Do all isotopes of an element have the same atomic number?

3 Answers
Jan 25, 2016

Yes, absolutely. Because, by definition, the element hydrogen is defined by #Z=1#, the atomic number, the number of positively charged particles in the atomic nucleus.


If there is 1 positively charged particle in the atomic nucleus, 1 proton, then we have the element hydrogen; if 2, then helium; if 23, then vanadium etc.

The atomic number, #Z#, thus determines elemental identity; and this relates to the number of protons, positively charged nuclear particles, contained in the nucleus. Of course, there may be relatively different numbers of NEUTRAL particles (i.e. NEUTRONS) contained in the nucleus, which gives rise to the existence of isotopes. For the simplest atom, hydrogen, we would normally encounter #""^1H#, the protium nucleus. A smaller percentage of hydrogen atoms would have 1 neutron in the nucleus, to give deuterium, i.e. #""^2H#; an even smaller percentage has 2 neutrons in the nucleus to give the tritium isotope, #""^3H#

It is important to understand these principles; they are basic and fundamental. Don't be backward in coming forward if you have an objection or query.

So the most important factoid: the atomic number determines elemental identity! Note that for all the Chemistry exams you will ever sit, you will and MUST be issued a Periodic Table, which will give you the average isotopic mass.

Jan 26, 2016

All atoms of the same element have the same atomic number.


The atomic number of an element is the number of protons in the nuclei of its atoms. All atoms of an element have the same atomic number. All isotopes of the same element, such as hydrogen, have the same number of protons (same atomic number) but different numbers of neutrons in their nuclei.

There are three isotopes of hydrogen; hydrogen-1, hydrogen-2, and hydrogen-3. The atomic number of hydrogen is 1. The atoms of hydrogen-1 have 1 proton and 0 neutrons in their nuclei, the atoms of hydrogen-2 have 1 proton and 1 neutron in their nuclei, and atoms of hydrogen-3 have 1 proton and 2 neutrons in their nuclei. Notice that all of the hydrogen isotopes have the same number of protons in their nuclei, regardless of the number of neutrons. Therefore, they all have the same atomic number, which, according to the periodic table, is 1.

Apr 9, 2018



An isotope of an element is just that same element but with a different number of neutrons, therefore different atomic mass.

A normal hydrogen atom has a mass of #1 \ "u"#, and so is denoted by #""^1H#.

Isotopes of hydrogen include deuterium and tritium, which are hydrogen-#2# and hydrogen-#3#, respectively. They are denoted by #""^2H# and #""^3H#.

Note that they are still the same element, as the atomic (proton) number is responsible on what element is currently there. It doesn't matter if we somehow create #""^1000H#, it will still be hydrogen, and react the same way in a chemical reaction like normal hydrogen would.