Sciencemadness Discussion Board

elements vs. atoms

r15h4bh.c - 16-8-2013 at 22:08

I learnt about atoms, elements, compounds, mixtures etc. a few years back but I recently had to revise atomic mass and reading through that section I had a small doubt.

I get what atoms are but I’m not sure what an element is. When someone says ‘oxygen’ are they referring to one single atom or to the element in its natural state like O2 or O3? And when you write the symbol for an element, like for hydrogen you write H then are we referring to the element in its natural state or just a single atom of hydrogen?

r15h4bh.c - 16-8-2013 at 22:33

Here's the question phrased better.

What exactly is a substance? On Wikipedia, it is defined as ‘a chemical substance is a form of matter that has constant chemical composition and characteristic properties.’ What does constant chemical composition mean? Can a single atom or molecule be called a substance?

What exactly is an element? The Wikipedia definition is ‘A chemical element is a pure chemical substance consisting of one type of atom.’ Take the example of oxygen. Does ‘oxygen’ refer to a single atom of oxygen or to the natural state of oxygen, either O2 or O3? Also, since it says ‘an element is a chemical substance,’ does element refer to a single atom (or molecule, depending on the answer to the previous question) or does it mean a bunch of many oxygen atoms (or molecules), since that’s what substance seems to imply to me.

What does ‘elemental X’ mean, for example ‘elemental Hydrogen’

[Edited on 17-8-2013 by r15h4bh.c]

[Edited on 17-8-2013 by r15h4bh.c]

Pulverulescent - 17-8-2013 at 04:32

Elemental hydrogen means the diatomic, most stable form!
O3 is an unstable allotrope, whereas O2 is much more stable . . .

r15h4bh.c - 17-8-2013 at 16:38

Thanks :) Other two q's?

violet sin - 17-8-2013 at 17:33

simply put, an element is a type of mater. it has distinct properties specific to that element. among others the number of proton's, neutrons and electrons per atom.

and atom is a single piece of that type. it is the smallest form an element that can still exhibit it's characteristic attributes.

a molecule is a single element or several elements in a stable form. O2, N2, S8, P4 single element examples. CH3OH is a molecule of methanol made from carbon, hydrogen and oxygen. which can be one single atom to many atoms depending on the molecule He, Ne, Ar, Kr, Xe are stable monatomic gasses at STP. meaning one atom of one element constitutes a whole molecule.

a substance is one to many different molecules in one form. glucose is a molecule on small scale and in large quantities is is also a substance. homogenous. pine resin is one substance on a large scale but if you look too closely it is many different substances mixed, heterogeneous.

all this should be in the first few pages of about any beginners chem book.

[Edited on 18-8-2013 by violet sin]

vmelkon - 17-8-2013 at 18:12

An element is a substance that is made of one type of atom. There isn't a rule that says you need to have a certain number of atoms at minimum, therefore, a single atom is a substance and it is an element.
There isn't a rule that says the substance needs to be monoatomic to be called an element. O2 and O3 are made of the element known as oxygen, therefore a test tube full of O2 gas can be said to contain the element known as oxygen.
Quote:
What exactly is a substance? On Wikipedia, it is defined as ‘a chemical substance is a form of matter that has constant chemical composition and characteristic properties.’ What does constant chemical composition mean?

Substance means matter. It can be a heterogeneous mix, a liquid, a gas, it can be anything. It is a very broad term.
Quote:
Can a single atom or molecule be called a substance?

Sure. There isn't a rule that says that you need to have a certain number of atoms at minimum. However, it is just a english word with no specifics like a lot of other words. What about a liquid? If I have a single molecule of H2O, is it a solid, a liquid or gas or unknown?

r15h4bh.c - 17-8-2013 at 23:19

Quote: Originally posted by violet sin  
simply put, an element is a type of mater. it has distinct properties specific to that element. among others the number of proton's, neutrons and electrons per atom.


But O (not combined with anything), O2 and O3 are all types of matter but they have different properties! But they can't be different elements. And can't different elements have the same number of neutrons? And the same number of electrons if they have formed a bond? This explanation doesn't help much :(
Quote: Originally posted by violet sin  
and atom is a single piece of that type. it is the smallest form an element that can still exhibit it's characteristic attributes.


