FIELD NOTES / 01
Atomic number, mass & isotopes
A small number with a big meaning.
An element is defined by its atomic number Z: the number of protons in its nucleus. Hydrogen has Z = 1; carbon has Z = 6. Changing the proton count changes the element.
The mass number A belongs to an isotope. It is the number of protons plus neutrons. Carbon-12 has 6 protons and 6 neutrons; carbon-14 has 6 protons and 8 neutrons.
Standard atomic weight describes the relative atomic masses and isotopic composition of normal terrestrial material. It is usually not an integer. It is dimensionless and is not the mass number of one isotope.
A = Z + N Mass number = protons + neutrons
Periodic table