Physics / Unit 10: Magnetism and Modern Physics
10.5 The Atom and the Nucleus
Notes
An atom has a tiny, dense nucleus of protons and neutrons, surrounded by electrons.
- Atomic number $Z$ = number of protons (this sets the element). Mass number $A$ = protons + neutrons.
- Neutrons $= A - Z$. Notation: $^{14}_{6}\text{C}$ has 6 protons and 8 neutrons.
- Isotopes are atoms of the same element with different numbers of neutrons.
- The strong nuclear force holds the nucleus together, overcoming the electric repulsion between protons.
- Electrons occupy energy levels. When an electron drops to a lower level, the atom emits light of a specific energy, giving each element its own line spectrum.
Practice
How many neutrons are in $^{235}_{92}\text{U}$?
Neutrons $= A - Z = 235 - 92 = 143$.
Carbon-12 and carbon-14 are isotopes. How do they differ?
Isotopes share the same number of protons but differ in neutrons.
What holds protons together in the nucleus even though they repel each other electrically?
The strong force is much stronger than the electric force at nuclear distances.
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