The nuclear atom · 核式原子模型
| English | 中文 | Pinyin · 拼音 |
|---|---|---|
| nucleus/ˈnjuːklɪəs/ | 原子核 | yuán zǐ hé |
| protons/ˈprəʊtɒnz/ | 质子 | zhì zi |
| neutrons/ˈnjuːtrɒnz/ | 中子 | zhōng zi |
| nuclide/ˈnjuːklaɪd/ | 核素 | hé sù |
| proton number/ˈprəʊtɒn ˈnʌmbə/ | 质子数 | zhì zi shù |
| nucleon number/ˈnjuːklɪən ˈnʌmbə/ | 核子数 | hé zǐ shù |
| nucleons/ˈnjuːklɪənz/ | 核子 | hé zǐ |
| isotopes/ˈaɪsətəʊps/ | 同位素 | tóng wèi sù |
| atomic mass unit/əˈtɒmɪk mæs ˈjuːnɪt/ | 原子质量单位 | yuán zi zhì liàng dān wèi |
Firing at gold foil
- Rutherford's team fired α-particles at a thin gold foil.
- Most went straight through — but a rare few bounced almost straight back.
- "As if you fired a shell at tissue paper and it came back at you."
轰击金箔
- 卢瑟福的团队用 α 粒子 轰击一片薄金箔。
- 大多数直接穿过——但极少数几乎直接弹了回来。
- "就像你朝一张薄纸开炮,炮弹却弹回来打中你。"
Nuclear atom evidence lab · 核式原子证据实验
Connect observations to the nuclear model of the atom. · 将观察结果与原子的核式模型联系起来。
What the scattering showed
- Most pass through → the atom is mostly empty space.
- A rare big deflection → a tiny, dense, positive nucleus 原子核 holds the charge and mass.
Most alpha-particles pass nearly straight through; a few are deflected sharply by the tiny nucleus
散射说明了什么
- 大多数穿过 → 原子 大部分是空的。
- 极少数大角度偏转 → 一个 微小、致密、带正电的原子核(nucleus) 集中了电荷和质量。
大多数 α 粒子几乎直线穿过;少数被微小的原子核大角度偏转
Most α-particles passing straight through the gold foil showed that the atom is: · 大多数 α 粒子直接穿过金箔表明原子是：
If most go straight through, there is little in the way — the atom is mostly empty, with the mass and charge concentrated in a tiny nucleus. · 如果大多数粒子直接穿过，说明路径上几乎没有阻碍——原子大部分是空的，质量和电荷集中在微小的原子核内。
The nuclear model
- Nucleus: protons 质子 (charge $+e$) and neutrons 中子 (no charge). Electrons ($-e$) surround it.
- The atom is $\sim 10^{-10}\ \text{m}$; the nucleus only $\sim 10^{-15}\ \text{m}$ — but holds nearly all the mass.
- A neutral atom has as many electrons as protons.
Simple models of a helium atom and a lithium atom (not to scale)
原子核模型
- 原子核:质子(protons)(电荷 $+e$)和 中子(neutrons)(不带电)。电子($-e$)环绕它。
- 原子约 $10^{-10}\ \text{m}$;原子核只有约 $10^{-15}\ \text{m}$——却几乎集中了全部质量。
- 中性原子的 电子 数等于质子数。
氦原子和锂原子的简单模型(未按比例)
Almost all of an atom's mass is in its nucleus. · 原子的几乎全部质量都集中在原子核中。
Protons and neutrons are ~1 u each; electrons are ~1/1836 u, so the nucleus holds nearly all the mass. · 质子和中子各约 1 u；电子约 1/1836 u，因此原子核包含了几乎所有的质量。
Notation
- A nuclide 核素 is written $^{A}_{Z}\text{X}$: $Z$ = proton number 质子数 (fixes the element), $A$ = nucleon number 核子数.
- Number of neutrons $N = A - Z$. Protons and neutrons together are nucleons 核子.
记号
- 核素写作 $^{A}_{Z}\text{X}$:$Z$ = 质子数(决定元素),$A$ = 核子数。
- 中子数 $N = A - Z$。质子和中子合称 核子(nucleons)。
Protons and neutrons together are called ____. · 质子和中子统称为 ____。
The nucleon number $A$ counts the protons plus neutrons in the nucleus. · 核子数 $A$ 计算的是原子核中质子与中子的总数。
How many neutrons are in $^{14}_{6}\text{C}$? · $^{14}_{6}\text{C}$ 中包含多少个中子？
$N = A - Z = 14 - 6 = 8$ neutrons. · $N = A - Z = 14 - 6 = 8$ 个中子。
Isotopes 同位素
- Isotopes have the same proton number $Z$ but different numbers of neutrons.
