45 votes
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Difference between shells, subshells and orbitals

Here's a graphic I use to explain the difference in my general chemistry courses: All electrons that have the same value for $n$ (the principle quantum number) are in the same shell Within a shell (...
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40 votes

Can an atom bond with more than 8 other atoms?

Yes, there are coordination complexes of large elements which have coordination numbers greater than eight. Some examples are: $\ce{[ReH9]^2-}$ with a tricapped trigonal prismatic structure. The nine ...
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34 votes
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Why does the energy gap for π - π* transitions shrink with the size of the pi-conjugated system?

Klaus Warzecha's answer pretty much answers your question. But I know that this subject is easier to understand if supported by some pictures. That's why I will take the same route as Klaus at ...
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31 votes

Can an atom bond with more than 8 other atoms?

14 coordination is claimed in $\ce{U(BH4)4}$ (ref_1, p. 268). The molecule exists as a polymer in the solid state. Six hydrogens from two of the $\ce{BH4}$ groups bond between the boron and uranium (...
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27 votes

What is the physical basis for Hund's first rule?

Disclaimer My following answer is the "traditional" explanation of Hund's first rule, which is based on a smaller value of $V_\mathrm{ee}$ (electron-electron repulsions) in the triplet state arising ...
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26 votes
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Is there an energy cost associated with flipping the spin of an electron?

I will try to describe what happens when two hydrogen atoms approach each other from infinity. At infinite separation the hydrogen atoms don't feel their mutual presence and each atom has one electron ...
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24 votes

Molecular orbital (MO) diagram for N2 and N2^-

I have been taught that the MO diagram is different for molecules with 14 or less electrons than the one used for molecules with 15 or more electrons. This is (partly) wrong because the change in ...
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23 votes
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Why is gold unreactive when only one electron is in the outer shell?

First off, gold does react. You can form stable gold alloys and gold compounds. It's just hard, mostly for reasons explained by the other answer The reason bulk gold solid is largely unreactive is ...
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23 votes
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Will adding up protons and electrons (without neutrons) create a new element?

Yes and no. Elements are defined by the number of protons only. It does not matter if (say) a carbon nucleus has six or seven (or eight) neutrons, they will all react the same.* With that, to create ...
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22 votes
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Why can Cu have an oxidation number of +2?

Atomic copper has the electron configuration $\ce{[Ar] 3d^{10} 4s^1}$. By removing one electron and producing $\ce{Cu^{+1}}$, an inert gas configuration $\ce{[Ar] 3d^{10} 4s^0}$ is produced. While ...
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22 votes
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Why do 3d orbitals have lesser energy than 4s orbitals in transition metals?

Disclaimer: I now believe this answer to be fully incorrect. Please consider un-upvoting it and/or downvoting it. I do not like seeing incorrect answers at +22. However, I will leave it up ...
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21 votes

What is SPDF configuration?

s, p, d, f and so on are the names given to the orbitals that hold the electrons in atoms. These orbitals have different shapes (e.g. electron density distributions in space) and energies (e.g. 1s is ...
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21 votes

Which relatively simple molecules violate the octet rule?

There are 3 types of octet rule "violations" or exceptions molecules with an odd number of electrons, such as nitric oxide (image source) molecules with less than 8 electrons around an atom, $\ce{...
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21 votes
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Why are protons more common than hydride ion?

This is because we live in a world dominated by oxygen and water. In other words, it is an oxidized world. Most metals occur naturally in the form of oxides, silicates, halides, or other derivatives. ...
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20 votes

Will adding up protons and electrons (without neutrons) create a new element?

Jan's answer is correct. I will try to fill in a few details about why neutrons are essential to creating stable nuclei. All stable isotopes excepting Hydrogen-1 have neutrons in their nuclei. ...
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20 votes
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What is the physical basis for Hund's first rule?

The lowest energy state has parallel spins to maximize the exchange energy. As you say, there's a Coulomb repulsion between two electrons to put them in the same orbital. There's also a quantum ...
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Difference between Coupled Cluster and Full CI

Full Coupled Cluster (FCC) vs. Full Configuration Interaction (FCI) The main theoretical difference is the way excitations are used. "Excitation" refers to putting one or more electrons in higher ...
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20 votes
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Degeneracy of second excited state of H-?

I think it is important to understand that for hydrogen atom (or any other one-electron system) all orbitals from the same shell have same energy. For instance, $E_\mathrm{2s} = E_\mathrm{2p}$, $E_\...
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17 votes

Why is gold unreactive when only one electron is in the outer shell?

Relativistic effects account for gold's lack of reactivity. Gold has a heavy enough nucleus that its electrons must travel at speeds nearing the speed of light to prevent them from falling into the ...
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17 votes
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What is the usage of orbitals more complex than f orbitals?

Surprisingly, I learned that there are also usages for orbitals g,h,i and even j. Actually, the letter "j" is not used, so it is s, p, d, f, g, h, i, k, l, etc. The higher angular momentum ...
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17 votes
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Hypervalency and the octet rule

Out there in the real world, university students and school pupils alike favour strict rules that are true as often as possible (or at least have clear, easily remembered exceptions), while their ...
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17 votes
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What do these labels for molecular electronic states mean?

As stated in this answer, these are irrep (irreducible representation) labels for molecular symmetry point groups. In the context of chemistry, point groups are usually introduced when learning about ...
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17 votes
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Help understanding how "steric effects" are distinct from "electronic effects"?

The normal distinction between "steric" and "electronic" is based on whether the effect is transmitted through space or through bonds All the normal physical interactions we ...
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16 votes

Can an atom bond with more than 8 other atoms?

There has been quite some interesting work by my former co-worker and my supervisor on metal-rich molecule with the co-ordination higher than eight. For more see Timo Bollermann, Thomas Cadenbach, ...
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15 votes

Can an atom have more than 8 valence electrons? If not, why is 8 the limit?

Why 8? has not really been addressed by the above answers, and while tangential to the question, it is somewhat important to be considered. In general, but not always, atoms react to form complete ...
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15 votes
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Why does K⁺ have 0 valence electrons?

What you say is correct. The [Ar] configuration we are left with does have 8 valence electrons. But I think it is just semantics. Elemental potassium has an [Ar] 4s1 electron configuration. One ...
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15 votes
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While filling electrons, we follow Aufbau principle, but not while removing them. Why is this so?

Usually when adding electrons based on the Aufbau principle, you go from one element to the next highest one, e.g. from $\ce{Ti}: \ce{[Ar] 4s^2 3d^2}$ to $\ce{V: [Ar] 4s^2 3d^3}$. Thus you add not ...
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14 votes

Can an atom bond with more than 8 other atoms?

In october 2015, 5 months after all the current answers were given, a new cluster compound witn C.N. 16, $\ce{CoB16-}$, has been reported [1]: Here we report the observation of a large metal-doped ...
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14 votes

Why do atoms need 8 electrons to stabilize?

The valence orbitals of atoms are composed of suborbitals (s and p) there is 1 s suborbital which is spherical and can hold 2 electrons (one with up spin and one with down spin). There are 3 p ...
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13 votes
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The energy gap between a pi-conjugated system with (2 bonding and 1 anti-bonding orbital) and (1 bonding and 2 anti-bonding) orbitals

If you want to really understand why energy levels of conjugated $\pi$ systems are the way they are I suggest you take a good look at the Hückel theory as @ssavec suggested - this method is only an ...
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