# How do you find the mass number and atomic number?

Jun 15, 2017

Most of the time these are tattooed on the Periodic Table.

#### Explanation:

Let's take a simple molecule, ${H}_{2} O$. By definition, all $H$ nuclei have ONE positively charged particle in their nuclei (i.e. $Z = 1$), and all $O$ nuclei have EIGHT positively charged particle in their nuclei (i.e. $Z = 8$). $Z$, $\text{the atomic number}$, defines the atom as a particular element: $Z = 1 , H$; $Z = 2 , H e$; $Z = 8 , O$;....... $Z = 77 , I r$; $\text{etc.}$

The mass number represents the number of MASSIVE particles, positive and neutral, that are present in the element's nucleus. The neutral particles, so-called $\text{neutrons}$, participate in VERY attractive interaction at almost impossibly short nuclear distances. And different numbers of neutrons give rise to the existence of $\text{isotopes}$.

Most hydrogen nuclei have NO neutrons present in their nucleus; this gives the ${\text{protium isotope," }}^{1} H$, which is approx. 99% abundant; a few hydrogen nuclei have ONE neutron present in their nuclei; this gives the ${\text{deuterium isotope," }}^{2} H$, which is less than 1% abundant; a smaller proportion of hydrogen nuclei contain TWO neutrons; and this is the ${\text{tritium isotope," }}^{3} H$.

As $Z$ grows, most heavier elements have a number of stable isotopes. The $\text{weighted average}$ of isotopic masses gives rise to the atomic masses printed on the Periodic Table. For a particular isotope, the mass number is simply the number of protons, $Z$, PLUS the number of neutrons present in that nucleus.

Anyway, you do have to have a grasp on these principles, but you don't have to remember atomic numbers, because a Periodic Table will be provided in every exam you sit in Chemistry and Physics. Good luck.