Inside the Atom: the Nuclear Model
Firing alpha particles at thin gold foil sent a few of them straight back, which no spread-out pudding of charge could do. The nuclear model that replaced it puts almost all the mass and all the positive charge in a tiny centre.
What a learner can do afterwards
- States what the scattering result was and why it ruled out the earlier model
- Describes the nuclear model's parts with their charges, and the scale gap between atom and nucleus
- Explains that a model is kept only while it fits the evidence, using this change as the example
1 · Read
You already know everything around you is made of atoms. Rutherford fired alpha particles at thin gold foil. Most sailed straight through, but a few bounced straight back. A spread out pudding of charge could only nudge particles, never fling them backward. That bounce killed the old model.
Nadia builds the new model from three parts. Positive protons and neutral neutrons pack into a tiny nucleus at the centre. Negative electrons move in the vast space around it. The atomic number counts protons, and the mass number adds protons and neutrons.
Atoms are mostly empty space, and the scale gap is hard to picture. If the nucleus were a marble in a stadium, the electrons would be dust circling high in the stands. Nearly all the mass sits in that marble, yet nearly all the volume is the empty stands. Electrons weigh far less than protons and neutrons.
A model survives only while it fits the evidence, and one sharp experiment can replace it. When fresh results break a model, scientists drop the model, not the results.
Most particles passed, a few bounced, so charge sits in a tiny nucleus.
The rewritten readings were written for this dataset.
More to read
Evidence changes atomic models
Experiments reshaped scientists' ideas about atoms. Dalton proposed solid spheres. Thomson placed electrons within evenly distributed positive charge. Rutherford tested this model by directing alpha particles at a thin sheet of gold. Most passed through; a small number reversed direction. Thomson's model could not explain these large deflections.
Rutherford concluded that a small nucleus contains the positive charge and nearly all the atom's mass. Electrons occupy the much larger surrounding region. Scientific models must account for observations. An experiment that contradicts a model can lead to its replacement.
Particles and atomic identity
Atoms contain positively charged protons and negatively charged electrons. Most also contain uncharged neutrons. Protons and neutrons occupy an atom's nucleus; electrons occupy the surrounding region. Neutral atoms have equal numbers of electrons and protons.
An element's identity depends on its proton count, called the atomic number. Adding its proton and neutron counts gives the mass number. Isotopes belong to one element but contain different numbers of neutrons. Their masses differ, while their chemical behaviour is generally very similar.
What atoms reveal about stars
An atom's nucleus is tiny compared with the region occupied by its electrons. Most of the atom is empty space, yet its nucleus holds almost its entire mass.
These structures help astronomers identify materials in stars. Changes between electron energy levels cause atoms to absorb or emit particular wavelengths of light. Each element produces a characteristic pattern. Analysing those patterns reveals the composition of distant stars without travelling to them.
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2 · Watch
En anglaisTake it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.