认识世界,从了解原子开始

认识世界，从了解原子开始

4种或5种元素构成——土、风、水、火（以及金，或意识），在古典时代，哲学家们开始建立理论学说，认为所有的物质实际上是由微小的、无形的、不可分割的原子构成的。

自从有人类文明开始，我们就一直在试图弄清楚，宇宙以及宇宙中一切是由什么构成的。古代的智者和哲学家们相信世界是由

从那时起，科学家们就开始针对原子展开一系列持续性的研究，希望揭开它神秘的面纱以及了解它的构成。到了20世纪，我们对原子的研究取得了进展，能够构建原子的精确模型。在过去的十几年里，我们对原子的了解更进一步，已经能够证实原子构成理论。

今天，原子的研究主要关注亚原子水平上物质结构和功能。不仅包括识别所有组成原子的亚原子粒子，而且还包括研究支配它们的基本力。这些基本力包括强核力，弱核力，电磁力和引力。接下来，我们要把我们目前所了解的原子情况进行分类讲解。

结构：

我们目前的原子模型分为三个组成部分：质子、中子和电子。质子带有单位正电荷、电子带有单位负电荷，中字没有电荷。根据粒子物理学的标准模型，质子和中子构成原子的原子核，电子围绕原子核做无规则运动。

由于电磁力的作用，原子中的电子被原子核中的质子吸引。电子只有受到外部能量的作用时，才会从它们的轨道上逃逸。电子距离原子核的轨道越近，吸引力越强；因此，电子逃逸就需要更强大的外部力。

电子在多个轨道上围绕原子核运转，每一个轨道上都有特定能量级的电子。电子通过吸收光子获取足够能量进入到新的量子态，从而可以改变状态达到更高的能量级。同样，高能量级的电子也可以通过辐射出光子、放出能量跃迁到低能量级。

当质子数与电子数相同时，这个原子就是电中性的。否则，就是带有正电荷或者负电荷的离子。距离原子核最远的电子可以转移到其它原子附近或成为原子之间的共享电子，原子能够与别的原子以共价键化合成为其它类型的化合物。

自然界中的基本粒子（电子）或复合粒子（质子和中子），这三类亚原子粒子都是费米子，费米子是自然界中构成物质的原材料。这意味着，电子有没有已知的内部结构，而质子和中子都是由其他亚原子粒子称为夸克构成的。原子中有两种类型的夸克，它们是基本电荷非整数的粒子。

质子是由两个“上型”夸克（每一个拥有+2/3电荷），和一个“下型”夸克（-1/3电荷）组成，而中子是由一个上型夸克和两个下型夸克组成。这种区别就是两类粒子电荷之间的差异，分别是+1 和0电荷，而电子的电荷是-1。

其它亚原子粒子包括轻子，它与费米子一道形成了物质的最基本构造块。在现有的原子模型中，一共存在六种类型的轻子：电子、μ子、τ粒子和与之相应的中微子。不同类型的轻子通常被称为“味”，通过它们的大小和电荷（能够影响其电磁相互作用水平）来加以区分。

还有一种规范玻色子，被称为“引力子”，是传递基本相互作用的媒介粒子。例如，胶子是一种负责传递强核力的玻色子，它们把夸克捆绑在一起。W 及 Z 玻色子是负责传递弱核力的基本粒子。光子是组成光的基本粒子，而希格斯玻色子则是用来解释W和Z玻色子非零质量的获得机制.

质量

组成原子的质子和中子构成了原子的绝大部分质量。电子是原子的组成粒子中质量最轻的，它的质量是9.11 x 10-31 kg，电子非常非常的小，就目前科学发展，电子仍观察不到。质子的质量是电子的1836倍，是1.6726×10-27kg，而中子是三个组成粒子中质量最大的，1.6929×10-27kg（是电子的质量1839倍)。

一个原子核（也称核子）中的质子和中子总数被称为质量数。例如，元素碳12的得名是因为它的质量数是12，源自其核子数是12(6个质子和6个中子)。元素也是基于其原子数进行排列的，与原子核中质子的数量形同。由此可知，碳的原子数为6。

