I am taking an Earth Science course now and currently reviewing some basic chemistry knowledge needed for further study on earh mineral and rocks. But I haven't touched chemistry for almost 2 years that I start to forget things, can someone help and clarify some basic chemistry stuff? I would appreciate! :)

1) Are the terms "element" and "atom" interchangeable? If not, why not?

2) Do the terms "substance" and "matter" mean exactly the same thing?

3) "Two of the eight common elements in the earth's crust are oxygen (O) and silicon (Si)"
http://inst.sfcc.edu/~gmead/minerals/crustal.gif

Why is oxygen expressed as O, instead of O2, in both the quote and diagram? Does that mean single O atom exist naturally in the crust?

4) Compound: a substance that results when the atoms of two or more elements are chemically combined
This is the definition of a "compound" from my text book and it's the same as all sources in the internet. According to the definition of a compound, O2 and N2 are not compounds, than what are they? elements?

1 no, an atom is the smallest smallest unit of matter available, while an element is a pure sample of just one type of atom, which can consist of as many atoms asn you want.

2 no, generally matter is the term used to describe all non energy in the universe, while substance is used to describe a compound or mixture, generally before you have discovered what it is, as then you would refer to it by its name. ie, if you were asked to identify a mystery substance given to you in a test.

3 that is mostly to do with the terms used. since they say 'atom' then they have to talk about the number of O atoms, rather than the amount of O₂ molecules. as far as i know, oxygen only naturally occurs as O₂

4 perhaps it should be "when two or more different elements are chemically bonded", basically, they arent compounds because they are only one type of atom.

A lot of the oxygen in the earth's crust in bound up in silicon oxide, where individual atoms of oxygen are bound to silion atoms - so it's not in the O2 form.

4) A compound is a substance that results when the atoms of two or more elements are chemically combined
A molecule is the smallest unit of a compound that still retains its properties.

If the above 2 definitions are both true, than O2, H2, N2 are all not (diatmoic) molecules, since they do not meet the requirement that "two or more elements", each of O2, H2, N2 has only one element chemically combined?! But why are they always referred to as diatmoic molecules?

A molecule is two or more atoms, of one or more elements bonded together; thus O2 and H2O are both molecules, but only H2O (not O2) is a compound because a compound consists of more than one element. Diatomic means that the molecule only has two atoms: "di" means "two" (di-atom).