5.1.1 The Atom
Structure of an Atom
An atom consists of a central nucleus surrounded by orbiting electrons.
- Central mass: The nucleus contains protons and neutrons.
- Protons are positively charged and neutrons are neutral.
- The orbiting electrons are negatively charged.
- Overall, the atom is electrically neutral because the number of protons equals the number of electrons.
Nuclear Force
The strong nuclear force binds protons and neutrons tightly together within the nucleus, overcoming the electrostatic repulsion between protons.
Alpha Particle Scattering Experiment
The alpha particle scattering experiment provided evidence for the existence of a small, dense, positively charged nucleus.
Method Summary
- A very thin sheet of gold foil was placed in the path of alpha particles.
- A movable detector was used to record scattering at different angles.
Observations
- Most alpha particles passed straight through with little or no deflection.
- A small number were deflected at large angles.
- A very few were reflected directly back toward the source.
Conclusions
- Most of the atom is empty space since most alpha particles passed through undeflected.
- The nucleus is small and dense — only a few alpha particles were reflected back.
- The nucleus is positively charged, repelling the positively charged alpha particles.
- Almost all the mass of the atom is concentrated in the nucleus.
Formation of Ions
Atoms can gain or lose electrons to form ions.
- Loss of electrons → formation of positive ions (cations).
- Gain of electrons → formation of negative ions (anions).
Key Points on Atomic Behaviour
- It is impossible to predict exactly when a particular nucleus will decay.
- Only the probability of decay can be estimated statistically.
- Each radioactive isotope has its own characteristic rate of decay.
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