How do you identify molecules? Are these an atom or molecule: Cl2, Br2, I2, P4, N2

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Elements are the base chemicals upon which all matter in the universe is based.  The smallest unit of an element is an atom.  Molecules are made up of multiple atoms bonded together.  These atoms (making the molecule) can be of different elements or the same element.  So if you are looking at a chemical formula and it contains a subscript number, you know that multiple atoms are involved and it is a molecule.  All of the examples you listed above are molecules due to the subscripts.  But a molecule can have multiple elements and still no subscript (NaCl for example).  Elements are abbreviated to a one or two letter symbol that starts with a capital letter.  So NaCl is a molecule with two different elements, Na and Cl.  Multiple capital letters in a chemical formula denote molecules, excluding single elements.

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