The nuclear atom
| English | Chinese | Pinyin |
|---|---|---|
| nucleus | 原子核 | yuán zǐ hé |
| proton | 质子 | zhì zi |
| neutron | 中子 | zhōng zi |
| nuclide | 核素 | hé sù |
| proton number | 质子数 | zhì zi shù |
| nucleon number | 核子数 | hé zǐ shù |
| nucleon | 核子 | hé zǐ |
| isotope | 同位素 | tóng wèi sù |
| atomic mass unit | 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."
Nuclear atom evidence lab
Connect observations to the nuclear model of the atom.
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 α-particles passing straight through the gold foil showed that the atom is:
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.
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.
Simple models of a helium atom and a lithium atom (not to scale)
Almost all of an atom's mass is in its nucleus.
Protons and neutrons are ~1 u each; electrons are ~1/1836 u, so the nucleus holds nearly all the mass.
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.
Most alpha-particles pass nearly straight through; a few are deflected sharply by the tiny nucleus
Protons and neutrons together are called ____.
The nucleon number $A$ counts the protons plus neutrons in the nucleus.
How many neutrons are in $^{14}_{6}\text{C}$?
$N = A - Z = 14 - 6 = 8$ neutrons.
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
Isotopes of an element have:
Same $Z$ (so same element and chemistry) but different $N$, giving a different mass number $A$.
What's conserved
- In any nuclear process, nucleon number $A$ and charge are conserved.
- These two rules let you balance every decay equation.
In a nuclear process, which quantities are always conserved?
Nucleon number and charge are conserved. Proton and neutron numbers can each change (e.g. in β-decay a neutron becomes a proton).
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 $1\ \text{u}$.
The unified atomic mass unit is defined so that carbon-12 has a mass of exactly 12 u.
Yes — that definition fixes $1\ \text{u} = 1.66 \times 10^{-27}\ \text{kg}$.
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; isotopes share $Z$
- nuclear processes conserve nucleon number and charge