- Example: $^{12}_{6}\text{C}$ and $^{14}_{6}\text{C}$ — same element, different mass.
Nuclide notation: nucleon number on top, proton number below
同位素
- 同位素(isotopes) 有 相同 的质子数 $Z$,但 不同 的中子数。
- 例子:$^{12}_{6}\text{C}$ 和 $^{14}_{6}\text{C}$——同一元素,不同质量。
核素记号:核子数在上,质子数在下
Isotopes of an element have: · 同一元素的同位素具有：
Same $Z$ (so same element and chemistry) but different $N$, giving a different mass number $A$. · 相同的 $Z$（因此是同一元素且化学性质相同），但不同的 $N$，导致质量数 $A$ 不同。
What's conserved
- In any nuclear process, nucleon number $A$ and charge are conserved.
- These two rules let you balance every decay equation: the top numbers must add up on both sides, and so must the bottom numbers.
什么守恒
- 在任何核过程中,核子数 $A$ 和 电荷 守恒。
- 这两条规则让你能配平每一个衰变方程:两边上面的数字之和必须相等,下面的数字之和也必须相等。
In a nuclear process, which quantities are always conserved? · 在核反应过程中，哪些物理量总是守恒的？
Nucleon number and charge are conserved. Proton and neutron numbers can each change (e.g. in β-decay a neutron becomes a proton). · 核子数和电荷守恒。质子和中子的数量各自可以改变（例如在 β-衰变中，一个中子变为一个质子）。
Match each emitted particle to how it is written in a decay equation. · 将每种发射粒子与其在衰变方程中的写法匹配。
The top number is the nucleon number and the bottom the charge; both must balance across the equation. · 上方数字是核子数，下方数字是电荷；方程两侧都必须平衡。
Worked example: tritium
Tritium, $^{3}_{1}\text{H}$, is a radioactive isotope of hydrogen that emits a β$^{-}$ particle.
- Particles in a neutral atom: $Z = 1$ proton; $N = 3 - 1 = 2$ neutrons; $1$ electron (equal to the protons).
- Decay equation: $^{3}_{1}\text{H} \to {}^{3}_{2}\text{He} + {}^{0}_{-1}\beta + \bar{\nu}$. Check: top $3 = 3 + 0$; bottom $1 = 2 + (-1)$. The extra particle is an antineutrino.
- Quarks in the nucleus: a proton is uud and a neutron is udd, so one proton and two neutrons contain $2 + 1 + 1 = 4$ up quarks and $1 + 2 + 2 = 5$ down quarks.
- Check the charge: $4\left(+\tfrac{2}{3}\right) + 5\left(-\tfrac{1}{3}\right) = \tfrac{8}{3} - \tfrac{5}{3} = +1$, the charge of the single proton.
例题:氚
氚 $^{3}_{1}\text{H}$ 是氢的放射性同位素,发射 β$^{-}$ 粒子。
- 中性原子中的粒子: $Z = 1$ 个质子;$N = 3 - 1 = 2$ 个中子;$1$ 个电子(等于质子数)。
- 衰变方程: $^{3}_{1}\text{H} \to {}^{3}_{2}\text{He} + {}^{0}_{-1}\beta + \bar{\nu}$。检查:上面 $3 = 3 + 0$;下面 $1 = 2 + (-1)$。多出来的粒子是 反中微子。
- 核内的夸克: 质子是 uud,中子是 udd,所以一个质子和两个中子含有 $2 + 1 + 1 = 4$ 个上夸克和 $1 + 2 + 2 = 5$ 个下夸克。
- 检查电荷: $4\left(+\tfrac{2}{3}\right) + 5\left(-\tfrac{1}{3}\right) = \tfrac{8}{3} - \tfrac{5}{3} = +1$,即那一个质子的电荷。
Worked example: a chain of decays
A nucleus of radium-228, $^{228}_{88}\text{Ra}$, decays through a series of steps in which five α-particles and four β$^{-}$ particles are emitted. Identify the final nucleus.
