# How right is defining elements with the number of protons? [closed]

Atomic number is the number of protons a certain atom has. It's the defining attribute of a certain element. But, How right is that?

From the viewpoint of chemistry, the definition is right. Because, for a certain chemical reaction, isotopes of a certain element will react in the same way.

For example, $\ce{H2 + 1/2O2 -> H2O}$ $\ce{ ^2H2 + 1/2O2-> ^2H2O}$

So, it's useful to define elements in terms of their number of protons.

But, other than reactivity in chemical reactions, many properties of an atom differ wildly due to difference of number of neutrons and electrons eg. half life of radioactive decay, type of radioactive decay, magnetic moment, stability, nuclear quantum states etc. So, even having the same number of protons can't ensure much similarity in some aspects.

I think to a rational perspective, the similarity of reactivity of isotopes in chemical reactions should weigh more than the other nuclear difference between isotopes. So, in that logic, the definition is right.

But, difference in nuclear properties should do matter. So, how right is defining elements with atomic number i.e. number of protons?

## closed as primarily opinion-based by Pritt Balagopal, Mithoron, Todd Minehardt, airhuff, Jon CusterJun 28 '17 at 21:31

Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise. If this question can be reworded to fit the rules in the help center, please edit the question.

• But aren't elements already defined via $^A_Z\ce{E}$ notation when it is necessary, i.e. in nuclear reactions? – andselisk Jun 28 '17 at 9:59
• @andselisk That is true. But, it's defined mainly by atomic number. And my question is why is that? – Mockingbird Jun 28 '17 at 10:20
• Look up isotope and nuclide. Isotope is primarily a chemical term and treats the proton number as more significant than the neutron number. Nuclide is a physicist's term and treats both equally. There are also terms isobar and isotone. – badjohn Jun 28 '17 at 11:31
• You just rehearsed the definition of element and isotope, and why we do have these two different, and why they are useful. Good. So what is the question? Why isotopes are not called elements and vice versa? – Greg Jun 28 '17 at 12:29
• You are trying to split hair. The terms are defined as they are, because those definitions are useful for the applications people had in mind when they made them. Chemistry, in this case. – Karl Jun 28 '17 at 19:47