原子的质量非常难测量，即使是质量最大的原子，它的“体重”也非常轻，没法用传统的重量单位来表示。因此，科学家们使用统一的原子质量单位u来表示，也被称为达尔顿（Da），它被定义为中性原子碳-12质量的十二分之一，大约相当于1.66×10-27千克。

化学家使用摩尔来表示物质的量，每1摩尔任何物质（微观物质，如分子，原子等）含有阿伏伽德罗常量（约6.02×10⊃2;⊃3;）个微粒。如果一个元素的一个原子质量是1 u，该元素的1摩尔原子的质量就接近一克。因为这个统一的原子质量单位的定义，因此每一个碳-12原子的原子质量是12 u，因此1摩尔碳-12原子的重力是0.012千克。

放射性衰变：

具有相同质子数的任何两个原子属于同种元素。但是具有相同数量质子的原子可以拥有数量不等的中子，这就是同一元素的不同同位素。这些同位素往往是不稳定的，所有原子序数大于82的原子都具有放射性。

当一个元素发生衰变时，其原子核通过辐射失去能量，辐射的粒子由α粒子（氦原子）β粒子（正电子）、γ射线（高频电磁能量）和内转换电子构成。不稳定元素衰变的速率被称为它的“半衰期”，放射性原子衰变至原来数量的一半所需的时间。

同位素的稳定性受质子与中子比值的影响。在地球上，339种不同类型的元素自然发生衰变，有254（约75%）被称为“稳定同位素”，即不衰变。另外34种放射性元素的半衰期超过8000万年，自太阳系早期就一直存在至今（因此他们被称为“原始元素”）。

最后，还有51种短生命周期元素是自然地发生衰变，作为其它元素衰变的“子元素”（例如核副产品）。此外，短生命周期放射性元素可能是地球上自然能量过程的结果，如宇宙射线的轰击（例如，发生在大气层中的碳-14）。

研究历史：

目前已知最早的原子理论出自古希腊和古印度，哲学家德谟克利特认为一切物质都是由微小的、不可分割的、不可毁灭的颗粒组成的。“原子”一词是由古希腊创造的，并产生了“原子论”学派。然而，这个理论更是一个哲学而非科学的概念。

直到19世纪，由于首个以证据为基础的实验，原子理论才明确地成为科学问题。例如，19世纪初，英国科学家约翰 道尔顿利用原子概念解释，为什么化学元素在某些可观察和可预测的方式下发生反应。

道尔顿首先研究了一个问题，为什么元素间的化合总是成整数和倍数的关系呢？道尔顿把这些事实总结概括加以分析，提出了物质是由具有一定质量的原子构成的。通过一系列的实验包括气体实验，道尔顿提出了著名的道尔顿原子论，这个著名的学说，成为现代物理和化学领域发展的基石。

这个理论可以归结为五个要点：元素（单质）是由被称为原子的粒子构成的；同一种元素的原子，其形状、质量和各种性质都是相同的；不同元素以其原子的质量为最基本的特征；元素的原子结合起来形成化合物；原子在化学反应中既不能被创造也不能被毁灭，它们在一切化学反应中保持其本性不变。

19世纪末，科学家们开始推测，原子是由一种以上基本单位构成的。大多数科学家直言，这个基本单位可能是已知最小的原子——氢的大小。然后，在1897年，利用阴极射线进行的一系列实验，物理学家汤普森宣布，他发现了一个基本单位，比氢原子的尺寸小1000倍，重量小1800倍。

他的实验也表明，它们与通过光电效应和放射性物质释放出的粒子是相同的。随后的实验也证明，这种基本单位携带电流穿过金属线，而且在原子内有负电荷。因此，这就是为什么这种粒子（一开始被命名为"corpuscle"），在1874年乔治 约翰斯通 斯托尼对它的预测之后，被改称为“电子”。

然而，汤姆森也假设电子分布与整个原子中（带正电荷的海洋里），创立了“梅子布丁模型。1909年，物理学家汉斯 吉格和厄内斯特 马斯登（在厄内斯特 卢瑟福的指导下）利用金属箔和α粒子进行了一项实验，证明了这个著名的“梅子布丁模型”是错误的。

