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difference between Elementary molecules and Compound molecules

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A molecule is formed when two or more atoms join together chemically. A compound is a molecule that contains at least two different elements. All compounds are molecules but not all molecules are compounds.

Molecular hydrogen (H2), molecular oxygen (O2) and molecular nitrogen (N2) are not compounds because each is composed of a single element. Water (H2O), carbon dioxide (CO2) and methane (CH4) are compounds because each is made from more than one element. The smallest bit of each of these substances would be referred to as a molecule. For example, a single molecule of molecular hydrogen is made from two atoms of hydrogen while a single molecule of water is made from two atoms of hydrogen and one atom of oxygen.

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Hi Prasenjeet,

I do not believe "elementary molecule" is still a common term in chemistry, but I did find the following wikipedia article that explains that Avogadro used to refer to atoms as "elementary molecules" and to compounds as "compound molecules" (http://en.wikipedia.org/wiki/History_of_molecular_theory#19th_century). If any other tutors know of a more recent distinction here, that would be appreciated.

Molecules are substances that are comprised of more than one atom. These atoms may be joined by ionic bonds or covalent bonds. Compounds, specifically, are molecules that combine more than one element; this excludes substances such as oxygen, hydrogen, and nitrogen, all of which naturally exist in a molecular form: O2, H2, and N2.

I hope this helps. Please let me know if you have any other questions.

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