Monday, January 02, 2006
Attraction, Repulsion and Potential Energy Soliloquy
Both AP and Honors Classes are in the midst of their Bonding Unit. Though most of you can express the connection between attraction and potential energy or repulsion and potential energy, others have not expressed that connection and, instead, have developed a completely disproportionate sense of the relevance and/or importance of the octet "rule". This "rule" has about as much contribution/relevance to potential energy lowering or stabilization as an eyelash contributes to the mass of an ELEPHANT!
Before I reiterate what I said in class about "Attraction, Repulsion and Potential Energy", let me reiterate why the octet "rule" exists. Take a fluorine atom, for example. A fluorine atom tends to bond to ONE and ONLY ONE other fluorine atom to form a diatomic fluorine molecule. Why is there no stable triatomic fluorine molecule? Once a fluorine atom shares its one unpaired electron with another fluorine atom, each F atom has eight valence electrons. Big deal. There is actually MORE repulsion (which is ALWAYS energy raising) when the eighth electron enters the valence shell. This extra repulsion is compensated for A LITTLE BIT due to the symmetric distribution of the electrons about the nucleus when the s and p sublevels are completely filled (due to the symmetric orientation of s and p orbitals). This symmetric distribution of electrons causes LESS repulsion compared to the amount of repulsion when there is an asymmetric electron distribution (i.e. when at atom has 6 or 7 valence electrons). If a third F atom bonded to the molecule, the central F atom would have 9 valence electrons! There is no "room" (too much repulsion...i.e. Pauli Exclusion Principle, Aufbau Principle) for a 9th electron in the 2nd principal energy level (2s2 2p6) so the next bonded electron would have to go to the much higher potential energy 3rd principal energy level. This would have a destabilizing effect (higher potential energy = destabilizing by definition!). So, that is why most (BUT NOT even ALL!) non-metal atoms stop bonding once they acquire an octet.
To really blow your mind, let me tell you what you actually already know: a sodium atom, BY ITSELF, is MORE STABLE than a sodium ION even though the sodium ION has an octet. Didn't think that you knew that? I bet you do. Here's why: Recall the definition of ionization energy. Look at the table of ionization energies. Locate sodium. Aha, so energy is actually REQUIRED to remove the valence electron from sodium? YES! Why? Simple: the positive sodium nucleus attracts the negative valence electron! Energy is required to overcome that attraction. Why doesn't sodium lose two electrons then? The octet rule? NOPE! The second ionization energy of Na is so high because the second electron lost "feels" a Zeff of +9 AND it is in a closer (to the nucleus) principal energy level; therefore, too much energy is required for Na to lose a second electron.
You object: Why is the sodium ion so stable and unreactive? ahaaa! Sodium ions are NEVER by themselves! They are bonded (VERY STABILIZING!!!) to any and all surrounding anions!...or they are surrounded by the partially negatively charged oxygen end of a water molecule (very stabilizing) in any aqueous solution or mixture (saliva, blood, the ocean).
Now, the main reason for potential energy lowering WHENEVER a bond is formed: (write this out and MEMORIZE it if you have to)
Covalent Bonds: the (negative) electrons that are SHARED between the (positive) nuclei will ALWAYS have a potential energy LOWERING effect (whether OR NOT there is an octet of electrons i.e. in Hydrogen) because:
1. positive particles attract negative particles and vice versa
2. attraction is BY DEFINITION potential energy LOWERING!
3. relative to having a valence electron around one nucleUS, there is MORE ATTRACTION when a valence electron is shared/located BETWEEN TWO NUCLEI because there is more positive surrounding negative and vice versa!
Ionic Bonds: again, the octet rule is practically IRRELEVANT to ionic bonding:
the actual potential energy lowering effect is due almost SOLELY to:
1. the cations are the positive particles and the anions are the negative particles in all ionic bonds.
2. positive particles attract negative particles and vice versa
3. attraction is BY DEFINITION potential energy LOWERING!
4. since electrons are NOT shared in ionic bonds, each cation can have 4, 6, maybe 8 BONDS to surrounding anions and vice versa! Each individual bond is between TWO fully charged IONS of opposite charge.
