Different textbooks use different terminologies when discussing the same chemical compounds, for example when discussing the minerals...

  1. Elements / Compounds

    • Sulfide minerals consist of metallic elements in chemical combination with the element sulfur
    • Phosphate are a group of minerals of one or more metallic elements chemically associated with the phosphate compound PO4
  2. Ions

    • Sulfide ions (S$^2$-) bond with a number of positive ions to form the sulfides
    • Phosphorous in the form of phosphate ions (PO4$^3$-) binds with positive ions to form the phosphate minerals

Are element/compounds and ions the same thing?
If they are the same, why the two terms?
Which is better terminology?

I'm just after a basic understanding of how elements form rocks and being able to differentiate between this terminology is beyond my layman abilities.


1 Answer 1

  • Elements are just one specific type of atom. Magnesium, oxygen, sulfur, etc. These are all elements.

  • Compounds are several different types of atoms (that is, elements) bonded together. For example, magnesium oxide (MgO), or iron sulfide (FeS).

  • Ions are charged atoms or compounds (also known as species in some cases). Ions can be monoatomic (consisting of only one atom, or element) or polyatomic (consisting of more than one type of atom). Cations are positively charged (like Ca2+) and anions are negatively charged (like O2-). Polyatomic ions that occur in geological contexts are usually anions, with carbonate (CO32-) and sulfate (SO42-) being two good examples.

All ions are elements (or combination of them). Not all elements are ions. We geologists and geochemists use these terms interchangeably all the time. I think that using ions may be slightly better, particularly in the case of sulfur. Because sulfur in minerals can be either in sulfate or sulfide form, just saying "sulfur" gives little information. The properties of sulfate and sulfide minerals are completely different, so it's actually useful to know.

Sulfide ions (S2-) bond with a number of positive ions to form the sulfides

Not strictly correct, because the bonding in many sulfide minerals is a combination of covalent and metallic bonding, and it only has a little ionic character. Still, even in cases when the bonding is not ionic (such as in sulfide minerals), we still often talk of them of ions because it's convenient and easy.

Sulfide minerals consist of metallic elements in chemical combination with elemental sulfur

This statement is not correct. When we speak of "elemental" elements, we mean that they are monoatomic compounds (or minerals). In the case of sulfur, it will be the yellow sulfur mineral. Gold, copper and silver and other example of "elemental" minerals. Once the sulfur is bonded to other metals, it is no longer elemental. It is part of a sulfide mineral.

  • $\begingroup$ Why is talking of "them" as ions more convenient/easy than speaking of them as compounds or elements? $\endgroup$
    – G. Gip
    Jul 22, 2016 at 11:06
  • $\begingroup$ "All ions are elements (or combination of them). Not all elements are ions". What are elements called that are not ions? $\endgroup$
    – G. Gip
    Jul 22, 2016 at 11:11
  • $\begingroup$ Apologies. elemental sulfur was a typo. In the book it actually says element sulfur. Is that statement still incorrect? $\endgroup$
    – G. Gip
    Jul 22, 2016 at 11:13
  • 1
    $\begingroup$ To take salt as a common example, on land the salt is solid - the sodium and chloride ions are combined, so it is conventional to talk of elemental proportions. In seawater, however, the sodium and chloride largely dissociate into their separate ions. So whether we talk of elements or ions largely depends upon the context. But do not confuse chlorine the element with chloride, the charged element. they behave dramatically differently! $\endgroup$ Jul 22, 2016 at 12:21
  • 1
    $\begingroup$ Does an ion remain always an ion? For example, do carbonate ions cease to be ions when bonded; OR does a carbonate ion remain an ion when bonded because it retains a charge or something? I'm trying to figure out if I should refer to a carbonate ion differently when it is a bond, perhaps as a "carbonate compound", and only as a carbonate ion if its dissociated in water. $\endgroup$
    – G. Gip
    Jul 22, 2016 at 18:06

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