Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Outer shells and reactions

  1. Nov 28, 2004 #1
    Outer shells and reactions....

    We say that atoms react in order to gain full outer shells - but why do they do this? Lets take water as an example - oxygen is very electronegative, so much so that when an oxygen atom collides with a hydrogen atom the outer electron of the hydrogen starts to orbit the oxygen some of the time as well, leading to a covalent bond. In fact, oxygen is so electron negative that the electron will spend more time around the oxygen , leading to the polarity of water. The oxygen with do this twice until it gains a full outer shell of 8 - but the it stops. Why does it stop forming bondsonce its outer shell is formed? Surely its ability to attract electrons wont be reduced that much by gaining just 2 electrons?

    Thanks in advance. :-)
  2. jcsd
  3. Nov 28, 2004 #2
    Sorry - one line there should say "The oxygen WILL do this twice until it gains a full outer shell of 8 - but THEN it stops"
  4. Nov 29, 2004 #3
    Well the question that you're asking is one of those questions that have no explanations. It's kind of like asking why the earth is round. Because it is. Just don't worry about it and just accept it as the way atoms work. :)
  5. Nov 29, 2004 #4
    But surely there must be a reason?
  6. Dec 1, 2004 #5
    No, there is an answer that guy just doesn't know it.

    Electrons are attracted to protons, and they orbit around the nucleus in shells. These shells can sometimes contain sub-shells. For example, the first shell contains only one sub-shell. As an electron gets further away from it's atom, it must have more "quantum energy." Electrons want to get as close to the nucleus as possible, but according to quantum physics, no two electrons can have the same "quantum energy." So, they orbit in shells. The electrons orbit in orbitals. The sub-shells have orbitals. For example, the 1 shell has an S orbital. Because it's an s orbital and it's the first shell it's labelled 1S. For 1 = First shell-, S = S orbital. An S orbital has the shape of a sphere. An orbital wants to fill it's self. Alright, so why would the atom want to have 8 electrons in it's outer most shell, good question. The second shell has two sub-shells. One sub-shell has an S orbital, and the second has three P orbitals. The reason it has three is because they can arrange themselves according to X,Y,Z. Each orbital has only two electrons, because no two electrons can have the same "quantum energy." So, for the valence shell of an atom with two shells, one S orbital and three P orbitals. Two electrons an orbital adds to...8. Hydogen, on the other hand, only has one shell. So, to fill it's valence shell, it only needs two electrons. It already has one - Hydogen = one proton, one electron - so, it only needs to bond with one atom to fill itself. Carbon, on the other hand, has two shells, so it needs 8 to fill it's valence shell. So....

    H C H Methane!!! CH4.

    If you were to count it up everyone's filled. The carbon atom has 6 electrons. 2 in it's first shell, and 4 in it's valence shell. It needs 8 in it's valence shell. So, it shares one with hydrogen, and the hydrogen shares one of the carbons. This gives the carbon an extra electron, and the hydrogen it's desired two. The carbon, then, bonds with three more to add to 8.

    HOH Water!!! H20. Oxygen has six valence electrons, meaning it needs 2 to gain, which it does with 2 hydrogen molecules.

    O=O Oxygen!!! O2.

    You're probably wondering, why is there an equals sign between the Oxygen molecules?
    This indicated a double bond. Oxygen has six valence electrons, when it bonds with another oxygen, it gets 7. That's not the desired 8. So, it makes a double bond, and they share two electrons each. Which adds to 8.

    O O Ozone!!! O3. Each one of these atoms share with each other, making 8.

    That's covelant bonding!!!
    This "quantum energy" I told you about is somewhat true. What's really true is that there are four "quantum numbers" that cannot match.
    The first is N.
    N is the energy of an electron. For example, an electron in the first shell would have an N of 1. An electron in the second shell would have an N of 2. An electron in the third shell would have an N of 3.
    N=1, means it's in the first shell.
    The second is L. It's actually a greek cursive L kind of like this. l. Okay. This sign is the orbital. L = N - 1. That's the equasion. So, if N = 1, then, L = 0. 0 is an S orbital.
    If N = 2, L can equal either 0 or 1. If it is 1, that's a P orbital. If N = 3, then that can be either 0,1 or 2. An S,P or...a D orbital.
    Now, the third quantum number is M. It is the orientation of the orbitals, you know XYZ.
    M can equal anything between -L and +L. For example if L is 1, then M can equal -1,0,1.
    This is 3 different ways of arranging the P orbital.
    Now the final one is Ms. For Spin. The spin of the electron can equal - 1/2 or 1/2.

    Okay, so let's look at the possible arrangements of some electrons.

    N L M Ms
    1 0 0 -1/2
    1 0 0 1/2 First shell, only can have two electrons.

    2 0 0 -1/2
    2 0 0 1/2
    2 1 -1 -1/2
    2 1 -1 1/2
    2 1 0 -1/2
    2 1 0 1/2
    2 1 1 -1/2
    2 1 1 1/2 Second shell, eight electrons, but none of them, nor the one's in the first shell have the same 4 quantum numbers.


    http://chemed.chem.purdue.edu/gench...h6/quantum.html [Broken]

    http://lectureonline.cl.msu.edu/~mm...od/electron.htm [Broken]
    Last edited by a moderator: May 1, 2017
  7. Dec 1, 2004 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    Think about from where the oxygen atom gets the two electrons - two hydrogen atoms.