That is the relationship among Attraction, Repulsion and Potential Energy. Pass it on.
p.s. as the AP class already knows, the next part of the bonding unit for Honors involves explaining physical properties of substances and mixtures. The cause(s) of attractions (potential energy lowering) and/or overcoming these attractions will be the key to EXPLAINING the rest of the unit. So, never forget, positive attracts negative and this attraction is potential energy lowering.
Before I reiterate what I said in class about "Attraction, Repulsion and Potential Energy", let me reiterate why the octet "rule" exists. Take a fluorine atom, for example. A fluorine atom tends to bond to ONE and ONLY ONE other fluorine atom to form a diatomic fluorine molecule. Why is there no stable triatomic fluorine molecule? Once a fluorine atom shares its one unpaired electron with another fluorine atom, each F atom has eight valence electrons. Big deal. There is actually MORE repulsion (which is ALWAYS energy raising) when the eighth electron enters the valence shell. This extra repulsion is compensated for A LITTLE BIT due to the symmetric distribution of the electrons about the nucleus when the s and p sublevels are completely filled (due to the symmetric orientation of s and p orbitals). This symmetric distribution of electrons causes LESS repulsion compared to the amount of repulsion when there is an asymmetric electron distribution (i.e. when at atom has 6 or 7 valence electrons). If a third F atom bonded to the molecule, the central F atom would have 9 valence electrons! There is no "room" (too much repulsion...i.e. Pauli Exclusion Principle, Aufbau Principle) for a 9th electron in the 2nd principal energy level (2s2 2p6) so the next bonded electron would have to go to the much higher potential energy 3rd principal energy level. This would have a destabilizing effect (higher potential energy = destabilizing by definition!). So, that is why most (BUT NOT even ALL!) non-metal atoms stop bonding once they acquire an octet.
To really blow your mind, let me tell you what you actually already know: a sodium atom, BY ITSELF, is MORE STABLE than a sodium ION even though the sodium ION has an octet. Didn't think that you knew that? I bet you do. Here's why: Recall the definition of ionization energy. Look at the table of ionization energies. Locate sodium. Aha, so energy is actually REQUIRED to remove the valence electron from sodium? YES! Why? Simple: the positive sodium nucleus attracts the negative valence electron! Energy is required to overcome that attraction. Why doesn't sodium lose two electrons then? The octet rule? NOPE! The second ionization energy of Na is so high because the second electron lost "feels" a Zeff of +9 AND it is in a closer (to the nucleus) principal energy level; therefore, too much energy is required for Na to lose a second electron.
You object: Why is the sodium ion so stable and unreactive? ahaaa! Sodium ions are NEVER by themselves! They are bonded (VERY STABILIZING!!!) to any and all surrounding anions!...or they are surrounded by the partially negatively charged oxygen end of a water molecule (very stabilizing) in any aqueous solution or mixture (saliva, blood, the ocean).
Now, the main reason for potential energy lowering WHENEVER a bond is formed: (write this out and MEMORIZE it if you have to)
Covalent Bonds: the (negative) electrons that are SHARED between the (positive) nuclei will ALWAYS have a potential energy LOWERING effect (whether OR NOT there is an octet of electrons i.e. in Hydrogen) because:
1. positive particles attract negative particles and vice versa
2. attraction is BY DEFINITION potential energy LOWERING!
3. relative to having a valence electron around one nucleUS, there is MORE ATTRACTION when a valence electron is shared/located BETWEEN TWO NUCLEI because there is more positive surrounding negative and vice versa!
Ionic Bonds: again, the octet rule is practically IRRELEVANT to ionic bonding:
the actual potential energy lowering effect is due almost SOLELY to:
1. the cations are the positive particles and the anions are the negative particles in all ionic bonds.
2. positive particles attract negative particles and vice versa
3. attraction is BY DEFINITION potential energy LOWERING!
4. since electrons are NOT shared in ionic bonds, each cation can have 4, 6, maybe 8 BONDS to surrounding anions and vice versa! Each individual bond is between TWO fully charged IONS of opposite charge.
That is the relationship among Attraction, Repulsion and Potential Energy. Pass it on.
p.s. as the AP class already knows, the next part of the bonding unit for Honors involves explaining physical properties of substances and mixtures. The cause(s) of attractions (potential energy lowering) and/or overcoming these attractions will be the key to EXPLAINING the rest of the unit. So, never forget, positive attracts negative and this attraction is potential energy lowering.
# posted by C @ 5:53 PM 
