What contributes more to an atoms mass:  electrons or protons??  Which
contributes more to an atom's size??

Dear Daisy,

Thanks for your question.  As one commenter noted, the protons
contribute significantly more to the mass of an atom or molecule.  The
classical (Bohr) model of the atom has the nucleus, comprised of
protons and neutrons, at the center, with the electrons orbiting
around the nucleus.  For this reason, the electrons contribute more to
the size of the atom of molecule.  The mass of an electron is about 9
x 10^-28 grams.  The mass of a proton is 1.7 x 10^-24 grams.  This
means that a proton has a mass that is greater than that of an
electron by
1.7 x 10^-24 grams / 9 x 10^-28 grams = 0.19 x 10^4 = 1900.
So, the proton is 1900 times more massive than the electron.  A more
accurate estimation of this ratio is 1836.1517.

In atoms larger than hydrogen, which has a single proton surrounded by
a single electron, there can also be neutrons in the nucleus, along
with the protons.  The neutrons contribute roughly the same to the
mass as the protons.


Here is a good page discussing atoms and their structure from Wikipedia:
http://en.wikipedia.org/wiki/Atom

This page also discusses the structure of the electrons around the
nucleus, including the various "orbitals" that the electrons can
occupy.  These orbitals correspond to energy.  Generally, the higher
the orbital, the greater the energy.

Here is an excerpt from this article discussing atom sizes:

"The size of an atom is not easily defined since the electron orbitals
just gradually go to zero as the distance from the nucleus increases.
For atoms that can form solid crystals, the distance between adjacent
nuclei can give an estimate of the atom size. For atoms that do not
form solid crystals other techniques are used, including theoretical
calculations. As an example, the size of a Hydrogen atom is estimated
to be approximately 1.2×10-10m. Compare this to the size of the Proton
which is the only particle in the nucleus of the Hydrogen atom which
is approximately 0.87×10-15m. Thus the ratio between the sizes of the
Hydrogen atom to its nucleus is about 100,000. Atoms of different
elements do vary in size, but the sizes are roughly the same to within
a factor of 2 or so. The reason for this is that elements with a large
positive charge on the nucleus attract the electrons to the center of
the atom more strongly."

Here is another excerpt that discusses atomic masses:

"The mass number, atomic mass number, or nucleon number of an element
is the total number of protons and neutrons in an atom of that
element, because each proton or neutron essentially has a mass of 1
amu. The number of neutrons in an atom has no effect on which element
it is. Each element can have numerous different atoms with the same
number of protons and electrons, but varying numbers of neutrons. Each
has the same atomic number but a different mass number. These are
called the isotopes of an element. When writing the name of an
isotope, the element name is followed by the mass number. For example,
carbon-14 contains 6 protons and 8 neutrons in each atom, for a total
mass number of 14.

The simplest atom is the hydrogen atom, which has atomic number 1 and
consists of one proton and one electron. The hydrogen isotope which
also contains 1 neutron is called deuterium or hydrogen-2; the
hydrogen isotope with 2 neutrons is called tritium or hydrogen-3.

The atomic mass listed for each element in the periodic table is an
average of the isotope masses found in nature, weighted by their
abundance."


Some other useful resources:

This is the MSN Encarta page on atoms:
http://uk.encarta.msn.com/encyclopedia_761567432/Atom.html

Richard Bowles page on atoms and isotopes:
http://richardbowles.tripod.com/chemistry/structure/structure.htm



You may also be interested in this Explore Learning site, which
includes an Element Builder.  This lets you add protons, neutrons, and
electrons interactively and then see what atom or ion you have
created.  It will also show you the chemical symbol, ionization state,
etc., for the atom you have created.  You can use the Element Builder
for free in 5 minutes blocks of time without paying.
http://www.explorelearning.com/View/correlations/States/CA/CA_S_CH.html


I hope this information was useful.  Please feel free to request clarification.

Best,

           -welte-ga

Proton's contribute more to the atom's mass.  There are equal numbers
of proton's as there are electrons, and the proton has a mass
thousands of times greater than that of the electron.

Your second question isn't very well posed, but I believe the answer
expected is that the electrons contribute more to the atom's size as
their location defines the outer extremities of the atom.