方程式数的是粒子个数：两个这个和一个那个反应。
But atoms are far too small to count, and there is no instrument that will hand you a number of atoms.
但原子实在太小，根本数不过来， 也没有任何仪器能直接告诉你原子的个数。
So chemists do the only thing they can. They weigh them instead, and use a conversion.
所以化学家做了唯一能做的事： 改成称重，再用一个换算关系。
That conversion is the mole.
这个换算关系就是摩尔。
One mole is a fixed, enormous number of particles, and it is the bridge between the balance in the lab and the equation on the page.
一摩尔是一个固定的、极其巨大的粒子数目，它是实验室里的天平和纸上的方程式之间的桥梁。
Unit one builds the atom, and then reads the periodic table from it.
第一单元先把原子搭起来，然后再用它来读懂元素周期表。
Let's begin.
让我们开始吧。
One mole is Avogadro's number of particles. Six point zero two two, times ten to the twenty third.
一摩尔就是阿伏伽德罗常数个粒子：6.022 乘以 10 的 23 次方。
It is a counting word, like a dozen, only vastly bigger.
它只是一个计数用的词，就像"一打"，只是大得多。
What makes it useful is the molar mass, the mass of one mole in grams, which you read straight off the periodic table.
而让它真正好用的是摩尔质量，也就是一摩尔物质的克数，你可以直接从元素周期表上读出来。
Divide grams by molar mass and you get moles. Multiply moles by molar mass and you get grams back.
用以克为单位的质量除以摩尔质量，就得到摩尔数；用摩尔数乘以摩尔质量，就换回克。
Picture the mole as the hub of a wheel. Grams on one spoke, particles on another, gas volume on a third, concentration on a fourth.
把摩尔想象成一个轮毂：一根辐条通向克，另一根通向粒子数，第三根通向气体体积， 第四根通向浓度。
You never go from grams to particles directly.
你从来不会直接从克算到粒子数，永远要经过摩尔。
Work one.
做一道。
How many moles, and how many molecules, are in thirty six grams of water?
36 克水里有多少摩尔？ 有多少个分子？
Water has a molar mass of eighteen grams per mole.
水的摩尔质量是每摩尔 18 克。
So divide. Thirty six over eighteen gives two moles.
所以做除法：36 除以 18 得到 2 摩尔。
That is the first step, and it is always the first step.
这是第一步，而且永远都是第一步。
Now for molecules, multiply by Avogadro's number. Two times six point zero two two times ten to the twenty third gives about one point two times ten to the twenty fourth molecules.
现在求分子数，乘以阿伏伽德罗常数：2 乘以 6.022 乘以 10 的 23 次方， 大约是 1.2 乘以 10 的 24 次方个分子。
Notice the route. Grams, then moles, then particles.
注意这条路线： 克，然后摩尔，然后粒子数。
Never skip the middle.
中间那一步绝不能跳过。
Now, atoms of one element are not all identical.
现在要注意，同一种元素的原子并不都完全相同。
Isotopes are atoms of the same element with different numbers of neutrons. Same chemistry, different mass.
同位素是同一种元素、 但中子数不同的原子：化学性质相同，质量不同。
A mass spectrometer separates them by mass and draws a mass spectrum.
质谱仪按质量把它们分开，画出一张质谱图。
Read it with two rules, and they are the same two rules you will use again later today. The position of each peak gives the mass of that isotope. The height of each peak gives its relative abundance, meaning what fraction of the sample it is.
用两条规则来读它，而且今天稍后你还会再用到这同样的两条规则： 每个峰的位置给出那个同位素的质量；每个峰的高度给出它的相对丰度， 也就是它在样品中占的比例。
The atomic mass printed on the periodic table is an abundance-weighted average of the isotopes.
元素周期表上印的原子质量，是各同位素按丰度加权的平均值。
Weighted, not a plain average.
是加权平均，不是简单平均。
Multiply each isotope's mass by its fraction, then add. Take chlorine.
把每个同位素的质量乘以它所占的比例，再相加。
It is about seventy five point eight percent chlorine thirty five, and twenty four point two percent chlorine thirty seven.
以氯为例： 大约 75.8% 是氯 35，24.2% 是氯 37。
That gives twenty six point five, plus eight point nine five, which is thirty five point five.
35 乘以 0.758 得 26.5； 37 乘以 0.242 得 8.95。 相加得到 35.5。
Notice where that lands. Much closer to thirty five, because chlorine thirty five is far more abundant.
注意这个结果落在哪里： 它离 35 比离 37 近得多，因为氯 35 的丰度大得多。
A plain average would give thirty six, which is wrong.
简单平均会给出 36，那是错的。
That is also why periodic table values are not whole numbers.
