# Is it possible for two different substances to contain the same number of protons but different atomic numbers?

May 31, 2017

Consider ${\text{NH}}_{3}$ and $\text{Ne}$. Both of these have $10$ protons:

$\text{N" -> "7 protons}$, atomic number of $7$
$3 \times \text{H" -> "3 protons}$, atomic number of $1$
$7 + 3 = \boldsymbol{\text{10 total protons}}$ in ${\text{NH}}_{3}$

$\text{Ne" -> bb"10 protons}$, atomic number of $10$

They are both substances... but they are not the same substance. ${\text{NH}}_{3}$ is a smelly gas, and $\text{Ne}$ is an inert noble gas.

On the other hand, if BOTH substances are the SAME KIND, then if they have the same number of protons, they cannot have differing atomic numbers.

If you have a $\text{Ne}$ atom, it must have $10$ protons, no matter what. If you choose an element to have an atomic number of $11$, it must be $\text{Na}$ and have $11$ protons, no matter what.

That is because we specifically chose elements, which contain only one kind of atom.