kJ/(g-atom) idea?

[robz]

Hi,
I was looking at a paper recently which used the units kJ/(g-atom) as a
measure of atomization enthalpy. I really don't understand this unit.
At first I thought it was simply the energy divided by the molar mass,
but I don't think this is true. Is anybody able to help me with this?

Thanks,
Rob

[t***@gmail.com]

"Gram-atom" is, I believe, an old term for mole. So these are the
normal units of kJ/mol.

[Lloyd Parker]

Post by t***@gmail.com
"Gram-atom" is, I believe, an old term for mole. So these are the
normal units of kJ/mol.

Gram-atom was used for atomic weights (source: Chemical Technician's Ready
Reference Handbook), and is the same as gram-atomic weight. They distinguish
it from mole, or gram molecular weight, which was reserved for molecules.

[Ron Jones]

Post by robz
Hi,
I was looking at a paper recently which used the units kJ/(g-atom) as
a measure of atomization enthalpy. I really don't understand this
unit. At first I thought it was simply the energy divided by the
molar mass, but I don't think this is true. Is anybody able to help
me with this?

More or less the same thing...
If it were a compound then it would be in kJ/mole, but only compounds have a
molecular weight, elements have an atomic weight, so for elements it's
kJ/(g-atom)

[t***@gmail.com]

Post by Ron Jones
If it were a compound then it would be in kJ/mole, but only compounds have a
molecular weight, elements have an atomic weight, so for elements it's
kJ/(g-atom)

That's odd. I have never heard that the term "mole" was reserved for
molecules.

My copy of the CRC (70th ed.) defines the mole as being "equal to the
amount of substance that contains as many elementary units as there are
atoms in 0.012 kg of carbon-12. The elementary units may be atoms,
molecules, ions, radicals, electrons, etc. and must be specified."

Cheers,

[Bob]

Post by t***@gmail.com

Post by Ron Jones
If it were a compound then it would be in kJ/mole, but only compounds have a
molecular weight, elements have an atomic weight, so for elements it's
kJ/(g-atom)

That's odd. I have never heard that the term "mole" was reserved for
molecules.

Good. It is an archaic distinction. Apparently it was retained in the
UK longer than it was here.

bob

[robz]

Thanks a lot everyone, this has helped me a lot.

rob

[///Owen\\\\\\]

Post by Bob

Post by t***@gmail.com

Post by Ron Jones
If it were a compound then it would be in kJ/mole, but only compounds have a
molecular weight, elements have an atomic weight, so for elements it's
kJ/(g-atom)

That's odd. I have never heard that the term "mole" was reserved for
molecules.

Good. It is an archaic distinction. Apparently it was retained in the
UK longer than it was here.
bob

The "gram atom", "gram molecule" and "gram ion"...... God, it was a long
time ago.

[r***@gmail.com]

And, lest this all be taken as without the possibility of ambiguity
(notwithstanding all the terminology), ;-) Some entity's enthalpy of
atomization of "1.0 kJ/(ounce-atom)" would refer to the enthalpy to
atomize as many formula units of the entity in question, as atoms in
0.75 pound of carbon 12.

If the entity referred to an element (Sn for example), as, say, alpha-
or beta- tin without any implied formula unit, it would be necessary to
accept the rather distressing limit of but one such formula unit in the
entire 0.075789 pound.

Or, a little simpler, if the entity referred to carbon as graphite or
diamond, one such formula unit in the entire 0.75000 pound.

[Bob]

Post by r***@gmail.com
And, lest this all be taken as without the possibility of ambiguity
(notwithstanding all the terminology), ;-) Some entity's enthalpy of
atomization of "1.0 kJ/(ounce-atom)" would refer to the enthalpy to
atomize as many formula units of the entity in question, as atoms in
0.75 pound of carbon 12.
If the entity referred to an element (Sn for example), as, say, alpha-
or beta- tin without any implied formula unit, it would be necessary

Nah. It is convention to use the common formula -- atom for (most)
elements or empirical formula for ionic compounds -- as the base of
determining what a mole is. A mole of diamond is 12 g. And there is no
ambiguity, because you say, a mole of C etc.

bob

Post by r***@gmail.com
to
accept the rather distressing limit of but one such formula unit in the
entire 0.075789 pound.
Or, a little simpler, if the entity referred to carbon as graphite or
diamond, one such formula unit in the entire 0.75000 pound.

[r***@gmail.com]

I certainly accept the convention of the g-mol, and the g-atom as its
synonym. They lead to an extra step in an SI world, to speak of the
number of entities in 0.012 kg of carbon-12.

Now there have been times in which the pound mol was discussed, and
there the base number of entities is the number of atoms in 12 lb of
carbon-12. (A tip of the hat to the times when knowing how many
atoms-- er, entities-- there were in a sample was more problematic than
knowing the number of entitities was identical in two samples.)

So, for the sake of discussion and a parallel example, I spoke of an
ounce-mol (or an ounce-atom) in a pound-standard world.

But then I realized (imagined?) the real basis for the whole thread--
it dealt with the example of atomization energies. Perhaps the heart
of the origin of the thread was wondering if "g-atom" called for some
additional partition of the energy! Now THAT deserved a "nah." ;-)

And I remembered that the sublimation energy of graphite had been
enough of a controversy to be interesting wholly outside semantics. So
I went around Robin Hood's barn to check out tin examples. (a grey
area!) And I discovered that a tin atom weighs to within 2%, ten times
a carbon atom...

Too much fun on the internet. Mea culpa! Mea culpa! (At least,
restraining myself from perverse speculation on the etymology of
"tin.")

However, before signing out I will mention... what I wish I had said.
The atomization energy per g-atom of carbon as (the most common)
fullerene is the energy to form sixty g-atom carbon atoms. The energy
to form a g-atom of carbon atoms from that fullerene is one sixtieth of
that energy. It matters not at all-- is totally irrelevant!-- whether
one uses the term "g-atom" or "g-mol" at any point within that
paragraph.

John