    So what does a hydrogen atom look like without an electron - a proton - with a + charge.

    The proton is sharing an electron with the oxygen atom, and each proton shares one electron, so the oxygen has two electrons to fill its outer valence shell.

    Have you studied atomic bonds yet? That might help answer the question.
    Last edited: Dec 1, 2004
  8. Dec 5, 2004 #7
    Yes I have studied it thankyou. :grumpy: I'm trying to ask a more fundamental question than that - why do atoms try to fill their outer shells at all?!

    Let us take an example - we have 2 hydrogen atoms floating around. Their nuclear charge is cancelled out by their electron charge. So why when they come near each other do they start to share electrons? Surely as they are both neutrally charged they would be happier just to keep their electrons - in fact surely wont the outer electrons be slightly repelled by each other as they are slightly closer than a nuclei and electron are?

    Thanks. :smile:
  9. Dec 5, 2004 #8
    To answer that question, you have to look at quantum mechanics and the motion of particles. Electro-magnetism is essential to understanding the balances of electron orbitals and why O takes 8 electrons,etc. I believe that quantum gravity and particle entanglement could explain why covalent bonds are formed from a chemical view. Why each element has to fill various orbitals to be complete comes from the quantum nature of particles. The balance between potential/kinetic energy b/t particles is also essential for covalent bond formation.
    Last edited: Dec 5, 2004
  10. Dec 6, 2004 #9
    I think you should ask this in the Physics forum. It has to do with quantum mechanics.
  11. Aug 26, 2009 #10
    Re: Outer shells and reactions....

    The reason why some atoms need to fill their outer electron shell to obtain an octet is because of their purpose on earth. Oxygen atoms are highly electronegative and if you think about it, the energy from the oxygen atoms provide us humans, and other living things, with life. When we breathe in oxygen, it is carried by the hemoglobin in our blood throughout the body, where it is used to generate energy by oxidation. The energy, which the oxygen provides us with comes from its electrons. If electronegativity did not exist and all the atoms would remain neutral due to their positive and negative charge, their would be no physical movement. Life would not be possible because everything would just stay still. Their would not be kinetic or potential energy. The purpose for the protons is to keep the electrons going around the atom. The purpose for the neutrons is to keep the protons from coliding into each other. Also, if you look at all the atoms that the human body uses, you will realize that they are all atoms, which have electronegativity. Carbon needs 4 more electrons, nitrogen needs 3, sulfur needs 2, oxygen needs 2, & hydrogen needs 1. Our bodies need all of these atoms in order to substain life. Now if you want to know why life exists... I cannot help you with that answer. Hope this answer helps anyone who is trying to figure out why an atom needs to fill their octet. :0)
    Last edited: Aug 26, 2009
  12. Aug 27, 2009 #11
    Re: Outer shells and reactions....

    When the atoms are sufficiently close together that there is significant overlap between their outer electron clouds, you have to start looking at molecular orbitals instead of atomic orbitals. Then you run into the old rule that
    [tex]bond~order = {{bonding~electrons - antibonding~electrons}\over{2}}[/tex], and lower energy (bonding) orbitals will fill before electrons go into higher energy (antibonding) orbitals.
    Last edited: Aug 27, 2009
  13. Aug 27, 2009 #12
    Re: Outer shells and reactions....

    The question this person was asking is why does an atom have to have 8 electrons on its outer shell when it already has sufficient electrons to compliment its protons in order to have a neutral charge...
    not where does the electron recide in or which orbitals do electrons use or about the electron cloud.
    If the person asked, how many shells does a certain atom have and where can you find an electron... or how many orbitals does a certain atom have... then, and only then, should you answer with anti, sigma, pi, s, py, px, pz, d, f, electron clouds... etc.
  14. Aug 28, 2009 #13
    Re: Outer shells and reactions....

    And the answer to that question is "because of the properties of the orbitals."

    The response "It is because of their purpose on earth" offers nothing in the way of explanation, and is very close to the banned topics of "physics according to the Bible/Koran/etc."

    Anyhow, it's not true that atoms always have 8 electrons in their outer shell. Hydrogen and helium are the obvious exceptions, along with plenty of other examples involving hybrid orbitals like [tex]sp^{3}d,sp^3{}d^{2},[/tex] etc. Look at sulfuric acid--the sulfur atom forms 6 bonds (4 sigma bonds + 2 pi bonds) , so it has 12 valence electrons.
    Last edited: Aug 28, 2009
  15. Aug 29, 2009 #14
    Re: Outer shells and reactions....

