Nuclear Constituents

Thursday, March 31, 2022

Historical Postulates

Physicists have historically postulated that the nucleus of an atom contains a certain number of electrons, to balance the nuclear charge of an atom ( $+Z$ ) with the atomic mass number $A$ . However, this prediction does not match the experimental results from the nuclear magnetic moment (which shows that protons have spin $1/2$ ).

Neutrons

The neutron was discovered as an nuclear constituent in 1932. In the proton-neutron model, atoms of atomic number $Z$ have $Z$ protons and $A-Z$ neutrons, such that the atomic mass number is $A$ . Chemical properties of an element depend only on the atomic number $Z$ .

Name	Symbol	Charge	Mass	Rest energy	Spin
Proton	$p$	$+e$	$1.007276~\text{u}$	$938.28~\text{MeV}$	$\frac{1}{2}$
Neutron	$n$	$+0$	$1.008665~\text{u}$	$939.57~\text{MeV}$	$\frac{1}{2}$

Isotopes

Atoms of the same atomic number with different mass numbers are called isotopes. Isotopes with atomic number $Z$ , mass number $A$ , and neutron number $N$ ( $N=A-Z$ ) are written as

$^A_ZX_N$

Since $X$ (a chemical symbol) and $Z$ hold the same information, $Z$ is not necessary. And with both the atomic number and mass number, $N$ is not necessary. Therefore, isotopes are compactly written as $^1H$ , $^2H$ , etc.