An atom is the form of an individual element, and vice versa.  There are over one hundered different elements, and since each element has subtle variations of itself (these variations of a particular element are called isotopes), there are well over three hundered different types of atom.  There is no such thing as a specific atom that isn’t a particular element; likewise, there is no such thing as a specific element or specific atom that isn’t a particular isotope.  Analogously, in the same way that crimson is a unique expression of red and red is a unique expression of color; an isotope is a unique expresion of a particular element, and an element is a unique expression of atom’s in general.

With the exception of hydrogen (given that it doesn’t have a neutron), all atoms are composed of three types of particles: protons, neutrons, and electrons.  Moreover, all atom’s have a nucleus, but only hydrogen has a nucleus that is bereft of a neutron.  All of the other atom’s have a nucleous that is comprised of at least one proton and one neutron bound together. These two particles are responsible for most of the atomic mass in an atom, whereas electrons hardly contribute any mass, given that an electron is approximately a thousand times smaller than a proton or neutron.  A hydrogen atom is only about a ten millionth of a millimeter in diameter, but the proton in the middle is a hundred thousand times smaller.  Atom’s contain so little actual material that they can barely be said to even exist.

I also like the different analogies that help us to understand the size of an atom:

• If you expand a hydrogen atom to the size of an apple, the apple would expand to the size of the Earth.
• If an atom were magnified until it was as large as a football stadium, the nucleus would be about the size of a grape.

Good points George.  I’d much rather prefer using analogies to convey size differences than numbers.  I’ll be sure to incoroporte those into this little essay I’m writing for school.

Question for who ever knows:  How can a person tell how many isotopes each element has?  Is there a way to tell by looking at the periodic table?

Question for who ever knows:  How can a person tell how many isotopes each element has?  Is there a way to tell by looking at the periodic table?

The CRC handbook lists the weights and half-lifes of known isotopes.  The mass listed on the periodic table is intended to account for naturally occuring isotope abundances.  So for example if you had 20 tonnes of carbon, some of it would be carbon-12, some carbon-13, some carbon-14, etc.  The known ratio of naturally occuring abundances is reflected in the stated “average” weight on the periodic table.

New isotopes are being discovered and created all the time.  Most non-naturally occuring ones are so radioactive they decay in fractions of a second.

-Scott

tscott - 10 October 2007 11:25 AM

Question for who ever knows:  How can a person tell how many isotopes each element has?  Is there a way to tell by looking at the periodic table?

The CRC handbook lists the weights and half-lifes of known isotopes. 

I’m not sure I’m ready for that stuff yet.  But let me ask you the most basic question I can think of: does every atomic configuration have a half life?  (If you have the time to have a discussion with me, please answer as briefly as possible, as that will allow for a more efficient dialogue.  Often a person will write a page to me, and I because I don’t understand a very basic thing, the person’s efforts go to waste)

The mass listed on the periodic table is intended to account for naturally occuring isotope abundances. 

I’m not quite sure what you’re referring to here.  For instance, consider the large version of the Periodic table on wikipedia.  Each element has a mass number.  Is that what you are referring to?  If so, then I have a vague idea of what you’re aiming at, and maybe we can discuss that.

So for example if you had 20 tonnes of carbon, some of it would be carbon-12, some carbon-13, some carbon-14, etc. 

I think I see what you mean here.  In any given tangible amount of element: Gold, Silver, Iron, Carbon, etc: you will find that the tangible element consists of individual elements that slightly vary.  Is that what you mean?

The known ratio of naturally occurring abundances is reflected in the stated “average” weight on the periodic table.

You lost me here.  Based on the periodic table that I linked to above, where can I see this average weight?  Does each element have an average weight?  Are you equating the average weight with the atomic mass?

Many periodic tables, like the one here, list the atomic weight of the element as well as its atomic number (i.e. “6” is the number for carbon, which is the number of electrons or the number of protons).  The “number” is the only value listed on the Wiki table that I saw at a glance The atomic weights, listed at the bottom of the element’s box on the table I pointed you at, is the value of the average weight of a collection of such atoms taken from out physical world.  So for example, carbon (C) has a weight of 12.011.  This is because in a given random sample of real carbon atoms, most of them will wiegh 12 AU, some will weigh 13AU and a few will weigh 14AU.

There are many isotopes which do not decay.  Carbon 12 is one example.  Again, the CRC, available at any library or school, will list half-lifes for isotopes that decay and will also indicate which isotopes are stable (or don’t decay).