古希臘哲學家、
19世紀的貴格會成員，
與諾貝爾獎科學家的共同點是甚麼？
儘管他們相隔超過2400年，
他們的貢獻都為了這個永恆的問題：
物質是由甚麼所組成的？
德謨克利特在公元前440年首次提出：
這世界上的一切都由微小粒子所組成，
粒子的四周都是空的。
他甚至推測粒子依它們組成的物質不同，
而有不同的大小與形狀。
他稱這些粒子為「atomos」-- 希臘語中的「不可分割」之意。
他的觀點不被當時 較受歡迎的哲學家接受，
例如亞里士多德就完全不同意，
他認為物質是由四個要素組成：
土、風、水、火，
其後大多數科學家紛紛遵從。
原子觀念被所有人遺忘，直至1808年，
一個名為約翰•道耳頓的貴格會老師， 他試圖挑戰亞里士多德的理論。
不同於德謨克利特的原子論是純粹的理論，
道爾頓展現同種物質都可被分解成
相同的比例的數種元素。
他的結論是不同的化合物
是由不同原子的元素所組合
每種元素的原子都有 特定的大小與質量，
而且原子既不能被創造、也不能被消滅。
雖然他的研究得到許多讚譽，
作為貴格會員，道爾頓 一生一直保持謙虛低調。
現在原子論被科學界接受了，
但接下來的重大進展，
卻直至近一個世紀之後才發生：
1897年物理學家J•J湯木生發現電子。
在我們可以稱之為巧克力餅乾模型原子，
湯木生的原子是負電荷的電子，
均勻分布在球狀正電雲中。
湯木生因發現電子 而榮獲1906年諾貝爾物理獎。
但他的原子模型沒有維持很久，
這是因為他有一些非常聰明的學生，
而歐尼斯特•拉塞福必列名其中，
他被稱為原子核物理學之父。
當研究X射線對氣體的影響時，
拉塞福決定更進一步研究原子，
他用帶正電荷的α粒子轟擊金箔。
在湯木生的原子模式中，
原子中均勻分布的正電荷
並不足以使α粒子發生偏轉。
結果應像是以一堆網球
射向一張薄薄的紙屏。
結果是：雖然大多數的顆粒都穿過金箔，
但是一些被反彈回來，
表示所金箔片更像有大網眼的厚網，
拉塞福的結論是原子中除了幾個電子外，
大部分空的，
大部分的質量集中在中央，
他稱之為原子核。
多數α粒子由原子中空部位通過，
但撞到帶正電荷原子核的就被散射開。
但原子論尚未完備，
1913年，湯木生的另一個學生尼爾斯•波耳
對拉塞福的核模型進行擴展修正。
他延續早期馬克斯•普朗克和 愛因斯坦的研究成果，
他提出：電子繞核的軌道
有固定的半徑與能量，
電子能夠從一層跳到另一個， 但不能存在於各層間的空間。
玻爾的行星模型成為主流的學說，
但很快，它也遇到了一些困難。
實驗已經證明， 電子不僅僅是一顆顆的粒子，
同時也會表現出波的特性，
並非局限於空間中的特定位置。
因此海森堡並提出了 著名測不準(不確定性)原理，
表示當測量一個電子時，
不可能同時確定
電子的確切位置與速度
電子不能被精確定位，
但存在於可能位置的範圍內，
此觀念啟發了現今原子的量子模型，
那是一套迷人的理論， 具有全新的複雜性，
然而其意涵尚未被完全掌握。
雖然我們對原子的理解不斷的進化，
但原子的存在為基本事實，
讓我們放煙火慶祝原子學說的勝利！
電子在環繞原子能階之間轉移時，
它們以特定波長的光釋放或吸收能量，
導致所有我們看到奇妙的色光。
我們可以想像德謨克利特 從什麼地方看著煙火，
心滿意足於經過了兩千多年，
他的學說一直都是正確的。
原子研究歷史 - Theresa Doud
What do an ancient Greek philosopher and a 19th century Quaker have in common with Nobel Prize-winning scientists? Although they are separated over 2,400 years of history, each of them contributed to answering the eternal question: what is stuff made of? It was around 440 BCE that Democritus first proposed that everything in the world was made up of tiny particles surrounded by empty space. And he even speculated that they vary in size and shape depending on the substance they compose. He called these particles "atomos," Greek for indivisible. His ideas were opposed by the more popular philosophers of his day. Aristotle, for instance, disagreed completely, stating instead that matter was made of four elements: earth, wind, water and fire, and most later scientists followed suit. Atoms would remain all but forgotten until 1808, when a Quaker teacher named John Dalton sought to challenge Aristotelian theory. Whereas Democritus's atomism had been purely theoretical, Dalton showed that common substances always broke down into the same elements in the same proportions. He concluded that the various compounds were combinations of atoms of different elements, each of a particular size and mass that could neither be created nor destroyed. Though he received many honors for his work, as a Quaker, Dalton lived modestly until the end of his days. Atomic theory was now accepted by the scientific community, but the next major advancement would not come until nearly a century later with the physicist J.J. Thompson's 1897 discovery of the electron. In what we might call the chocolate chip cookie model of the atom, he showed atoms as uniformly packed spheres of positive matter filled with negatively charged electrons. Thompson won a Nobel Prize in 1906 for his electron discovery, but his model of the atom didn't stick around long. This was because he happened to have some pretty smart students, including a certain Ernest Rutherford, who would become known as the father of the nuclear age. While studying the effects of X-rays on gases, Rutherford decided to investigate atoms more closely by shooting small, positively charged alpha particles at a sheet of gold foil. Under Thompson's model, the atom's thinly dispersed positive charge would not be enough to deflect the particles in any one place. The effect would have been like a bunch of tennis balls punching through a thin paper screen. But while most of the particles did pass through, some bounced right back, suggesting that the foil was more like a thick net with a very large mesh. Rutherford concluded that atoms consisted largely of empty space with just a few electrons, while most of the mass was concentrated in the center, which he termed the nucleus. The alpha particles passed through the gaps but bounced back from the dense, positively charged nucleus. But the atomic theory wasn't complete just yet. In 1913, another of Thompson's students by the name of Niels Bohr expanded on Rutherford's nuclear model. Drawing on earlier work by Max Planck and Albert Einstein he stipulated that electrons orbit the nucleus at fixed energies and distances, able to jump from one level to another, but not to exist in the space between. Bohr's planetary model took center stage, but soon, it too encountered some complications. Experiments had shown that rather than simply being discrete particles, electrons simultaneously behaved like waves, not being confined to a particular point in space. And in formulating his famous uncertainty principle, Werner Heisenberg showed it was impossible to determine both the exact position and speed of electrons as they moved around an atom. The idea that electrons cannot be pinpointed but exist within a range of possible locations gave rise to the current quantum model of the atom, a fascinating theory with a whole new set of complexities whose implications have yet to be fully grasped. Even though our understanding of atoms keeps changing, the basic fact of atoms remains, so let's celebrate the triumph of atomic theory with some fireworks. As electrons circling an atom shift between energy levels, they absorb or release energy in the form of specific wavelengths of light, resulting in all the marvelous colors we see. And we can imagine Democritus watching from somewhere, satisfied that over two millennia later, he turned out to have been right all along.
授課教師
陳永忠 ycchen@thu.edu.tw