Thursday, March 31, 2022
Physicists have historically postulated that the nucleus of an atom contains a certain number of electrons, to balance the nuclear charge of an atom () with the atomic mass number . However, this prediction does not match the experimental results from the nuclear magnetic moment (which shows that protons have spin ).
The neutron was discovered as an nuclear constituent in 1932. In the proton-neutron model, atoms of atomic number have protons and neutrons, such that the atomic mass number is . Chemical properties of an element depend only on the atomic number .
Atoms of the same atomic number with different mass numbers are called isotopes. Isotopes with atomic number , mass number , and neutron number () are written as
Since (a chemical symbol) and hold the same information, is not necessary. And with both the atomic number and mass number, is not necessary. Therefore, isotopes are compactly written as , , etc.