定义Definition
原子是化学变化中的最小粒子，保持元素的化学特性。原子由带正电的原子核和核外电子组成，原子核又由质子和中子构成。原子呈电中性，质子数等于电子数。原子半径约为 10⁻¹⁰ m，原子核半径约为 10⁻¹⁵ m，但原子质量几乎全部集中在原子核上。
An atom is the smallest particle of an element that keeps its chemical identity and takes part in chemical change. It consists of a positively charged nucleus, made of protons and neutrons, surrounded by electrons. Atoms are neutral, with equal numbers of protons and electrons. An atom is about 10⁻¹⁰ m across and its nucleus about 10⁻¹⁵ m, yet almost all the mass is in the nucleus.
三种基本粒子The three particles
质子：相对质量 1，电荷 +1；中子：相对质量 1，电荷 0；电子：相对质量约 1/1836，电荷 −1。质子数（原子序数 Z）决定元素种类，质子数与中子数之和为质量数 A。
Proton: relative mass 1, charge +1; neutron: relative mass 1, charge 0; electron: relative mass about 1/1836, charge −1. The number of protons, the atomic number Z, fixes the element, and protons plus neutrons give the mass number A.
原子模型的演变How the model developed
道尔顿提出原子是不可分割的实心球；汤姆孙发现电子，提出“葡萄干布丁”模型；卢瑟福的 α 粒子散射实验证明原子中心有体积很小、带正电的原子核；玻尔提出电子在特定能级上运动；后来量子力学用轨道描述电子的概率分布。
Dalton pictured atoms as indivisible solid spheres; Thomson discovered the electron and proposed the 'plum pudding' model; Rutherford's α-particle scattering showed a tiny positive nucleus at the centre; Bohr put electrons in fixed energy levels; quantum mechanics later described electrons by orbitals giving probability distributions.
年表Timeline
- 1808
道尔顿发表原子论
Dalton publishes his atomic theory
- 1897
汤姆孙发现电子
Thomson discovers the electron
- 1911
卢瑟福提出原子的核式结构
Rutherford proposes the nuclear atom
- 1932
查德威克发现中子
Chadwick discovers the neutron