[blokeybloke]

Hey guys,

I’m not too sure whether this topic is appropriate for this forum although I’m still having a lot of trouble understanding the definitions of elements and substances. In chemistry, we define elements as “species of atoms with the same number of protons in their nucleus” as well as “substances made up of one type of atom”. I do not know what the second definition means at all. What exactly is a substance? From the latter definition of an element it sounds like it is a collection of particles (i.e. atoms, molecules or ions). But then again, we also refer to “water” as a molecule that is also a substance. All this just seems so contradictory and it is extremely  confusing to me. I would really appreciate it a lot if someone could help me out here. Thanks a lot in advance.

[Borek]

I am afraid we won't be able to add much more than we already told you in the previous thread: http://www.chemicalforums.com/index.php?topic=97416.0

[-_-zzzz]

I am afraid we won't be able to add much more than we already told you in the previous thread: http://www.chemicalforums.com/index.php?topic=97416.0

Ok sure that's fine. Although I am confused as to what it means when people say certain elements "exist" as atoms or diatomic molecules. Could you explain with reference to the definition of a substance that we have gone over? Thank you very much in advance.

[Borek]

Although I am confused as to what it means when people say certain elements "exist" as atoms or diatomic molecules. Could you explain with reference to the definition of a substance that we have gone over? Thank you very much in advance.

Back to marbles - imagine all green marbles are called "nitrogen atoms". Then, if you have a collection of green marbles, you have some nitrogen, or you have some nitrogen atoms. Now, thing is, these green marbles have tendency to connect to each other, so that they look like kind of a dumbbell. We call these dumbbells "nitrogen" as well. When it matters and we need precision we should name them specifically "nitrogen atoms" or "nitrogen molecules", but most often we will just say "nitrogen", as the exact form is typically obvious from the context.

Separate nitrogen atoms (while they can exist in some exotic conditions) are very rare, they have very strong tendency to combine into molecules. So nitrogen as a "substance" (collection of objects) will be almost always collection of nitrogen molecules. We also often say "nitrogen" when we really mean "gas made of nitrogen molecules" - again, in most cases context gives the clues. Whenever in doubt you can always use more descriptive name (thus "atomic nitrogen gas" will be perfectly understood and unambiguous, although every chemist will think about gas of high temperature and/or very low pressure, as these are the only conditions where such gas is reasonably stable).

[blokeybloke]

Although I am confused as to what it means when people say certain elements "exist" as atoms or diatomic molecules. Could you explain with reference to the definition of a substance that we have gone over? Thank you very much in advance.

Back to marbles - imagine all green marbles are called "nitrogen atoms". Then, if you have a collection of green marbles, you have some nitrogen, or you have some nitrogen atoms. Now, thing is, these green marbles have tendency to connect to each other, so that they look like kind of a dumbbell. We call these dumbbells "nitrogen" as well. When it matters and we need precision we should name them specifically "nitrogen atoms" or "nitrogen molecules", but most often we will just say "nitrogen", as the exact form is typically obvious from the context.

Separate nitrogen atoms (while they can exist in some exotic conditions) are very rare, they have very strong tendency to combine into molecules. So nitrogen as a "substance" (collection of objects) will be almost always collection of nitrogen molecules. We also often say "nitrogen" when we really mean "gas made of nitrogen molecules" - again, in most cases context gives the clues. Whenever in doubt you can always use more descriptive name (thus "atomic nitrogen gas" will be perfectly understood and unambiguous, although every chemist will think about gas of high temperature and/or very low pressure, as these are the only conditions where such gas is reasonably stable).

Thanks a lot Borek. Sorry to bother you with these questions. But I am starting to look at trends in the period table. When we say that reactivity of metals decreases down a group, are we talking about types of atoms or the solid metal objects? How can I tell in most cases?

[Borek]

We rarely deal with single atoms in chemistry (and their properties are often different from the properties of a bulk substance).