    Am glad to find such a question. I also had the same but a little bit different.
    As follows, when an atom recieves an electron(mostly these electronegative atom) there is energy changes, and as it gains, i believe (may be someone can help me out) it becomes more unstable than before due to more elecron-elecron repulsion . Now taking an example of Oxygen its very unstable with two electrons added! Why is it having that tendency of recieving 2 electrons? Including other electron negative atoms!
  16. Aug 29, 2009 #15
    Re: Outer shells and reactions....

    stating the facts regarding the orbitals does not really go into the reasons why an atom prefers to be electronegative over being neutral. If you re-read the question and think about what the person asked originally, you will understand that your orbital answer does not answer their question. Their question is not about the s, p, d, f subshells.
    The person specifically asked, why, if they have all their protons and electrons which cause them to be neutral, do they still attach themselves to other atoms in order to gain more electronegativity? They also asked, wouldn't the negative charges repel each other?

    If the person asked about the rules that govern the placement of electrons in atomic orbitals, then I would have answered with -
    * the aufbau principle, which states that orbitals are filled in order of increasing energy
    * the pauli exclusion principle, which states that each orbital can hold two electrons of opposite spin, &
    * Hund's rule, which states an electron will occupy an empty degenerate orbital before it will pair up with an electron that is already present in an orbital

    I could have gone into all my chemistry text books and I could have answered directly from there... but this person's question is more fundamental as the person stated under their reply that they know the rules but what they are asking is a more fundamental question.

    nonmetal atoms do have purposes in life and that is to sustain life by providing energy (electronegativity), which they lose when they enter into the body of a life form, which uses the energy to move and live.
    And metal atoms, which have positive charges instead of negative due to the loss of their electrons to nonmetal atoms, also have their purposes, which is to make up the earth in which life is sustained.

    Religion, which is man made, has nothing to do with biological science, which is the science of life. Religion has to do with beliefs of a higher power... the creator... the end of the world, the end of man, paradise, THE SOUL (nothing to do with life), etc...

    My answer had nothing to do with the creator or a higher power or so on an so forth. My answer specifically pointed out biological science, which is the study of life, which includes the nucleus, protons, neutrons, electrons, atoms, molecules, the cell, tissue, organs, organ system, organisms, the ecosystem, and the biosphere.

    Maybe the problem with your understanding is that you have possibly not studied biology and so you dont understand chemicals in a more fundamental manner.
    You are possibly only educated in chemistry, which only specifies the properties and rules of atomic chemicals, orbitals, shells, subshells, etc... instead of going more in depth of what they actually do.

    I have studied biology and I know that atoms have a greater purpose. We, as physical beings, use many of the nonmetals, and even some metals as enzyme catalysts, in our bodies in order to move, think, breath, grow, etc...

    What other greater purpose does an atom have other than life and earth?

    You are looking inside a square box instead of seing the bigger picture.
    Last edited: Aug 29, 2009
  17. Aug 31, 2009 #16
    Re: Outer shells and reactions....

    Only a creationist (or now apparently a biologist) would be arrogant enough to assume that the tiny fraction of matter comprising Earth's biomass is the reason for the existence of the rest of the matter on earth, let alone the whole universe.

    Atoms do not have "purpose". They have measurable physical properties.

    "Purpose" implies they were intended for some use, and intent implies some sort of intelligent design of matter.

    In fact the answer to the original question has nothing to do with biology at all, but is related to the fact that the "outer" shells are only partially shielded from the nuclear charge. Because of this atoms tend to draw extra electrons into the outer shells. Since energy levels are quantized, eventually adding more electrons means electrons need to occupy a higher-energy shell--and if that shell has a higher energy than the electron's initial state, it won't happen.

    That much just covers simple ions. Molecular bonds are a slightly different story in that molecular orbitals are being formed instead of simply transfering electrons from one atomic orbital to another, but still the same basic idea.
  18. Sep 1, 2009 #17
    Re: Outer shells and reactions....

    PhaseShifter hit the nail on the head here.

    The electronegativity still persists Cheman, but bonding cannot occur due to the high energy nature of the upper level shells. However, intermolecular forces due to the electronegative nature of oxygen and electropositive nature of hydrogen do persist and are the source of the strong hydrogen bonds that make water have the physical properties we are used to.
  19. Sep 19, 2009 #18
    Re: Outer shells and reactions....

    On the Question on why does an atom only gains an octet number of electrons in the valence shell!

    Its this way just ask your self why do atoms lose or gain electrons? According to what I think is that Atoms lose electrons since they have electrons in the outermost shell that are very weak/ of higher energy i can say so whenever an out side potential ie The electronegative atoms come across, they can easily be taken and after that the cation formed is very stable since it has noble gas configuration and not only even more since it has more nuclear charge than the preceding noble gas!!

    For the electronegative atoms, they gain electrons because not that they gain more stability but they are stable with the excess electrons than the metals can do! So this gain in electrons has limits to the basis of stability, and usually the limit lies under the valence shell, and it is as follows;

    Taking an example Oxygen of configuration [He]2S2 2Px2 2Px1 2Px1 can at most gain 2 electrons since adding excess according to paulis exclusion principle the electron is added to the next outer shell that for Oxygen doesn't exist, and even if it where there it would form a very unstable atom so more gain of electrons is stopped!!

    And please may you read my question thread that falls under answers to your thread. It has something related to it!!
  20. Sep 30, 2009 #19
    Re: Outer shells and reactions....

    Well, the Earth is round due to gravity.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook