PDA

View Full Version : why does number of protons affect chemical properties so much?


vinniepaz
12-20-2004, 12:10 AM
As I understand it, the only thing on a chemical level differentiating the different elements from each other is number of protons, and thus electrons. But how does adding one proton change something from say, carbon to nitrogen?

I would guess there is something huge here that I'm misunderstanding.

HMS Irruncible
12-20-2004, 12:29 AM
As I understand it, the only thing on a chemical level differentiating the different elements from each other is number of protons, and thus electrons. But how does adding one proton change something from say, carbon to nitrogen?

I would guess there is something huge here that I'm misunderstanding.
All chemical reactions depend on interactions between electrons. Or, more specifically, their energy level configuration and their attraction for the positively-charged nucleus. Really, that's the definition of "carbon" and "nitrogen" right there... one has 6 protons, the other has 7. Everything else you know about carbon and nitrogen stems from that fact.

Electron configuration is arguably the most influential factor. I don't have time to explain how it works, but suffice it to say that elements that have the same electron configuration are chemically similar. Long ago a scientist noticed that every 8th element (as counted by atomic number) had similar chemical properties. In other words, chemical properties occur in periods of 8, so you could build something called a "periodic" table that organized elements according to chemical properties. This discovery was what led to the discovery of the chemical importance of electrons.

If your question of "why does a single proton make a difference between a black solid and an odorless, colorless gas", I'm afraid I can't provide a good explanation except to say what should be clear by now... that it's (mostly) about the electrons.

friedo
12-20-2004, 12:33 AM
To belabor NattoGuy's point, what really matters is the configuration of the electrons. Atoms want to have a neutral charge, so if you add more protons, you need to add more electrons also. As you add electrons, certain patterns in their structure begin to repeat themselves, which is why we can group them into categories like metals, noble gases, and so on. Elements in those categories behave in a similar fashion, due to their electron configurations.

For more, sign up for Chem 101. :)

Roland Deschain
12-20-2004, 01:01 AM
As I understand it, the only thing on a chemical level differentiating the different elements from each other is number of protons, and thus electrons. But how does adding one proton change something from say, carbon to nitrogen?

I would guess there is something huge here that I'm misunderstanding.
Well I'm no chemist but,it's not just protons. If something has more protons it also has more neutrons (although the reverse is not true since we have Isotopes). Also, keep in mind that although protons have the opposite (and approximately equal) charge as electrons they are much larger and reside in the core of the atom rather than in the "space/orbitals" around the atom as do electrons. However, the question as to "why" they make such a huge difference is a good, subtle, and posibly profound one. I think it has something to do with the electromagnetic properties of the protons. Keep in mind that within a column (such as the noble gasses) that it is the electrons (and specifically the outer of valence electrons) that most determine their chemical properties. I leave it to the chemists out there to better elucidate the unique role of Protons in determining different elements

Alan Smithee
12-20-2004, 01:30 AM
IANAC, but I think it's important to remember that you are never dealing with isolated, individual atoms of an element. Most of the properties that you associate with an element come from the way atoms of the element react to other atoms (including other atoms of the same element).

IOW, the difference between carbon and nitrogen comes not from the fact that nitrogen has one more proton, but that atoms of nitrogen join together in pairs to make N2 (I think), while carbon atoms each form bonds with four neighboring atoms, creating lattices of variable regularity. Diatomic molecules like nitrogen, oxygen, hydrogen, etc., are frequently gasses at room temperature (as are monatomic elements, like the noble gasses, and small compounds like CO, CO2, and in some conditions H2O.) Carbon forms large chunks of coal or diamond that are much to large to be gaseous, but don't have a smaller mollecular stucture. (In a sense, the whole diamond or hunk of coal is a single molecule.)

Metals also have a loose crystaline structure--positive ions form regular lattices, but instead of forming tight bonds with their neighbors, they "float" in a sort of "sea of electrons" that belongs commonly to all of them (hence metal's excelent conductivity). Again, it's not the properties if the individual atoms you notice, so much as the way they interact with one another. That interaction is directly related to the behavior of the electrons, which in turn is directly related to the number of protons in the relevant nuclei.

Roland Deschain
12-20-2004, 01:53 AM
Good point, thus much of what see as properties of the atoms in a given element are instead "emergent properties" resulting from the unique electromagnetic interplay of the those atoms between one another. We see the same phenominum with compounds. Consider that both Na (sodium) and Cl (chloride) are explosive and poisonous in their elemental states. However, Sodium chloride is tasty. On the other hand Carbon, and Nitrogen are essentially harmless in their elemental form buy cyanide is deadly (the same thing can be said of Carbon and Oxygen and I'm sure you can think of other examples).

