Read nuclear notation and classify isotope relationships.
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Method and worked examples
A nucleus contains protons and neutrons, collectively nucleons. In the notation ZXA,A is mass number and Z is atomic number. Proton number is Z and neutron number is N=A−Z. A neutral atom has Z electrons; an ion’s electron count changes without changing its nucleus. Most atomic mass is concentrated in the much smaller nucleus.
Isotopes share Z but differ in neutron number. Their similar electron arrangements give similar ordinary chemistry, while nuclear and some other physical properties can differ. The attractive nuclear interaction is very short-range, whereas protons repel electrically. Stable neutron-to-proton patterns change with nuclear size; heavy stable nuclei generally need more neutrons than protons. Composition alone does not assign an exact half-life.
Example 1
A neutral atom has A=23 and Z=11. Find proton, neutron and electron numbers.
Protons =11. Neutrons =23−11=12. A neutral atom has 11 electrons.
Example 2
Are nuclei with (A,Z)=(12,6) and (14,6) isotopes?
Yes. Both have six protons and are carbon, but their neutron numbers are six and eight.
Watch out:Different proton numbers define different elements. Isotopes of one element have different neutron numbers.
Nuclei, isotopes and stability practice | Year 11 Physics | Ascent Arcade