与达尔顿的原子模型一致，他们认为α粒子穿过金属箔时会发生偏离。但是许多粒子偏离的角度大于90°。为了解释这个现象，卢瑟福提出原子的正电荷集中在原子中心很小的核内。

1913、物理学家尼尔斯 玻尔提出了一个模型，其中电子围绕原子核运转，但只能在有限的一组轨道内。他还提出，电子可以在轨道之间跃迁，只有在光子吸收或辐射一定的固定能量时才能做到。这不仅细化卢瑟福提出的模型，而且也将量子概念引入原子结构中。

随着质谱仪的发展（根据带电粒子在电磁场中能够偏转的原理，利用磁铁偏转离子束轨迹），能够更加精确地测量原子质量、化学家弗朗西斯威廉阿斯顿使用这个仪器显示出同位素具有不同的质量。1932年物理学家詹姆士 查德威克开展了后续工作，发现了中子，并且解释了为什么许多化学元素会有不同原子质量的许多同位素存在。

20世纪初，原子的量子理论得到进一步发展。1922年德国物理学家奥托·施特恩和瓦尔特·格拉赫进行了一项实验，实验中，一束银原子通过磁场逐渐分为两束运动，证实了原子角动量的量子化。

这就是著名的施特恩-格拉赫实验，实验中原子束分为两部分，取决于原子自旋方向是向上还是向下。1926年，物理学家欧文 薛定谔利用粒子的波动性学说开发了一个数学模型，认为电子是三维波形而不是单纯的粒子。

使用波形来描述粒子的结果是，在任意给定的时间里，不可能从数学上获得粒子的位置和动量准确值。同年，沃纳海森堡求解这个问题，将其称之为“不确定性原理”。根据海森堡的研究，对于一个给定位置的精确测量，我们只能获得一系列可能的动量值，反之亦然。

在上世纪30年代，由于奥托 哈恩、迈特纳和弗里斯的实验，物理学家发现了核裂变。哈恩的实验目的是希望创造一种超铀元素，涉及到将中子转移到铀原子上。相反，这一过程变成他的样本铀-92（Ur92）变为两个新的元素–钡（B56）和氪（Kr27）。

迈特纳和弗里斯证实了这项实验，并将其归因于铀原子的裂变，形成具有相同原子重量的两个元素，这个过程通过破坏原子键，释放了大量的能量。在随后的几年中，开始将这一过程进行军事化研究（即核武器），并由美国在1945年创造出了第一颗原子弹。

20世纪50年代，随着改进的粒子加速器和粒子探测器的发展，能够让科学家们研究高能量下原子运动的影响。由此，建立了粒子物理学的标准模型，这个模型目前已经成功解释了原子核的性质、亚原子粒子理论上的存在，以及支配其相互作用的力量。

现代实验：

20世纪下半叶以来，原子学说和量子力学领域产生了许多全新的、令人兴奋的科学发现。例如，对希格斯玻色子的漫长寻找取得了重大突破，2012年欧洲核子研究组织(CERN)的研究人员在瑞士宣布这个“上帝粒子”被发现。

在最近十年里，物理学家花费了大量的时间和精力发展统一场论（又名，大统一理论或万有理论）。从本质上说，自从标准模型首次提出以来，科学家们就试图了解宇宙的四个基本力（引力、强核力、弱核力和电磁力）如何一起发挥作用。

重力可以用爱因斯坦的相对论来解释，核力和电磁力可以用量子理论来解释，但是没有一种理论能够解释这四个基本力如何共同发挥作用。多年来，科学家们为了解决这个问题，提出了从弦理论到圈量子引力理论的一系列理论。到目前为止，这些理论都没有突破性进展。

我们对原子的认识经历了很长的一段路，从将它看作是一种惰性固体与其它原子机械地相互作用的经典模型，到原子是由行为不可预测的高能粒子组成的现代理论。虽然这些理论主导了几千年前，但是现在我们对所有物质基本结构的知识已经显著提升了。

然而，有关原子仍然存在许多尚未解决的谜团。随着时间的推移，和我们持续的努力，最终能够解开原子剩余的秘密。也许我们的新发现又会引发出更多的问题——这些新问题可能比之前困扰我们的难题更加棘手！

“英文原文”

What are the parts of an atom?

Since the beginning of time, human beings have sought to understand what the universe and everything within it is made up of. And while ancient magi and philosophers conceived of a world composed of four or five elements – earth, air, water, fire (and metal, or consciousness) – by classical antiquity, philosophers began to theorize that all matter was actually made up of tiny, invisible, and indivisible atoms.

Since that time, scientists have engaged in a process of ongoing discovery with the atom, hoping to discover its true nature and makeup. By the 20th century, our understanding became refined to the point that we were able to construct an accurate model of it. And within the past decade, our understanding has advanced even further, to the point that we have come to confirm the existence of almost all of its theorized parts.

Today, atomic research is focused on studying the structure and the function of matter at the subatomic level. This not only consists of identifying all the subatomic particles that are thought to make up an atom, but investigating the forces that govern them. These include strong nuclear forces, weak nuclear forces, electromagnetism and gravity. Here is a breakdown of all that we've come to learn about the atom so far…

Structure:

Our current model of the atom can be broken down into three constituents parts – protons, neutron, and electrons. Each of these parts has an associated charge, with protons carrying a positive charge, electrons having a negative charge, and neutrons possessing no net charge. In accordance with the Standard Model of particle physics, protons and neutrons make up the nucleus of the atom, while electrons orbit it in a "cloud".

The electrons in an atom are attracted to the protons in the nucleus by the electromagnetic force. Electrons can escape from their orbit, but only in response to an external source of energy being applied. The closer orbit of the electron to the nucleus, the greater the attractive force; hence, the stronger the external force needed to cause an electron to escape.

Electrons orbit the nucleus in multiple orbits, each of which corresponds to a particular energy level of the electron. The electron can change its state to a higher energy level by absorbing a photon with sufficient energy to boost it into the new quantum state. Likewise, an electron in a higher energy state can drop to a lower energy state while radiating the excess energy as a photon.

Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms that have either a deficit or a surplus of electrons are called ions. Electrons that are farthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism, atoms are able to bond into molecules and other types of chemical compounds.

All three of these subatomic particles are Fermions, a class of particle associated with matter that is either elementary (electrons) or composite (protons and neutrons) in nature. This means that electrons have no known internal structure, whereas protons and neutrons are made up of other subatomic particles. called quarks. There are two types of quarks in atoms, which have a fractional electric charge.

Protons are composed of two "up" quarks (each with a charge of +2/3) and one "down" quark (-1/3), while neutrons consist of one up quark and two down quarks. This distinction accounts for the difference in charge between the two particles, which works out to a charge of +1 and 0 respectively, while electrons have a charge of -1.

Other subatomic particles include Leptons, which combine with Fermions to form the building blocks of matter. There are six leptons in the present atomic model: the electron, muon, and tau particles, and their associated neutrinos. The different varieties of the Lepton particles, commonly called "flavors", are differentiated by their sizes and charges, which effects the level of their electromagnetic interactions.

Then, there are Gauge Bosons, which are known as "force carriers" since they mediate physical forces. For instance, gluons are responsible for the strong nuclear force that holds quarks together while W and Z bosons (still hypothetical) are believed to be responsible for the weak nuclear force behind electromagnetism. Photons are the elementary particle that makes up light, while the Higgs Boson is responsible for giving the W and Z bosons their mass.

Mass:

The majority of an atoms' mass comes from the protons and neutrons that make up its nucleus. Electrons are the least massive of an atom's constituent particles, with a mass of 9.11 x 10-31 kg and a size too small to be measured by current techniques. Protons have a mass that is 1,836 times that of the electron, at 1.6726×10-27 kg, while neutrons are the most massive of the three, at 1.6929×10-27 kg (1,839 times the mass of the electron).

The total number of protons and neutrons in an atoms' nucleus (called "nucleons") is called the mass number. For example, the element Carbon-12 is so-named because it has a mass number of 12 – derived from its 12 nucleons (six protons and six neutrons). However, elements are also arranged based on their atomic numbers, which is the same as the number of protons found in the nucleus. In this case, Carbon has an atomic number of 6.

