The nuclear atom
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."
What the scattering showed
- Most pass through → the atom is mostly empty space.
- A rare big deflection → a tiny, dense, positive nucleus 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 (charge $+e$) and neutrons (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.
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 is written $^{A}_{Z}\text{X}$: $Z$ = proton number (fixes the element), $A$ = nucleon number.
- Number of neutrons $N = A - Z$. Protons and neutrons together are nucleons.
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
- 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.
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
- 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