Heller Highwater
12-20-2004, 02:06 AM
One of the things that pretty much everyone to reply so far has mentioned has been that it is the electrons that determine the chemical properties, and not the protons. The reason for this comes from the structure of the atom itself. Once the electron was discovered (by JJ Thomson), people figured that there had to be some corresponding positive particle (the proton), since it was known that atoms were electrically neutral. (The discovery of the neutron didn't come until much later.) However, nobody knew for certain what the distribution of protons and electrons was until Rutherford performed his gold foil experiment. This showed that the positive part of the atom was confined to a very tiny space (the nucleus), compared to the total volume. As an example, the radius of the hydrogen atom is about half an Angstrom (10-10 m), while the nucleus is only a few femtometers (10-15 m) in size.

So, what does this mean for atomic and molecular interactions? Well, it means that if you want the protons to interact, you're gonna' need to really small objects (the nuclei) to collide, compared to something about a trillion times larger for electron interactions. (I'll note here that I'm simplifying things somewhat, but hopefully you get the idea. Please forgive me.) Of course, in a neutral atom, the number of protons and electrons is the same, so we choose to classify elements based on their proton number. However, a carbon atom with one extra electron will behave in roughly the same manner (i.e., react in the same way) as a neutral nitrogen atom, or an oxygen atom with one electron missing.

Similarly, we say that only the "outer shell" electrons participate in reactions (not technically true, but a decent approximation for a lot of important elements like carbon and oxygen), which means that elements with the same number of valence electrons will react similarly. This was the basis on which Mendeleev (the scientist mentioned by NattoGuy) created his periodic table. Of course, at that time, the concept of electron shells, or even electrons (they wouldn't be discovered for almost another 20 years), but the idea of categorizing elements based on their chemical properties is a very fundamental one. As an aside, I believe Mendeleev even predicted the existance of an element, prior to its discovery, based on a "hole" in his table. I don't have a cite for that at the moment, but I'll try to find one.

Smeghead
12-20-2004, 02:07 AM
An atom is really nothing more than a clump of electrical charges, when you get right down to it. So the number of those charges is clearly going to be important.

CurtC
12-20-2004, 12:19 PM
Well I'm no chemist but,it's not just protons. If something has more protons it also has more neutrons...But don't different isotopes of the same element share the same basic chemical properties?

I don't really agree with the statements here that the electrons give an element its "personality," because electrons come and go. What stays behind are the protons, and if one of those goes, you have a completely different kind of substance.

vinniepaz
12-20-2004, 01:48 PM
Thanks for all the answers.

But, when something becomes an ion, it loses or gains electrons, correct? And does it not also retain it's chemical properties?

rowrrbazzle
12-20-2004, 03:03 PM
As an aside, I believe Mendeleev even predicted the existance of an element, prior to its discovery, based on a "hole" in his table. I don't have a cite for that at the moment, but I'll try to find one.Germanium. He called it eka-silicon. http://www.infoplease.com/ce6/sci/A0820627.html

HMS Irruncible
12-20-2004, 04:02 PM
Thanks for all the answers.

But, when something becomes an ion, it loses or gains electrons, correct? And does it not also retain it's chemical properties?
Yes, the electron count changes, but the electron configuration stays within one of several known states for that element.

To explain this simply in rank layman's terms takes a lot more education than I have. But there exists a total of 25 possible zones called "orbitals" that electrons can occupy. They are both energy levels and probability functions, and they are filled in sequence. Most importantly, they determine the size, shape, and reaction behavior of the electron cloud. The electron orbital configuration is what gives the element all of its chemical, physical, and optical properties.
Even if no electrons are present, the number of protons governs how electrons will "fall" into the orbitals.

Take a look at some software-generated graphics (http://www.shef.ac.uk/chemistry/orbitron/AOs/2p/index.html) of what orbitals look like. Even if you strip away all the electrons as in a plasma state, the atom still has these zones, these theoretical orbitals that govern how the electrons will behave around the atom. And the atom follows very deterministic guidelines about filling the orbitals... The atom is most stable when each orbital has two electrons and all orbitals in an energy level are filled. Orbitals are filled from the lowest energy to the highest, in sequence. All orbitals in an energy level must contain one electron before any one can have two electrons.

I can't think of any better analogy than to say it is something like a roulette wheel. A wheel 30cm in diameter with 30 slots and 1 ball will always have the same probability set of outcomes. If you add more balls the results will change, but they change in a known way for that wheel.

Whew, I sure respect my chemistry teacher a lot more now. This stuff is hard to explain.