Again, I doubt a single atom of Oxygen would exhibit the same characteristic attributes as O2 would.
Quote: Originally posted by violet sin  
a molecule is a single element or several elements in a stable form. O2, N2, S8, P4 single element examples. CH3OH is a molecule of methanol made from carbon, hydrogen and oxygen. which can be one single atom to many atoms depending on the molecule He, Ne, Ar, Kr, Xe are stable monatomic gasses at STP. meaning one atom of one element constitutes a whole molecule.


I don't have a problem understanding molecules.
Quote: Originally posted by violet sin  
a substance is one to many different molecules in one form. glucose is a molecule on small scale and in large quantities is is also a substance. homogenous. pine resin is one substance on a large scale but if you look too closely it is many different substances mixed, heterogeneous.


I didn't get the glucose and pine resin examples. :(
Quote: Originally posted by violet sin  
all this should be in the first few pages of about any beginners chem book.


I know, I read carefully but I don't get what the definition implies.
Quote: Originally posted by vmelkon  
An element is a substance that is made of one type of atom. There isn't a rule that says you need to have a certain number of atoms at minimum, therefore, a single atom is a substance and it is an element.


So an atom of O (not combined with anything), a molecule of O2, a molecule of O3 and a test tube of O2 all consist of only one type of atom, so they are all elements? If someone says, "The element Oxygen..." which form are they referring to? Does it depend on context?
Quote: Originally posted by vmelkon  
There isn't a rule that says the substance needs to be monoatomic to be called an element. O2 and O3 are made of the element known as oxygen, therefore a test tube full of O2 gas can be said to contain the element known as oxygen.


Again, by that first line are you saying that an atom of O (not combined with anything), a molecule of O2, a molecule of O3 and a test tube of O2 all consist of only one type of atom, so they are all elements? But then in the second line you say O2 and O3 are made of the element known as oxygen, not are the element oxygen. To me, this seems to imply that just O is the element and O2 and O3 are combinations of the element, but not the actual element itself. And in the last line when you say,
Quote: Originally posted by vmelkon  
therefore a test tube full of O2 gas can be said to contain the element known as oxygen.


What are you referring to by 'the element known as oxygen'? Are you referring to all the molecules of O2? Are referring to each molecule of O2? Or to each atom of oxygen?
Quote: Originally posted by vmelkon  
Substance means matter. It can be a heterogeneous mix, a liquid, a gas, it can be anything. It is a very broad term.


All the sources that I've read say that a chemical substance is not a mixture. For example on the wiki page for chemical substance it says 'It cannot be separated into components by physical separation methods' Obviously mixtures can be separated by physical methods. Then again on the disambiguation page for substance it says that substance may be referring to matter or chemical substance. So I guess within chemistry mixtures are not chemical substances?
Quote: Originally posted by vmelkon  
Sure. There isn't a rule that says that you need to have a certain number of atoms at minimum. However, it is just a english word with no specifics like a lot of other words. What about a liquid? If I have a single molecule of H2O, is it a solid, a liquid or gas or unknown?


Okay, so an atom is a substance too. Don't get what you're trying to get across with the 'single molecule of water' thing.

Thanks for your help :D

[Edited on 18-8-2013 by r15h4bh.c]

violet sin - 18-8-2013 at 11:23

I said simply put because there are exceptions and limitations on the rules. O, O2 and O3 have different chemical reactivities. but they are able to react like oxygen, just at diff rates. and yes there are overlaps in neutron, proton with some isotopes. but that is not really important if you don't get the rest.

a single atom of oxygen is the smallest form oxygen can come in and still be oxygen. of course it acts different. normally it is in O2 and less frequently O3. a single O is 1/2 of an O2 molecule so it has already been fed enough energy to break the inter-atomic bond. so it is looking to link back up with a partner. O3 is a standard O2 + O. so its got the desire of the single O to want to join something, AND the desire of the O2 to get back to a comfortable diatomic molecule. so kinda push pull at the same time. again details.

