What is the shape of a molecule? - George Zaidan and Charles Morton

翻译人员: Gabriella Hu 校对人员: Claire Zhang

What is the shape of a molecule?

分子是什么形状？

Well, a molecule is mostly empty space.

分子内部大部分都是空的。

Almost all of its mass is concentrated

几乎所有的质量都集中在

in the extremely dense nuclei of its atoms.

原子极其致密的核上。

And its electrons,

而它的电子，

which determine how the atoms

则决定原子之间

are bonded to each other,

如何相互结合，

are more like clouds of negative charge

它们比较像带负电荷的云，

than individual, discrete particles.

而不是一颗颗单独的粒子。

So, a molecule doesn't have a shape

因此，分子并不像

in the same way that, for example,

雕塑一样

a statue has a shape.

有着特定的形状。

But for every molecule,

但是对于每一个分子

there's at least one way

至少存在一种方式

to arrange the nuclei and electrons

排列原子核和电子，

so as to maximize the attraction

使得相异电荷之间的

of opposite charges

吸引力最大化，

and minimize the repulsion

相同电荷之间的排斥力

of like charges.

最小化。

Now, let's assume that the only electrons

现在我们假设唯一与分子形状

that matter to a molecule's shape

有关的电子

are the outermost ones from each participating atom.

是每个原子最外层上的电子。

And let's also assume

同时我们也假设

that the electron clouds in between atoms,

在原子之间的电子云，

in other words, a molecule's bonds,

也就是分子键，

are shaped kind of like sausages.

形状像一根香肠。

Remember that nuclei are positively charged

记住原子核是带有正电荷的，

and electrons are negatively charged,

而电子是带负电的，

and if all of a molecule's nuclei

而且如果所有分子的核

were bunched up together

都堆积到了一起，

or all of its electrons were bunched up together,

或者它所有的电子都堆积到了一起，

they would just repel each other and fly apart,

它们会相互排斥并且飞散，

and that doesn't help anyone.

那就没什么用了。

In 1776, Alessandro Volta,

1776年，亚历山德罗·伏打

decades before he would eventually invent batteries,

在他发明电池数十年前

discovered methane.

发现了甲烷。

Now, the chemical formula of methane is CH4.

甲烷的化学式是CH4。

And this formula tells us

这个化学式告诉我们

that every molecule of methane

每个甲烷分子

is made up of one carbon and four hydrogen atoms,

都是由一个碳原子和四个氢原子构成的，

but it doesn't tell us what's bonded to what

但是它并没有告诉我们 哪个原子与哪个原子连在一起，

or how they atoms are arranged in 3D space.

或者在三维空间中这些原子是如何排列的。

From their electron configurations,

从它们的电子排布中

we know that carbon can bond

我们知道碳原子能与

with up to four other atoms

其他四个原子相连，

and that each hydrogen can only bond

而且每个氢原子只能与

with one other atom.

一个原子相连。

So, we can guess

因此我们能够猜测

that the carbon should be the central atom

碳原子应该是中心原子，

bonded to all the hydrogens.

与所有的氢原子相连。

Now, each bond represents

现在，每个键代表

the sharing of two electrons

共用两个电子，

and we draw each shared pair of electrons as a line.

我们用一条线表示一对共用电子。

So, now we have a flat representation

于是，我们现在得到了这个原子的

of this molecule,

平面示意图，

but how would it look in three dimensions?

但是在三维空间中它看起来是什么样的？

We can reasonably say

我们能够合理地说

that because each of these bonds

因为每个键都是

is a region of negative electric charge

一个带负电的区域，

and like charges repel each other,

同性互斥，

the most favorable configuration of atoms

最可能的原子构型

would maximize the distance between bonds.

应该使得键与键之间的距离最大。

And to get all the bonds

而要使得所有的键之间的

as far away from each other as possible,

距离尽可能地远，

the optimal shape is this.

最合适的形状是这样子的。

This is called a tetrahedron.

这叫做四面体。

Now, depending on the different atoms involved,

根据不同的原子，

you can actually get lots of different shapes.

你可以得到很多种不同的形状。

Ammonia, or NH3, is shaped like a pyramid.

氨，NH3，是金字塔形。

Carbon dioxide, or CO2, is a straight line.

二氧化碳，CO2，是直线形。

Water, H2O, is bent like your elbow would be bent.

水，H2O，像弯折的手臂。

And chlorine trifluoride, or ClF3,

而三氟化氯，ClF3，

is shaped like the letter T.

形状像字母T。

Remember that what we've been doing here

记住，我们现在做的

is expanding on our model of atoms and electrons

是根据我们的原子和电子模型

to build up to 3D shapes.

建立分子的三维形状。

We'd have to do experiments

我们必须通过做实验

to figure out if these molecules

来确认这些分子的形状

actually do have the shapes we predict.

是否和我们预测的一样。

Spoiler alert:

剧透一下：

most of the do, but some of them don't.

很多是符合的，但是也有一些与预测不符。

Now, shapes get more complicated

因为随着原子数目的增加，

as you increase the number of atoms.

分子的形状变得越来越复杂。

All the examples we just talked about

我们前面提到的所有例子

had one obviously central atom,

只有一个明显的中心原子。

but most molecules,

但是大部分的分子并没有，

from relatively small pharmaceuticals

小到药物小分子，

all the way up to long polymers

大到很长的多聚体，

like DNA or proteins, don't.

比如DNA，都没有中心原子。

The key thing to remember

最关键的是

is that bonded atoms will arrange themselves

参与连结的原子会自动排列，

to maximize the attraction between opposite charges

使得异种电荷之间的吸引力达到最大，

and minimize the repulsion between like charges.

同种电荷之间的排斥力达到最小。

Some molecules even have two or more

有些分子甚至有两种或者更多的

stable arrangements of atoms,

稳定的原子排列方式，

and we can actually get really cool chemistry

在不同的构型之间转换，

from the switches between those configurations,

我们会看到化学是多么酷炫，

even when the composition of that molecule,

即使分子的构成，

that's to say the number and identity of its atoms,

即其中原子的数量和种类

has not changed at all.

根本没有改变。