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Chemical Bonds - various questions

  1. Jan 15, 2017 #1
    A group 2 metal always forms 2+ ions, since the second electron is harder to remove how come you don't find 1+ ions of these in ionic compound formations, How does it always manage to lose both outer electrons?

    So it takes a certain amount of energy to break a bond, how does water molecules provide the energy to break the bond in NaCl and dissolve it?

    Is a Van der Waals binding very transient or does it remain?

    A bond is never COMPLETELY ionic, it's always shared to some degree, but is it always within the realms of the anions outer shell?

    I know this next question have been answered in a million places on line but I just can't grasp it. The release of energy when forming a bond. When a Cl atom picks up an extra electron and forms a full shell and an ionic bond with a cation it releases energy, WHERE from is this energy taken? If the Cl atom, where in the Cl atom was this energy before?
     
  2. jcsd
  3. Jan 16, 2017 #2

    DrDu

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    ad 1: Consider the following disproportionation reaction ## \mathrm{2 MgCl} \rightleftharpoons \mathrm{Mg+MgCl_2}##. Clearly, going to the right, you have to ionize Mg##^+## another time, but this is partly compensated by the other Mg##^+## taking up the electron. Furthermore, Mg##^{2+}## is smaller than Mg##^+## so that the attraction between the Mg##^{2+}## ions and the Cl##^-## ions will be higher in the lattice. Furthermore, the electrons in ##Mg## are stabilized by metallic bonding. The net effect is that the equilibrium will lie on the far right.
    ad 2: The water molecules have a dipole moment and the dipole-charge interaction is considerable. You should also not forget about the strong entropic component of this reaction as the ions in solution have much more degrees of freedom to move than in the crystal.
    ad 3: A van der Waals bond is usually weak but may nevertheless be stable.
    ad 4: I don't understand your question.
    ad 5: This energy is mostly due to the attraction between the negative anion and positively charged cation.
     
  4. Jan 16, 2017 #3
    I will start with your last question. Consider a bond forming between Cl- and H+ ions. You need to look at the total energy of the system (the bond formation): it is lower than the energy possessed by each ion. In other words, the total energy of the Cl anion and the H cation is higher than the H-Cl bond once formed. The difference in energy is released to the surroundings.

    Going up the list, it is true that there are no 100% ionic or covalent bonds. They do exist along a continuum; I do not understand your question however.

    As DrDu stated, Van der Waals forces are very weak, but can be stable. Induced molecular interactions are transient.

    When NaCl (s) is placed in water, there is complete dissociation into Na cations and Cl anions. The attractive forces between the water (with a permanent dipole) and the Cl anions and Na cations lower the energy of the system as they form. The partial positive charge on water molecules is attracted to the anions, and the partial negative charge on water is attracted to the cations. These attractions are stronger than the lattice energy. Entropy plays a small part also.

    171saltdissolve2.gif

    As far as Mg forming 2+ ions, again, you must look at the overall energy change of the system. The first and second ionization energies of Mg being oxidized to Mg2+ are more than compensated for by the energy released by a non metal accepting the electrons (electron affinity). Group 2 metal lose 2 electrons so that they will have a full outer shell of valence electrons. I do not understand DrDu's answer.
     
  5. Jan 17, 2017 #4

    DrDu

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    As far as I understood, the question was not, why e.g. Mg forms Mg2+ with an excess of oxidizing agent, but why we don't observe compounds containing monovalent alkaline earth elements, e.g. MgCl. This is what I tried to answer.
     
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