I have read the Chem.SE question about the difference between 'compound', 'mixture', 'element' and 'molecule', but I would like to know the precise definitions for more terms (see below). From the article, these are the definitions:

  • Elements are classes of atoms. Atoms of the same element are similar (if not identical) in their physical and chemical properties (but be aware of Isotopes which are physical variations among atoms of the same element).

  • Molecules are a group of atoms covalently bonded to each other.

  • Compounds are made up of two or more elements. Compound is often used interchangeably with the word substance.

  • Mixtures are collectives of different molecules or atoms.

How are compounds substances? Isn't 'substances' a general term for any chemical thing?

These are the terms that I would like definitions for:

  1. chemical species
  2. substance
  3. particle
  4. atom/isotope
  5. salt

I am rather confused about these terms, because my tutor uses particles and molecules (and elements) as if they were the same thing. As for the rest, I want to know how to distinguish between general (such as chemical species, substance) and specific terms (such as particle, atom, isotope, salt).

  • 2
    $\begingroup$ Have you checked the IUPAC gold book for the definitions? IUPAC probably collects the most rigorous and well-defined terminology, to the extent possible. For example, you can find chemical species, but particle is too general of a term and has to be broken down into subsets if you want the word to have a more specific meaning. $\endgroup$ Aug 15, 2014 at 13:34
  • $\begingroup$ You are already misstating a few things. $\endgroup$
    – iad22agp
    Aug 15, 2014 at 18:56
  • $\begingroup$ Elements are not just classes of atoms - all atoms of an element must have the same number of protons (atomic number). There are other classes of atoms that are not elements, such as the class of atoms that are radioactive, the class of atoms that are bosons, etc. $\endgroup$
    – iad22agp
    Aug 15, 2014 at 19:06
  • $\begingroup$ I see, perhaps you would also like to correct the answer from which I adopted these definitions? $\endgroup$ Aug 15, 2014 at 23:13
  • $\begingroup$ That question was already answered, although perhaps the answer could be improved upon. Specifically, to add that a substance can be either an element or a compound. You are getting into the philosophy of science in addressing these definitions. $\endgroup$
    – iad22agp
    Aug 16, 2014 at 16:22

1 Answer 1


A chemical species consists of chemically identical atoms, molecules or ions (more generally molecular entities) and is identified as a single species in a spectroscopic experiment. The latter part of the definition is important, because, for example, a compound which is identified as a single chemical species by NMR spectroscopy can still contain different enantiomers (that are separate molecular entities) which cannot be distinguished by an achiral spectroscopic method.

A substance is matter which has a specific and distinguishable chemical composition and specific properties. From this follows that every pure element and chemical compound is a substance. For example, pure $\ce{Fe}$ and $\ce{CH4}$ are substances, while an aqueous solution of $\ce{NaCl}$ and gasoline are not. The latter two are a mixture of two ($\ce{NaCl}$ and $\ce{H2O}$) or more different substances and have therefore properties which depend on the composition.

Particles can be defined as microscopic constituents of matter, like atoms or nuclei. The term can also refer to even smaller constituents like electrons, protons, or neutrons, which are called elementary particles. It is also used in the definition of an atom, the smallest particle which is characteristic for a chemical element. Atoms with the same number of protons $Z$ (and equal number of electrons) represent the same chemical element. Isotopes are atoms of the same chemical element which have different mass numbers, i.e. the same number of protons $Z$, but different numbers of neutrons in the nucleus. In the three isotopes of hydrogen, for example, the nuclei contain different number of neutrons (0, 1 and 2).

A salt is a chemical compound which consists of cations and anions. It should be distinguished from a zwitterionic compound, which contains charged but overall neutral molecules as building blocks. An example for the latter is trimethylamine N-oxide.


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