这也是元素周期表上的数值不是整数的原因。
Next, going from measurements to a formula.
接下来，从测量数据得到化学式。
The percent composition of a compound is each element's share of the total mass.
一个化合物的百分组成， 是每种元素在总质量中所占的份额。
From that you can find the empirical formula, which is the simplest whole number ratio of atoms.
由它可以求出实验式，也就是原子个数的最简整数比。 三个步骤，永远不变。
Three steps, and they never change. One. Assume one hundred grams, so each percent becomes a mass in grams.
第一，假设有 100 克，这样每个百分数就变成以克为单位的质量。
Two. Convert each mass to moles by dividing by that element's molar mass. Three.
第二，把每个质量除以该元素的摩尔质量，换算成摩尔数。
Divide every answer by the smallest of them, and read off the ratio.
第三，把每个结果都除以其中最小的那个，读出比值。
Then the molecular formula is a whole number multiple of the empirical one, and you find that multiple by comparing the molar masses.
然后分子式就是实验式的整数倍，而这个倍数可以通过比较摩尔质量来求出。
Work it.
来做这道题。
A compound is forty percent carbon, six point seven percent hydrogen, and fifty three point three percent oxygen by mass.
一个化合物按质量计含 40% 碳、6.7% 氢、53.3% 氧。 假设有 100 克。
Assume one hundred grams, then convert to moles. Forty over twelve gives three point three three. Six point seven over one gives six point seven.
现在换算成摩尔数：40 除以 12 得 3.33；6.7 除以 1 得 6.7；53.3 除以 16 得 3.33。
Fifty three point three over sixteen gives three point three three.
现在都除以最小的那个，也就是 3.33，得到 1、2、1。
Now divide by the smallest.
所以实验式是 CH₂O。
That gives one, two, one, so the empirical formula is C H two O.
现在假设真实的摩尔质量是每摩尔 180 克。
Now suppose the real molar mass is one hundred and eighty grams per mole.
实验式单元的质量是 30， 180 除以 30 等于 6。
The empirical unit weighs thirty, and one hundred and eighty divided by thirty is six. Multiply everything by six, and you get glucose.
所以把每个数都乘以 6，得到 C₆H₁₂O₆，也就是葡萄糖。
One contrast before we move on.
在往下讲之前先做一个对比。
A pure compound has a fixed formula, because its atoms are locked in a fixed ratio.
纯化合物有固定的化学式，因为它的原子被锁定在固定的比例上。
A mixture does not.
混合物则不是。
A mixture has variable composition, and its components keep their own identities.
混合物的组成是可变的，而且它的各组分保持各自的性质。
You can have more salt or less salt in the same water.
同样的水里，盐可以多一点，也可以少一点。
So you cannot write a formula for a mixture. You describe it with fractions instead, the mass fraction or the mole fraction of each component.
所以你没法给混合物写化学式， 而要用分数来描述它：每个组分的质量分数或摩尔分数。
And spectroscopy can measure how much of each is present.
而光谱学可以测出每种组分各有多少。
Now inside the atom.
现在进到原子内部。
An atom is a tiny nucleus of protons and neutrons, and almost all the volume is empty space with electrons in it.
原子是一个由质子和中子组成的极小的原子核， 而几乎全部的体积都是里面装着电子的空隙。
Those electrons sit in shells, and each shell has subshells labelled s, p, d and f.
这些电子待在电子层里， 每个电子层又有标记为 s、p、d、f 的亚层。
The electron configuration lists how they fill, lowest energy first.
电子排布列出它们如何按能量由低到高填充。
Why do the energies differ?
那么能量为什么会不同？
Coulomb's law.
靠库仑定律。
An electron closer to the nucleus feels a stronger pull, so it is harder to remove.
离原子核更近的电子受到更强的吸引， 所以被抓得更牢，更难移走。
An electron further out, or shielded from the full nuclear charge, is held loosely.
离得更远的电子，或者被内层电子屏蔽、 感受不到全部核电荷的电子，就被抓得松。
Those loose outer ones are the valence electrons, and they do essentially all of the chemistry.
那些松散的外层电子就是价电子， 而化学反应基本上全靠它们。
Work one.
做一道。
Write the electron configuration of sulfur, which has sixteen electrons.
写出硫的电子排布，硫有 16 个电子。 按顺序填，边填边数。
Fill in order, and keep a running count. Two in the first s subshell, two in the second s, six in the second p. That is ten so far.
第一层 s 亚层 2 个，第二层 s 亚层 2 个，第二层 p 亚层 6 个，到这里是 10 个。
Two in the third s makes twelve, and four in the third p makes sixteen. Done. Now read the chemistry off it.
