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Name: Lisa Status: educator Grade: 6-8 Location: NH Country: USA Date: Spring 2012

I am helping my students learn the difference between atoms/molecules, compounds/elements. We have been drawing circles touching and not touching to represent chemically bonded and not chemically bonded. Please let me know if I am explaining this correctly: A container with one black circle is a container holding one element. A container with 2 black circles not touching is a container holding one element. A container with 2 black circles touching is a container holding one molecule, but it can also be described as holding one element. A container holding one black circle, and two more black circles that are touching is a container holding one element. I know oxygen can be found in the form of 02, if it is in a box, does the box still contain one element or can you say it only contains one molecule? Oxygen can be found in the form of 03 (ozone). If one molecule of 03 is in the box, does the box contain one element, or does it contain only a molecule or can you describe it both ways?

Hi Lisa,

Both ways in this case. We think of a circle as a single atom of an element, which could be a molecule. Two circles just touching are two individual (singlet) atoms bumping into each other. Bound atoms, defined as a molecule, are partially overlapping circles.

The black circles, a single “molecule” of an element, oxygen, are a bit amiss because oxygen does not exist as a single molecule, for any sustainable length of time, in nature.

Oxygen exists as two elemental oxygen(O) atoms, bound together to form the molecule, oxygen, O2. The container still contains one element, oxygen, but as an oxygen molecule. These we would represent as overlapping circles.

Ozone, O3, exists as three atoms of oxygen bound together. Here, the container still contains one element, oxygen, but as an ozone molecule… three overlapping circles.

If you think of the black circles as atoms, it may help any confusion and just let the molecules fall where they may. No matter how many, if all you see is black = one atom, one element.

Letting the molecules fall, Peter E. Hughes, Ph.D. Milford, NH

Elements can exist in both atomic and molecular form. The element Oxygen exists as O, O2 and O3. It can also combine with other elements to form other molecules which could not be described as elements.

Greg Bradburn

Hi Lisa-

I think it’s great that you are helping your students understand these concepts. Yes, I think you have it correct. And to answer your questions: you can describe it both ways. If you say one “kind” or “type” of element that may help you keep it straight.

For example, when you have a molecule of O2 in your box, you still have one type of element (and two atoms of that element). If you have two atoms of Oxygen separated (not bonded), you still have one type of element (and two atoms of that element). If you have an O3 molecule, you have one type of element and 3 atoms of Oxygen. If you have O2 and O, you have one molecule of O2 and one free atom of O, all the same type of element and a total of three atoms of Oxygen. If you have three free atoms (not touching or bonded), you have one type of element and three atoms of Oxygen.

I hope this helps. Patricia Rowe, MS, MS Ed

Lisa, You are correct across the board. As long as the element is not with another element then it is called an element. If You have a pile of pure iron on your desk it is elemental iron. This is even though there are many many atoms of Fe. In the case of oxygen you have atomic oxygen (one atom) and molecular oxygen (two or more atoms joined of same element). For oxygen there are multiple forms of molecular oxygen (allotropes) as you mention. So in your final example you can say indeed it is one element as well as on type of molecule. Now if you had O2 and O3 both you would have one element but two different molecules.

Hope this helps, Brad Sieve


Atoms differ from molecules in that atoms are single, individual entities, while molecules are bonded atoms. Compounds differ from elements in that compounds contain more than one atom, whereas an element contains the same atoms. Thus O is an atom, O2 is a molecule - but still considered an element (only one type of atom), whereas CO2 is a compound and a molecule because there are two different types of atoms in it.

Greg (Roberto Gregorius) Canisius College

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