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In our chemistry book is written that ozone ($\ce{O3}$) is a chemical element. Also our chemistry book gives a definition that agrees with that. But our teacher doesn't agree.

His reasons:

  1. If ozone is a chemical element why it isn't in the periodic table?
  2. It is not true with the things that Robert Boyle has written in his book.

Then what is the best definition for chemical element and is ozone a chemical element?

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    $\begingroup$ goldbook.iupac.org/C01022.html: an element is a "species of atoms with the same number of protons in the nucleus". Ozone isn't an atom. Ozone is an allotrope of oxygen. Oxygen is the element. $\endgroup$ – orthocresol Sep 27 '16 at 17:03
  • $\begingroup$ Well, we shouldn't be basing chemical knowledge solely on something that one chemist from a really long time ago wrote in his book. Chemistry has advanced a long way since the 1600s and even some of the best current models and theories break down at some point, so the second point is moot. $\endgroup$ – 86BCP2432T Sep 27 '16 at 17:03
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You might want to read up on this Wikipedia article

My answer is a bit oversimplified, but I think it would serve your purpose.

When you refer to a chemical element, as it is, you'd be referring to one of the 118 elements in the Periodic Table. Something's a chemical element if it can't be broken down into more atoms that can be located on the periodic table. Ozone is not an element for the same reason dioxygen is not an element.....it can be broken down into three identical constituent particles, which you can locate on the Periodic Table (oxygen).

As @orthocresol pointed out, ozone is an allotrope of elemental oxygen.

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$\ce{O3}$ is not a chemical element, but is a molecular form or allotrope of an element. Sulfur, for example, exists in different allotropes: puckered rings of $\ce{S8}$, a red triatomic ($\ce{S3}$) gaseous form, similar to ozone, etc.

Oxygen can exist temporarily in monatomic form, as ordinary diatomic oxygen in air, as triatomic ozone, even as $\ce{O8}$, a red solid. All are the same element, just different allotropes.

That said, it was difficult for the early chemists to prove that different allotropes were really the same element. Antoine Lavoisier had to burn diamonds to show that they were made of carbon, like graphite... and that experiment rather annoyed the new egalitarian French Republic. It goes to show that chemistry can be dangerous.

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