The actual mass of an atom at rest is very difficult to measure, as even the most massive of atoms are too light to express in conventional units. As such, scientists often use the unified atomic mass unit (u) – also called dalton (Da) – which is defined as a twelfth of the mass of a free neutral atom of carbon-12, which is approximately 1.66×10-27kg.

Chemists also use moles, a unit defined as one mole of any element always having the same number of atoms (about 6.022×1023). This number was chosen so that if an element has an atomic mass of 1 u, a mole of atoms of that element has a mass close to one gram. Because of the definition of the unified atomic mass unit, each carbon-12 atom has an atomic mass of exactly 12 u, and so a mole of carbon-12 atoms weighs exactly 0.012 kg.

Radioactive Decay:

Any two atoms that have the same number of protons belong to the same chemical element. But atoms with an equal number of protons can have a different number of neutrons, which are defined as being different isotopes of the same element. These isotopes are often unstable, and all those with an atomic number greater than 82 are known to be radioactive.

When an element undergoes decay, its nucleus loses energy by emitting radiation – which can consist of alpha particles (helium atoms), beta particles (positrons), gamma rays (high-frequency electromagnetic energy) and conversion electrons. The rate at which an unstable element decays is known as its "half-life", which is the amount of time required for the element to fall to half its initial value.

The stability of an isotope is affected by the ratio of protons to neutrons. Of the 339 different types of elements that occur naturally on Earth, 254 (about 75%) have been labelled as "stable isotopes" – i.e. not subject to decay. An additional 34 radioactive elements have half-lives longer than 80 million years, and have also been in existence since the early Solar System (hence why they are called "primordial elements").

Finally, an additional 51 short-lived elements are known to occur naturally, as "daughter elements" (i.e. nuclear by-products) of the decay of other elements (such as radium from uranium). In addition, short-lived radioactive elements can be the result of natural energetic processes on Earth, such as cosmic ray bombardment (for example, carbon-14, which occurs in our atmosphere).

History of Study:

The earliest known examples of atomic theory come from ancient Greece and India, where philosophers such as Democritus postulated that all matter was composed of tiny, indivisible and indestructible units. The term "atom" was coined in ancient Greece and gave rise to the school of thought known as "atomism". However, this theory was more of a philosophical concept than a scientific one.

It was not until the 19th century that the theory of atoms became articulated as a scientific matter, with the first evidence-based experiments being conducted. For example, in the early 1800's, English scientist John Dalton used the concept of the atom to explain why chemical elements reacted in certain observable and predictable ways.

Dalton began with the question of why elements reacted in ratios of small whole numbers, and concluded that these reactions occurred in whole number multiples of discrete units—in other words, atoms. Through a series of experiments involving gases, Dalton went on to developed what is known as Dalton's Atomic Theory, which remains one of the cornerstones of modern physics and chemistry.

The theory comes down to five premises: elements, in their purest state, consist of particles called atoms; atoms of a specific element are all the same, down to the very last atom; atoms of different elements can be told apart by their atomic weights; atoms of elements unite to form chemical compounds; atoms can neither be created or destroyed in chemical reaction, only the grouping ever changes.

By the late 19th century, scientists began to theorize that the atom was made up of more than one fundamental unit. However, most scientists ventured that this unit would be the size of the smallest known atom – hydrogen. And then in 1897, through a series of experiments using cathode rays, physicist J.J. Thompson announced that he had discovered a unit that was 1000 times smaller and 1800 times lighter than a hydrogen atom.

His experiments also showed that they were identical to particles given off by the photoelectric effect and by radioactive materials. Subsequent experiments revealed that this particle carried electric current through metal wires and negative electric charges within atoms. Hence why the particle – which was originally named a "corpuscle" – was later changed to "electron", after the particle George Johnstone Stoney's predicted in 1874.

However, Thomson also postulated that electrons were distributed throughout the atom, which was a uniform sea of positive charge. This became known as the "plum pudding model", which would later be proven wrong. This took place in 1909, when physicists Hans Gieger and Ernest Marsden (under the direction of Ernest Rutherfod) conducted their experiment using metal foil and alpha particles.

