定义Definition
原子序数是原子核中的质子数，符号 Z，也等于核电荷数和中性原子的核外电子数。每种元素有唯一的原子序数，元素在周期表中按原子序数递增的顺序排列。
The atomic number, Z, is the number of protons in an atom's nucleus; it also equals the nuclear charge and the number of electrons in a neutral atom. Each element has its own atomic number, and the periodic table lists elements in order of increasing atomic number.
定义元素It defines the element
原子序数决定一个原子属于哪种元素：所有 Z = 6 的原子都是碳，无论其中子数是 6、7 还是 8。原子得失电子形成离子后原子序数不变；只有核反应才会改变原子序数，使一种元素变为另一种元素。
Atomic number decides which element an atom is: every atom with Z = 6 is carbon, whether it has 6, 7 or 8 neutrons. Gaining or losing electrons to form ions leaves Z unchanged; only nuclear reactions change it, turning one element into another.
莫塞莱的发现Moseley's discovery
门捷列夫按原子量排列元素，但为了符合性质的周期性，有几对元素不得不颠倒顺序，如碲（127.6）排在碘（126.9）之前，钴排在镍之前。1913 年莫塞莱测量各元素的特征 X 射线，发现频率的平方根与一个逐个递增的整数成正比，这个整数就是核电荷数。按原子序数排列，这些反常都消失了，并预言了尚未发现的元素。
Mendeleev arranged elements by atomic weight but had to reverse a few pairs to keep the periodic pattern, putting tellurium (127.6) before iodine (126.9) and cobalt before nickel. In 1913 Moseley measured the characteristic X-rays of the elements and found the square root of the frequency was proportional to a whole number rising by one from element to element: the nuclear charge. Ordering by atomic number removed the anomalies and predicted missing elements.
表示方法Notation
原子序数写在元素符号的左下角，质量数写在左上角，如 ²³⁵₉₂U。在核反应方程式中，两边原子序数之和守恒。
Atomic number is written at the lower left of the symbol and mass number at the upper left, as in ²³⁵₉₂U. In nuclear equations the atomic numbers on each side add up to the same total.
年表Timeline
- 1869
门捷列夫按原子量排列元素，发表周期表
Mendeleev publishes his periodic table ordered by atomic weight
- 1911
范登布鲁克提出元素序号等于核电荷数
van den Broek suggests an element's place number equals its nuclear charge
- 1913
莫塞莱用 X 射线谱证实原子序数等于核电荷数
Moseley confirms with X-ray spectra that atomic number equals nuclear charge
相关公式Formulas
例题Worked example
Cu（Z = 29）：λ(Kα) ≈ 1/(0.75×1.097×10⁷×28²) = 0.155 nm（实测 0.154 nm）。
Cu Kα predicted 0.155 nm (measured 0.154 nm).