chemists understood that molecules were made of atoms

化學家了解，分子是由原子構成的

connected through chemical bonds.

原子之間由化學鍵連接

However, the three-dimensional shapes of molecules

但對分子的三維結構

were utterly unclear, since they couldn't be observed directly.

則一無所知，因為無法直接觀察到

Molecules were represented using simple connectivity graphs

以前用簡單的連接圖表示分子

like the one you see here.

就像圖上的這個

It was clear to savvy chemists of the mid-19th century

許多 19 世紀中期 聰明的化學家都清楚

that these flat representations couldn't explain

這種平面表示法，無法解釋

many of their observations.

許多他們觀察到的現象

But chemical theory hadn't provided a satisfactory explanation

但化學理論又不足以

for the three-dimensional structures of molecules.

對分子的三維結構提出滿意的解釋

In 1874, the chemist Van't Hoff published a remarkable hypothesis:

1874 年，化學家范特霍夫 (Van't Hoff) 發表一個驚人的假說：

the four bonds of a saturated carbon atom

飽和碳原子的四個鍵

point to the corners of a tetrahedron.

會指向四面體的四個角

It would take over 25 years

後來花了 25 年以上時間

for the quantum revolution to theoretically validate his hypothesis.

量子革命才用理論解釋了這個假說

But Van't Hoff supported his theory using optical rotation.

但是范特霍夫利用旋光效應 (optical rotation) 來支持他的理論

Van't Hoff noticed that only compounds containing a central carbon

范特霍夫發現只有以碳為中心

bound to four different atoms or groups

鍵結四種不同原子或化學基的 這類化合物

rotated plane-polarized light.

才會旋轉平面偏極光 (plane-polarized light)

Clearly there's something unique about this class of compounds.

顯然這類化合物有某些獨特的性質

Take a look at the two molecules you see here.

看看這裡的兩個分子

Each one is characterized by a central, tetrahedral carbon atom

每個的中心，都是四個鍵的碳

bound to four different atoms:

連接到四個不同的原子

bromine, chlorine, fluorine, and hydrogen.

溴 Br、氯 Cl、氟 F、氫 H

We might be tempted to conclude that the two molecules

如果我們只考慮組成的成分

are the same, if we just concern ourselves with what they're made of.

我們可能會認定這兩個分子相同

However, let's see if we can overlay the two molecules

然而，若想重合這兩個分子

perfectly to really prove that they're the same.

以完美證明它們是相同的

We have free license to rotate and translate both of the molecules

我們可任意旋轉、平移這兩個分子

as we wish. Remarkably though,

引人注意的是

no matter how we move the molecules,

不論我們怎麼移動這兩個分子

we find that perfect superposition is impossible to achieve.

我們發現，不可能完美重合

Now take a look at your hands.

現在看看你的手

Notice that your two hands have all the same parts:

請注意，你的兩隻手零件完全相同

a thumb, fingers, a palm, etc.

有姆指、其他手指、手掌等

Like our two molecules under study,

就像我們剛討論的那兩個分子

both of your hands are made of the same stuff.

你的雙手都由相同的東西組成

Furthermore, the distances between stuff in both of your hands are the same.

此外，各部位間的距離也都相同

The index finger is next to the middle finger,

食指在中指旁邊

which is next to the ring finger, etc.

中指旁邊是無名指等

The same is true of our hypothetical molecules.

我們假想的分子也是如此

All of their internal distances

所有的內部距離也都相同

are the same. Despite the similarities between them,

雖然它們非常相似

your hands, and our molecules,

你的手、還有這些分子

are certainly not the same.

確實不完全一樣

Try superimposing your hands on one another.

試著將你的雙手重疊

Just like our molecules from before,

就像剛剛分子那樣

you'll find that it can't be done perfectly.

你會發現，不可能完全重合

Now, point your palms toward one another.

現在，將你的手掌遙遙相對

Wiggle both of your index fingers.

動一下你的兩隻食指

Notice that your left hand looks as if it's looking

注意你的左手，看起來就好像是

in a mirror at your right.

透過鏡子在看著右手

In other words, your hands are mirror images.

換句話說，你的雙手是彼此的鏡像

The same can be said of our molecules.

我們的分子同樣如此

We can turn them so that one looks at the other

我們可以旋轉方向 讓一個分子看起來是另一個的鏡像

as in a mirror. Your hands - and our molecules -

你的手、還有我們的分子

possess a spatial property in common called chirality,

都擁有一種空間性質 一般稱為「手性」 (chirality)

or handedness.

或「掌性」 (handedness)

Chirality means exactly what we've just described:

手性正是我們剛剛所描述的

a chiral object is not the same as its mirror image.

手性物體與鏡像不同

Chiral objects are very special in both chemistry and everyday life.

手性的物體在化學和日常生活中 都非常特殊

Screws, for example, are also chiral.

比如說螺絲釘，也是手性的

That's why we need the terms right-handed and left-handed screws.

所以我們有左旋螺絲、右旋螺絲

And believe it or not, certain types of light

信不信由你，某些種類的光

can behave like chiral screws.

也有類似螺絲那樣的手性

Packed into every linear, plane-polarized beam of light

平面偏極光的每個光束中

are right-handed and left-handed parts

都有左旋和右旋的光

that rotate together to produce plane polarization.

它們會一起旋轉，產生平面偏極光

Chiral molecules, placed in a beam of such light,

手性分子若用這種光束照射

interact differently with the two chiral components.

兩種手性分子，會有不同反應

As a result, one component of the light gets temporarily slowed down

結果是其中一種光，相較於另一種

relative to the other. The effect on the light beam

會暫時減速，光束與原本相較

is a rotation of its plane from the original one,

偏極平面會產生旋轉

otherwise known as optical rotation.

也就是旋光效應

Van't Hoff and later chemists realized that the chiral nature

范特霍夫以及後來的化學家發現

of tetrahedral carbons can explain this fascinating phenomenon.

四面體碳原子的手性 可解釋這個有趣的現象

Chirality is responsible for all kinds of other fascinating effects

手性也在其他有趣效應中 扮演重要角色

in chemistry, and everyday life.

不論是化學、或日常生活

Humans tend to love symmetry

人類比較喜愛對稱

and so if you look around you, you'll find that chiral objects

因此如果你看看四周，你會發現

made by humans are rare.

人造的手性物體非常少見

But chiral molecules are absolutely everywhere.

但手性分子卻是無所不在

Phenomena as separate as optical rotation,

各種不同現象，如旋光效應、 像是旋光效應、

Screwing together furniture,

鎖傢俱的螺絲、

and clapping your hands

以及拍手

all involve this intriguing spatial property.

都和這令人著迷的空間性質有關