Fire alpha particles at a thin gold foil and almost all sail through, but a rare few bounce straight back. That single experiment revealed that nearly all an atom mass sits in a minute, positively charged nucleus.
Alpha-particle scattering showed the atom has a tiny, dense, positively charged nucleus surrounded by mostly empty space. A nuclide is written as nucleon number A over proton number Z. Isotopes share the same Z but differ in A. In every nuclear change, nucleon number and charge are conserved. Alpha radiation is a helium nucleus (charge +2e), beta-minus is a fast electron (charge −e), and gamma is a high-energy photon (no charge or mass).
Fire alpha particles past a nucleus and vary the aim. A near-head-on approach is repelled through a large angle; a distant one barely bends, just as Rutherford found.
Each differs in composition, charge and penetration.
| Radiation | Composition | Charge |
|---|---|---|
| α | helium nucleus (2p + 2n) | charge +2e |
| β⁻ | fast electron | charge −e |
| γ | high-energy photon | no charge, no mass |
Four quick checks tied to this lesson. Each correct answer earns XP and lights this skill on your star map.
Alpha-particle scattering showed that the nucleus is:
In the notation with nucleon number A above proton number Z, A is the number of:
Isotopes of an element have the same number of protons but different numbers of:
An alpha particle consists of:
Two quantities must balance on both sides of every nuclear equation.
Most alpha particles pass straight through the foil because the atom is mostly empty space; only a near head-on approach to the tiny nucleus gives a large deflection. In a decay equation, balance both the nucleon number A and the charge Z. In beta-minus decay the nucleon number stays the same (a neutron turns into a proton), so only Z changes, rising by one.
Unlocks once the checks above are done. Worth more XP, written to AS Paper 1 and 2 standard.
A uranium nucleus (A = 238, Z = 92) emits an alpha particle. The resulting nucleus has:
When a nucleus undergoes beta-minus decay, its proton number Z:
In the alpha-scattering experiment, the large-angle deflection of a few alpha particles is evidence that the nucleus is:
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