第三层 s 亚层 2 个，到 12 个；第三层 p 亚层 4 个，到 16 个。
The outermost shell is the third, and it holds two plus four, which is six valence electrons.
填完了。 现在从中读出化学性质：最外层是第三层，里面有 2 加 4，也就是 6 个价电子。
Six is two short of a full eight.
6 比满的 8 少 2。
So sulfur tends to gain two electrons, which is exactly why it forms an ion with a two minus charge.
所以硫倾向于得到 2 个电子， 这正是它会形成带 2 负电荷的离子的原因。
And there is an experiment that shows this directly.
而且有一个实验能直接把这一切显示出来。
Photoelectron spectroscopy measures the energy needed to remove electrons from each subshell.
光电子能谱测量的是 从每个亚层移走电子所需要的能量。
Here is the spectrum of neon.
这是氖的谱图。
Every peak is one subshell, and you read it with the same two rules as the mass spectrum.
谱图上的每一个峰对应一个亚层， 而你读它用的是和质谱完全相同的两条规则。
The position of a peak gives the binding energy, and binding energy rises toward the left, so a peak further left means an electron held more tightly, which means it came from a shell closer to the nucleus.
峰的位置给出结合能，而结合能是向左增大的，所以位置更靠左的峰意味着电子被抓得更牢， 也就意味着它来自更靠近原子核的电子层。
The height gives the number of electrons in that subshell.
峰的高度给出那个亚层里的电子个数。
The tallest peak here holds six, so that peak is the two p subshell.
这里最高的峰对应 6 个电子，所以那个峰就是 2p 亚层。
So you can read the whole configuration off the spectrum, and confirm the shell structure by experiment.
所以你可以直接从谱图上读出一种元素的完整电子排布， 用实验而不是靠规则来确认电子层结构。
Now the payoff.
现在是收获的时候。
Every periodic trend comes from just two competing effects. Nuclear charge, which pulls electrons in, and shielding by inner electrons, which pushes the effect of that charge back out.
每一条周期性趋势都只来自两个互相竞争的效应： 核电荷把电子往里拉，而内层电子的屏蔽把这个电荷的作用往外推。
Go across a period. The nuclear charge rises but the shielding barely changes, so the pull gets stronger.
沿着一个周期往右走：核电荷上升，而屏蔽几乎不变，所以吸引变强。
Go down a group. You add whole new shells, so the outer electrons sit further out and are better shielded.
沿着一个族往下走：你加上了全新的电子层，所以外层电子位置更靠外、被屏蔽得更好。
Now read the three trends.
现在读这三条趋势。
Atomic radius decreases across and increases down.
原子半径向右减小、向下增大。
Ionization energy, the energy to remove an electron, does the opposite. It increases across and decreases down.
电离能，也就是移走一个电子所需的能量，正好相反：向右增大、向下减小。
And electronegativity, the pull on shared electrons in a bond, increases across and up, toward fluorine.
而电负性，也就是对成键共用电子的吸引力，向右和向上增大，指向氟。
Finally, put the trends to work.
最后，把这些趋势用起来。
Atoms gain, lose or share valence electrons to reach a stable configuration.
原子通过得到、失去或共用价电子来达到稳定的电子排布。
Metals have low ionization energy and lose electrons easily, forming positive ions called cations.
金属在左边，电离能低，容易失去电子，形成带正电的离子，叫做阳离子。
Nonmetals are strongly electronegative and gain electrons, forming negative ions called anions.
非金属在右边，电负性强，会得到电子，形成带负电的离子，叫做阴离子。
Opposite charges attract, and the result is an ionic compound.
异性电荷相吸，结果就是离子化合物。
To write its formula, balance the charges so the whole thing is neutral.
要写出它的化学式， 就让电荷平衡，使整体呈电中性。
Aluminium forms a three plus ion and oxide is two minus.
铝形成 3 正的离子，氧化物是 2 负。
Two aluminiums give plus six, three oxides give minus six, they cancel, and the formula is aluminium two, oxygen three.
两个铝给出正 6，三个氧化物给出负 6，正好抵消，所以化学式是 Al₂O₃。
Three marks students lose.
学生最常丢分的三个地方。
First, always go through moles. Grams to moles to particles, never grams straight to particles.
第一，永远要经过摩尔：克到摩尔再到粒子数， 绝不能从克直接跳到粒子数。
Second, average atomic mass is abundance-weighted, so it sits near the more abundant isotope, not halfway.
第二，平均原子质量是按丰度加权的，所以它靠近丰度更大的那个同位素，而不是在正中间。
Third, when a question asks why a trend goes that way, answer with nuclear charge and shielding.
第三，当题目问某条趋势为什么是这样时，要用核电荷和屏蔽来回答。
Naming the trend is not explaining it.
把趋势的名字说出来并不等于解释它。