glucose n pine resin examples were of homogeneous and heterogeneous substances. and by way of that I was explaining that a single molecule of a substance can and/or can not be representative of a substance. in a pure substance like glucose, one single molecule of glucose represents the whole. where as with pine resin, it is only pine resin untill you look too close. then it is a mix of many things. so a single molecule of a substance CAN be the smallest size that it still acts like that substance. like the atom, element example. and yes this is a generalization here. there are exemptions.

r15h4bh.c - 18-8-2013 at 16:20

Thanks :D So basically all three forms are oxygen? But if you just say 'oxygen reacts with...' then how do you know which form of oxygen they are talking about? And can all three forms be considered elements?

violet sin - 18-8-2013 at 17:42

all three are representative forms of oxygen that exist here yes. as well as the isotopes of oxygen. when some one says that oxygen reacts with XXX, you are often told where it comes from. oxygen from the air for instance, can be represented as 1/2 O2, O or just O2 depending on the circumstances, weather or not the equation was multiplied to fit. the level of the reader (or subject matter) comes into play here. if you know enough it will be obvious where the oxygen comes from and how it is being used.

in beginning chem, often times you are asked to balance an equation where XXX is happening. they want you to remember that O comes in the normal atmospheric form of O2. that it isn't just floating around by it self in highly reactive free monatomic oxygen. this is to reinforce the idea that chemicals seek a balance of charge. oxygen by is self as a single atom isn't balanced with the octet rule. before the oxygen can react as a single atom in a rxn, you have to supply the energy to split the O2. there is a lot to chemistry and a lot that they will teach you one semester, then go back the following semester and say, " you remember when we told you XXXX? well ya that was basically a lie." you are taught chem in a generalized form. the concepts are important. after you get the concepts, they tell you what the exceptions are and why. lie, truth, lie, truth etc. until you get it. so basically it is hard to dig too far into the subject until you know the building blocks and can see with more clarity. and its hard to explain to some one also. but I fully understand being interested in a subject and not wanting to wait to know more. so just hang in there and get the basics down so the rest can fall in place.

I love chem and I like to try to help out in understanding so I hope this helps you a bit. but my way of explaining stuff doesn't work for everyone. and even what I typed up has generalizations. what amazed me about chem was the fact that water is far from this pure simple thing I thought it was.
-Violet Sin-

woelen - 19-8-2013 at 00:19

I would define 'element' as a type of atom. I try to explain by means of an analogy with Lego-bricks.

Lego has many types of bricks. They can have different sizes and different colors, e.g. 1x2 red, 2x2 red, 2x4 red, 1x2 blue, 1x2 black. Each of these types can be considered 'lego-elements'.

In this analogy, an atom is a particular instance of an element. E.g. a bag can contain 10 pieces of 1x2 red bricks. This bag then has 10 'lego atoms' of the 1x2 red type.

'Lego molecules' can be regarded as constructions made of 'lego atoms', i.e. bricks. Each type of brick allows a limited set of connections to other bricks and this limits the types of possible constructions. This is the same as with real atoms. Each type of atom only allows a certain number and type of connections to other atoms, so not every combination is possible.

In chemistry, there are appr. 100 types of atoms, i.e. appr. 100 elements. The simplest molecules are single-atom molecules. For example, helium makes single atom molecules. Most elements, however, do not easily build single-atom molecules. E.g. the element oxygen under normal conditions makes either diatomic molecules (the oxygen we breathe from the air), or triatomic molecules (ozone). Sulphur makes ring-shaped molecules, consisting of 8 sulphur atoms. Metal-elements (e.g. iron, copper) make structures of unlimited size, with zillions of atoms arranged in a 3D-structure bonded to each other with a so-called metal bond (atoms immersed in a 'sea' of electrons) and all these zillions of atoms for a lump of the metal.
Finally, and most commonly, the largest number of compounds are molecules consisting of multiple elements. A simple one is H-O-H (water), in living materials there can be tremendously complex molecules, made up from hundreds or even thousands of individual atoms.

phlogiston - 19-8-2013 at 02:21

- In the definitions used in chemistry a substance is a pure compound. It contains only molecules of one kind. Mixtures should not be called substances. Examples of substances: salt, oyxgen. Examples of mixtures that are, therefore, not substances: sea water, air, gasoline

- An element is a substance that can't be broken down into two or more simpler substances. They are composed of only a single type of atom. All pure elements are subtances.

