Re: Why does the number of protons determine the element?

Hello Sam:

As you probably know, an atom of an element starts out electrically neutral, i.e., the number of electrons (each carrying a "-1" electric charge) equals the number of protons (each carrying a "+1" charge) in the nucleus. When atoms take part in chemical reactions, two possibilities occur. One possibility is that atoms of metals lose some of their electrons, becoming positively-charged atoms (cations) as a consequence, while atoms of non-metals gain those electrons, becoming negatively- charged atoms (anions). Since "opposite charges attract," the cations and anions will be "bound" to each other by electrostatic forces called "ionic bonds." Huge collections of these combinations (each of which is called a "formula unit") form what are called ionic compounds. On the other hands, if non-metal atoms combine with other non-metal atoms, electrons are not lost or gained, but are shared among the atoms. The electrons, in effect, form an "electron glue" which binds the atoms to each other. These types of attractions are called covalent forces or covalent bonds. The atomic combinations in this case are called molecules, and huge collections of these molecules form what are called molecular elements (such as dioxygen or difluorine) or molecular (covalent) compounds (such as sulfur dioxide or hydrogen monofluoride). In spite of all the chaos that occurs when the electrons (which turn out to be the most energetic, outermost electrons of the original atoms - - hence are called valence electrons) trade places or form the "glue," the atoms themselves retain their identity, since the numbers of protons in their nuclei remain the same during chemical reactions. Perhaps this is the reason why protons define what an element (or an atom of an element) is, rather than the number of electrons (or, as you phrased it, ". . . why is it the number of protons that determines the element?"). A more fundamental reason for this identity were the original experiments using X-rays, carried out on elements by a British scientist, H.G.J. Moseley in 1913(check out his biography at the following URLs:

http://www.uh.edu/engines/epi717.htm
http://www.absoluteastronomy.com/encyclopedia/h/he/henry_moseley.htm

He showed that the characteristic "periodic properties" of elements were fundamentally associated with the positive charge in the nucleus (i.e., the atomic number, Z). This was experimental and interpretational work of the highest order, and had he not been killed in World War I (in those times the British government sent their scientists to the front lines), he would undoubtedly been awarded the Nobel Prize for Physics and/or Chemistry (needless to say, government scientists are no longer sent "to the front").

Regards,
P.M.Fichte
Professor of Chemistry
Coker College
Hartsville, SC