定义Definition
原子核是原子中心由质子和中子（统称核子）组成的带正电的微小核心。它的半径约为 10⁻¹⁵–10⁻¹⁴ m，只有原子半径的约十万分之一，却集中了原子 99.9% 以上的质量。
The atomic nucleus is the tiny, positively charged core at the centre of an atom, made of protons and neutrons (together called nucleons). Its radius is about 10⁻¹⁵–10⁻¹⁴ m, roughly a hundred-thousandth of the atom's, yet it holds more than 99.9% of the atom's mass.
发现Discovery
1909 年盖革和马斯登用 α 粒子轰击金箔，发现绝大多数 α 粒子直接穿过，极少数发生大角度偏转甚至被弹回。1911 年卢瑟福据此提出：原子的正电荷和绝大部分质量集中在一个很小的核上，电子在核外很大的空间中运动。
In 1909 Geiger and Marsden fired alpha particles at gold foil and found that almost all passed straight through while a very few were deflected through large angles or even bounced back. In 1911 Rutherford concluded that an atom's positive charge and nearly all its mass are concentrated in a tiny nucleus, with electrons occupying the large space around it.
组成与表示Composition and notation
原子核含 Z 个质子和 N 个中子，质量数 A = Z + N。核素用 ᴬ_ZX 表示，如 ³⁷₁₇Cl 含 17 个质子、20 个中子。质子之间有静电排斥，核子之间靠短程的强核力结合在一起。
A nucleus contains Z protons and N neutrons, with mass number A = Z + N. A nuclide is written ᴬ_ZX; ³⁷₁₇Cl, for example, has 17 protons and 20 neutrons. Protons repel one another electrostatically, and the nucleons are held together by the short-range strong nuclear force.
结合能与稳定性Binding energy and stability
原子核的质量略小于组成它的核子质量之和，这一质量亏损对应于结合能（E = mc²）。平均每个核子的结合能在铁-56、镍-62 附近最大，因此重核裂变和轻核聚变都能释放能量。中子与质子数之比不合适的核会发生放射性衰变。
A nucleus has slightly less mass than its separate nucleons; this mass defect corresponds to the binding energy (E = mc²). Binding energy per nucleon peaks near iron-56 and nickel-62, so both fission of heavy nuclei and fusion of light ones release energy. Nuclei with an unsuitable neutron-to-proton ratio decay radioactively.
年表Timeline
- 1909
盖革–马斯登 α 粒子散射实验
Geiger–Marsden alpha-scattering experiment
- 1911
卢瑟福提出原子的核式结构
Rutherford proposes the nuclear model of the atom
- 1932
发现中子后，原子核由质子和中子组成的模型确立
After the neutron's discovery, the proton–neutron model of the nucleus is established