- Nucleon number: each α removes $4$, each β$^{-}$ removes none: $A = 228 - 5 \times 4 = 208$.
- Proton number: each α removes $2$, each β$^{-}$ adds $1$: $Z = 88 - 10 + 4 = 82$.
- Answer: $^{208}_{82}\text{Pb}$ — lead.
- Check: the order of the decays does not matter; only the totals do. Charge and nucleon number are conserved at every single step, so they are conserved overall.
例题:一串衰变
一个镭-228 原子核 $^{228}_{88}\text{Ra}$ 经过一系列衰变,共发射 五 个 α 粒子和 四 个 β$^{-}$ 粒子。确定最终的原子核。
- 核子数: 每个 α 减少 $4$,每个 β$^{-}$ 不减少:$A = 228 - 5 \times 4 = 208$。
- 质子数: 每个 α 减少 $2$,每个 β$^{-}$ 增加 $1$:$Z = 88 - 10 + 4 = 82$。
- 答案: $^{208}_{82}\text{Pb}$——铅。
- 检查: 衰变的先后顺序无关紧要;只有总数才重要。电荷和核子数在每一步都守恒,所以整体也守恒。
A nucleus with proton number $92$ emits three α-particles and two β⁻ particles. What is the proton number of the final nucleus? · 一个质子数为 $92$ 的原子核发射了三个 α粒子和两个 β⁻粒子。最终原子核的质子数是多少？
Each α lowers $Z$ by $2$ and each β⁻ raises it by $1$: $92 - 6 + 2 = 88$. · 每个 α粒子使 $Z$ 减少 $2$，每个 β⁻粒子使其增加 $1$：结果为 $92 - 6 + 2 = 88$。
A neutral atom has $Z$ electrons — the proton number, never $A$. Isotopes differ only in neutrons, so their chemistry is identical. In a decay equation balance both rows: a β$^{-}$ is written $^{0}_{-1}\beta$, so it leaves $A$ unchanged and raises $Z$ by one. And "the nucleus is tiny" means about $10^{-15}\ \text{m}$ — a hundred thousand times smaller than the atom, not ten.
中性原子有 $Z$ 个电子——是 质子 数,绝不是 $A$。同位素 只 在中子数上不同,所以它们的化学性质完全相同。配平衰变方程时 两行 都要配平:β$^{-}$ 写作 $^{0}_{-1}\beta$,所以它不改变 $A$,使 $Z$ 增加一。另外,"原子核很小"指的是约 $10^{-15}\ \text{m}$——比原子小十万倍,而不是十倍。
Atomic mass unit 原子质量单位
- The unit $\text{u}$ is set so that $^{12}_{6}\text{C}$ has mass exactly $12\ \text{u}$.
- $1\ \text{u} = 1.66 \times 10^{-27}\ \text{kg}$; a proton and a neutron are each about $1\ \text{u}$, an α-particle about $4\ \text{u}$ with charge $+2e$.
原子质量单位
- 单位 $\text{u}$ 的规定使 $^{12}_{6}\text{C}$ 的质量 恰好 为 $12\ \text{u}$。
- $1\ \text{u} = 1.66 \times 10^{-27}\ \text{kg}$;质子和中子各约 $1\ \text{u}$,α 粒子约 $4\ \text{u}$、电荷 $+2e$。
The unified atomic mass unit is defined so that carbon-12 has a mass of exactly 12 u. · 统一原子质量单位的定义使得碳-12 的质量恰好为 12 u。
Yes — that definition fixes $1\ \text{u} = 1.66 \times 10^{-27}\ \text{kg}$. · 是的——该定义确定了 $1\ \text{u} = 1.66 \times 10^{-27}\ \text{kg}$。
You've got it
- α-scattering → atom is mostly empty with a tiny dense nucleus
- $^{A}_{Z}\text{X}$: $Z$ protons, $A$ nucleons, $N = A - Z$ neutrons, $Z$ electrons when neutral; isotopes share $Z$
- nuclear processes conserve nucleon number and charge — balance both rows of every equation
你掌握了
- α 散射 → 原子 大部分是空的,有一个 微小致密的原子核
- $^{A}_{Z}\text{X}$:$Z$ 个质子,$A$ 个核子,$N = A - Z$ 个中子,中性时 $Z$ 个电子;同位素共享 $Z$
- 核过程守恒 核子数 和 电荷——配平每个方程的两行