Consistent with Dalton's atomic model, they believed that the alpha particles would pass straight through the foil with little deflection. However, many of the particles were deflected at angles greater than 90°. To explain this, Rutherford proposed that the positive charge of the atom is concentrated in a tiny nucleus at the center.

In 1913, physicist Niels Bohr proposed a model where electrons orbited the nucleus, but could only do so in a finite set of orbits. He also proposed that electrons could jump between orbits, but only in discrete changes of energy corresponding to the absorption or radiation of a photon. This not only refined Rutherford's proposed model, but also gave rise to the concept of a quantized atom, where matter behaved in discreet packets.

The development of the mass spectrometer – which uses a magnet to bend the trajectory of a beam of ions – allowed the mass of atoms to be measured with increased accuracy. Chemist Francis William Aston used this instrument to show that isotopes had different masses. This in turn was followed up by physicist James Chadwick, who in 1932 proposed the neutron as a way of explaining the existence of isotopes.

Throughout the early 20th century, the quantum nature of atoms was developed further. In 1922, German physicists Otto Stern and Walther Gerlach conducted an experiment where a beam of silver atoms was directed through a magnetic field, which was intended to split the beam between the direction of the atoms angular momentum (or spin).

Known as the Stern–Gerlach Experiment, the results was that the beam split in two parts, depending on whether or not the spin of the atoms was oriented up or down. In 1926, physicist Erwin Schrodinger used the idea of particles behaving like waves to develop a mathematical model that described electrons as three-dimensional waveforms rather than mere particles.

A consequence of using waveforms to describe particles is that it is mathematically impossible to obtain precise values for both the position and momentum of a particle at any given time. That same year, Werner Heisenberg formulated this problem and called it the "uncertainty principle". According to Heisenberg, for a given accurate measurement of position, one can only obtain a range of probable values for momentum, and vice versa.

In the 1930s, physicists discovered nuclear fission, thanks to the experiments of Otto Hahn, Lise Meitner and Otto Frisch. Hahn's experiments involved directing neutrons onto uranium atoms in the hopes of creating a transuranium element. Instead, the process turned his sample of uranium-92 (Ur92) into two new elements – barium (B56) and krypton (Kr27).

Meitner and Frisch verified the experiment and attributed it to the uranium atoms splitting to form two element with the same total atomic weight, a process which also released a considerable amount of energy by breaking the atomic bonds. In the years that followed, research into the possible weaponization of this process began (i.e. nuclear weapons) and led to the construction of the first atomic bombs in the US by 1945.

In the 1950s, the development of improved particle accelerators and particle detectors allowed scientists to study the impacts of atoms moving at high energies. From this, the Standard Model of particle physics was developed, which has so far successfully explained the properties of the nucleus, the existence of theorized subatomic particles, and the forces that govern their interactions.

Since the latter half of the 20th century, many new and exciting discoveries have been with regards to atomic theory and quantum mechanics. For example, in 2012, the long search for the Higgs Boson led to a breakthrough where researchers working at the European Organization for Nuclear Research (CERN) in Switzerland announced its discovery.

In recent decades, a great deal of time and energy has been dedicated by physicists to the development of a unified field theory (aka. Grand Unifying Theory or Theory of Everything). In essence, since the Standard Model was first proposed, scientists have sought to understand how the four fundamental forces of the universe (gravity, strong and weak nuclear forces, and electromagnetism) work together.

Whereas gravity can be understood using Einstein's theories of relativity, and nuclear forces and electromagnetism can be understood using quantum theory, neither theory can account for all four forces working together. Attempts to resolve this have led to a number of proposed theories over the years, ranging from String Theory to Loop Quantum Gravity. To date, none of these theories have led to a breakthrough.

Our understanding of the atom has come a long way, from classical models that saw it as an inert solid that interacted with other atoms mechanically, to modern theories where atoms are composed of energetic particles that behave unpredictably. While it has taken several thousand years, our knowledge of the fundamental structure of all matter has advanced considerably.

And yet, there remain many mysteries that are yet to be resolved. With time and continued efforts, we may finally unlock the last remaining secrets of the atom. Then again, it could very well be that any new discoveries we make will only give rise to more questions – and they could be even more confounding than the ones that came before!