Roches
12-20-2004, 04:15 PM
CurtC: Isotopes of the same element have the same chemical properties, for the most part. There are exceptions -- deuterium, for example, has around twice the atomic weight of hydrogen, so it does react differently in reactions where atomic weight is a factor. Also, some isotopes are more useful than others for certain chemists who need to determine the structures of their products by NMR spectroscopy, which uses the nuclear 'spin' of compounds to determine the structure of compounds by examining the influence of the magnetic field surrounding the nuclei. Some isotopes are invisible to NMR spectroscopy, which can be either a good thing (solvents used for this technique have to be 'invisible', or the solvent would be the only thing you saw) or a bad thing (when an important atom is missing from the results).

vinniepaz: Right, when a neutral atom loses or gains electrons it becomes an ion. But ions are extremely reactive, and most exist for only a fraction of a second (a very small fraction of a second -- less than it takes for a chemical bond to vibrate) before they react with something that stabilizes their charge. Ions don't really resemble the adjacent elements that have the same electronic configuration, because ions have positive or negative charge, while the adjacent elements do not. Their reactivity is characterized more by the electrical charge than by the resemblance to the electron configuration of an adjacent element. Once the ion has reacted and stabilized its negative or positive charge (even if it formally retains that charge, as in ionic compounds), it continues to resemble the neutral element.

HMS Irruncible
12-20-2004, 05:39 PM
vinniepaz: Right, when a neutral atom loses or gains electrons it becomes an ion. But ions are extremely reactive, and most exist for only a fraction of a second (a very small fraction of a second -- less than it takes for a chemical bond to vibrate) before they react with something that stabilizes their charge.
Is that really true? By definition, aqueous solutions of acids, bases, or salts are all composed mainly of ions. You can keep a bottle of OH- or H30+ in perpetuity, as long as the bottle is sealed and made of a chemically inert substance.

Alan Smithee
12-20-2004, 11:07 PM
But don't different isotopes of the same element share the same basic chemical properties?

I don't really agree with the statements here that the electrons give an element its "personality," because electrons come and go. What stays behind are the protons, and if one of those goes, you have a completely different kind of substance.Thanks for all the answers.

But, when something becomes an ion, it loses or gains electrons, correct? And does it not also retain it's chemical properties?
Remember that the physical properties you associate with a substance are emergent from the ways in which individual atoms within the substance interact with one another. Atoms interact with one another through their electrons. It is precicely the way in which the electrons come and go that create the substance's physical properties, and it is the potential for those particular comings and goings that are its chemical properties.

For example, it is a chemical property of sodium that it tends to lose one of its electrons. It can hardly be said to have lost this property when it actually does lose its electron, say to chlorine, which has the chemical property of tending to grab electrons. But its physical properties certainly have changed! In elemental form, sodium is a metal. Its nuclei and their closely held electrons float as ions in a sort of sea of loosely held outer electrons. (Remember my description of metals above?) Give those electrons to chlorine, however, and the ions rigidly align thelmselves with the neighboring chlorine ions in edible transparent cubic crystals--about as different a substance as you could imagine! (It's best to have this happen slowly, or you'll get quite an explosion as the ions align themselves energetically!) Strip the electrons away entirely, and you have plasma, a bunch of positive ions flying around repelling each other--an even more bizzare substance completely unlike a metal.

So you do get a completely different substance when electrons come and go, but because the protons more-or-less define the ways they come and go (and because protons tend to stay put, relatively speaking) it makes sense for chemists to think of these completely different substances as composed of the same constant set of elements, defined by the numbers of protons.

Heller Highwater
12-21-2004, 01:37 AM
I wanna' echo Alan Smithee's comments that changing the number of electrons for a fixed proton number (i.e., creating an ion) absolutely changes the chemical properties. This is the reason that chemists have to note whether, for example, the element being studied is the sodium atom or the sodium ion. In fact, to get all the information, we usually need to not only identify that there is an ion present, but what the charge on that ion is. There's a world of difference between sodium in the plus one state, and in the minus one state. (That is, with one less or one more electron than the neutral atom, respectively.)

That said, in any of those cases, we still call the atom or ion "sodium" (or whatever element you may happen to have). Even though there is no proton exchange or interaction in chemical interactions, we have defined the identity of the element based on the number of protons. Moreover, because protons (and neutrons) are so heavy compared to electrons, each being about 2000 times more massive, there are ways of determining the number of protons present that don't really depend on the electrons (again, I'm simplifying a bit), which lets us identify the element regardless of the overall charge.

Oh, and thanks rowrrbazzle for finding that cite about germanium. I thought I remembered my high school chem teacher saying something about that, but it's been six or seven years, so I wasn't sure, me being absent-minded enough as it is. ;)

Send questions for Cecil Adams to: cecil@straightdope.com

Send comments about this website to: webmaster@straightdope.com

Terms of Use / Privacy Policy

Advertise on the Straight Dope!
(Your direct line to thousands of the smartest, hippest people on the planet, plus a few total dipsticks.)

Copyright 2018 STM Reader, LLC.