- Some elements, such as oxygen can exist in several forms, (O<sub>2</sub> and O<sub>3</sub>;). These different forms are called allotropes. They can have very different properties and different names (oxygen and ozone, or graphite and diamond), but sometimes it is less obvious (at room temperature there are actually three allotropes of iron: ferrite, austenite and delta-iron).

- The type of atom is defined by the amount of positive charge in its nucleus, ie. the number of protons. All atoms of one element have exactly the same number of protons in their nuclei. The number of neutrons may vary, and as you correctly say there can be atoms of two different elements that have identical numbers of neutrons. Atoms of one element with different numbers of neutrons are called 'isotopes'. This is irrelevant to your questions about substances, etc. If it confuses you, forget about atomic structure for now untill you figured out the other concepts.

- The symbol for an element is used simply as an abbreviation for the element. It should be clear from the context what is meant (a single atom, the elemental substance, etc).

[Edited on 19-8-2013 by phlogiston]

r15h4bh.c - 19-8-2013 at 17:05

Thanks so much everyone! :D Now the definition and scope of 'element' make much more sense. I suppose what really confused me is that according to the definition 'A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number' O, O2 are all chemical substances with one type of atom with the same atomic number, so it follows they're both the same element, but whenever someone is talking about the properties of an element they only ever refer to the non combined form. So statements like these on the wiki page for oxygen tripped me up, 'two atoms of the element bind to form dioxygen, a colorless, odorless, tasteless diatomic gas with the formula O2' which seems to say that the non combined form is the element. Then there's this 'is a highly reactive nonmetallic element that readily forms compounds (notably oxides) with most elements except the noble gases Helium and Neon' which is again talking about non combined oxygen.

Quote: Originally posted by violet sin  
I love chem and I like to try to help out in understanding so I hope this helps you a bit. but my way of explaining stuff doesn't work for everyone. and even what I typed up has generalizations. what amazed me about chem was the fact that water is far from this pure simple thing I thought it was.
-Violet Sin-


What is the answer to the water question? I think that solids, liquids and gases only apply to many particles? Otherwise you couldn't apply stuff like 'solids have large inter particle forces of attraction' Or maybe it depends on the temperature? But I think I read somewhere that in water there can be particles with really high energy and low energy and they keep bouncing into each other or something...
Quote: Originally posted by phlogiston  
- An element is a substance that can't be broken down into two or more simpler substances. They are composed of only a single type of atom. All pure elements are subtances.


What makes a substance more simple than another? And do you mean break down using physical methods only or chemical methods also? Because then you can break down O3 into O2 and O and then how do you decide which one is simpler? :P

Thanks again everyone :)

phlogiston - 19-8-2013 at 17:31

Quote: Originally posted by r15h4bh.c  

Quote: Originally posted by phlogiston  
- An element is a substance that can't be broken down into two or more simpler substances. They are composed of only a single type of atom. All pure elements are subtances.


What makes a substance more simple than another? And do you mean break down using physical methods only or chemical methods also? Because then you can break down O3 into O2 and O and then how do you decide which one is simpler? :P


Excellent question! The answer takes us back to the early history of modern chemistry. John Dalton was the first to propose this concept of elements as we know them today, in the early 1800's , laying the foundations for 'atomic theory'. He proposed a controversial "rule of greatest simplicity" : When atoms combine in only one ratio, "..it must be presumed to be a binary one, unless some cause appear to the contrary"

In fact, it doesn't hold up. For example, it led him to assume 'OH' as the molecular formula for water. Still, his theory eventually led to the modern concept of elements.

Whether 'chemical' or 'physical' means depends on your definition of both. Any means that excludes interactions with the atomic nucleus. I think 'chemical' is most accurate. Electrochemical and thermal means of decomposition are allowed.

[Edited on 20-8-2013 by phlogiston]

r15h4bh.c - 20-8-2013 at 04:54

My definition of physical and chemical is the basic one where physical means filtration, magnets, distillation etc. and chemical means chemical reactions. If chemical is most accurate then can't O2 be broken down into two or more substances, making it a...not-element (I don't know what to call it :P)?