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Resources for learning Chemistry

Based on various other Stack exchange site (Mandarin Chinese, Russian and German), we adapt this project here for chemistry, since it's a great idea to have all kinds of resources in one place. This ...
58
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4answers
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Why do elements in columns 6 and 11 assume 'abnormal' electron configurations?

When I look around for why copper and chromium only have one electron in their outermost s orbital and 5/10 in their outermost d orbital, I'm bombarded with the fact that they are more stable with a ...
49
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1answer
4k views

Why does F replace the axial bond in PCl5?

Why does $\ce{F}$ replace an axial bond in $\ce{PCl5}$? I realize that it would be more stable there than at equatorial bond, but what is the reason of its stability? Similarly in $\ce{AB4}$ type of ...
160
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8answers
124k views

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

According to some chemistry textbooks, the maximum number of valence electrons for an atom is 8, but the reason for this is not explained. So, can an atom have more than 8 valence electrons? If ...
60
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2answers
19k views

What is Bent's rule?

I'm all bent out of shape trying to figure out what Bent's rule means. I have several formulations of it, and the most common formulation is also the hardest to understand. Atomic s character ...
187
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3answers
22k views

Why is gold golden?

Bulk gold has a very characteristic warm yellow shine to it, whereas almost all other metals have a grey or silvery color. Where does this come from? I have heard that this property arises from ...
50
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6answers
39k views

Ortho-effect in substituted aromatic acids and bases

When comparing o,m,p-toluidine basicities, the ortho effect is believed to explain why o-toluidine is weaker. But when comparing o,m,p-toluic acid basicities, the ortho effect is stated as a reason ...
21
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2answers
26k views

Why does the 3rd electron shell start filling up with scandium?

The electron configuration of calcium is 2, 8, 8, 2, where up to that point each shell, asides from the first shell counts up to 8 - why then does scandium have an electron configuration of 2, 8, 9, 2?...
36
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2answers
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How can the dipole moment of carbon monoxide be rationalised by molecular orbital theory?

Despite the fact that oxygen is much more electronegative than carbon, the bond in $\ce{CO}$ presents a weak dipole moment. This observation can easily be explained using the concept of "dative bond", ...
32
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4answers
3k views

What is resonance, and are resonance structures real?

My teacher told me about resonance and explained it as different structures which are flipping back and forth and that we only observe a sort of average structure. How does this work? Why do the ...
41
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8answers
22k views

Why do compounds like SF6 and SF4 exist but SH6 and SH4 don't?

Both $\ce{SF6}$ and $\ce{SH6}$ and $\ce{SF4}$ and $\ce{SH4}$ have the same central atom and the same hybridization, but my teacher specifically mentioned that $\ce{SH6}$ and $\ce{SH4}$ don't exist. I'...
18
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1answer
4k views

How can the intense color of potassium permanganate be explained with molecular orbital theory?

In the permanganate ion, manganese is in the $+7$ oxidation state, therefore it is a $\ce{d^0}$ ion. $\ce{d^0}$ and $\ce{d^1^0}$ ions don't absorb visible spectrum radiation because there are no ...
33
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5answers
7k views

Fundamental forces behind covalent bonding

I understand that covalent bonding is an equilibrium state between attractive and repulsive forces, but which one of fundamental forces actually causes atoms to attract each other? Also, am I right ...
25
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2answers
5k views

Why does cyclopropane react with bromine?

In my exam, I was asked why cyclopropane could decolourise bromine water (indicating that it reacted with the bromine). All I could guess was that it is related to the high angle strain in ...
32
votes
2answers
28k views

How can I predict if a reaction will occur between any two (or more) substances?

I am new to chemistry and I find it fascinating. I am trying to learn about chemical reactions and I was wondering if there was an easy way to quickly tell if any combination of chemical substances ...
29
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3answers
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Why is the bond angle H-P-H smaller than H-N-H?

$\ce{N}$ & $\ce{P}$ are in the same group. Both $\ce{NH3}$ and $\ce{PH3}$ have one lone pair and according to VSEPR theory, both the central atoms are predicted to be $\ce{sp^3}$ hybridized. But ...
27
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4answers
3k views

DFT Functional Selection Criteria

I have a very very general question: In DFT functional selection , mostly people speak about the most recent ones. For example my professor always asks: " which DFT Functional did you select ? " ...
36
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4answers
34k views

Why does bond angle decrease in the order H2O, H2S, H2Se?

I know that bond angle decreases in the order $\ce{H2O}$, $\ce{H2S}$ and $\ce{H2Se}$. I wish to know the reason for this. I think this is because of the lone pair repulsion but how?
23
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5answers
23k views

Bonding in the phosphate ion

I'm looking for an explanation of the bonding in the phosphate (PO43−) ion: (Image courtesy of Wikipedia) Phosphorus (15P) - being the fifteenth element - has fifteen electrons, five valence ...
14
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1answer
12k views

Is KF the most ionic compound?

I saw somewhere (can't recall where) that KF is the most ionic compound. I expected CsF. Does the greater polarizability of Cs allow it to more easily form covalent bonds compared to K? Does this ...
46
votes
3answers
598k views

How do I figure out the hybridization of a particular atom in a molecule?

I'm learning how to apply the VSEPR theory to Lewis structures and in my homework, I'm being asked to provide the hybridization of the central atom in each Lewis structure I've drawn. I've drawn out ...
40
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3answers
2k views

Unit of the equilibrium constant: contradiction of Bridgman's theorem?

The following equation is standard in thermodynamics: $$ \Delta G^\circ=-RT\log(K) $$ where $K$ is the equilibrium constant. In dimensional analysis, Bridgman's theorem tells us that the argument ...
91
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7answers
51k views

Is a negative pH level physically possible?

A friend of mine was looking over the definition of pH, and wondering if it is possible to have a negative pH level. From the equation below, it certainly seems possible—just have a $1.1$ (or ...
32
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2answers
2k views

Hypervalency and the octet rule

I realize that the octet rule is more a suggestion than a rule, and that it applies mainly to non-transition metal compounds. Still, compounds that don't have an octet, like $\ce{BH3}$, tend to ...
16
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1answer
892 views

What makes C=O more stable that C(OH)₂

When there are $2$ hydroxy groups on same carbon, they tend to undergo dehydration ( of course with exceptions like chloral where $\ce H$ bonding prevails). Why does this happen? We are breaking a ...
14
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3answers
1k views

Is hydrogen bonding generally defined to include only three period two elements?

Traditionally, hydrogen bonding has been defined to only include interactions between a positively polarized hydrogen and three period-two elements: nitrogen, oxygen, and fluorine. Why was this ...
33
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4answers
37k views

The reason behind the steep rise in pH in the acid base titration curve

Most books refer to a steep rise in pH when a titration reaches the equivalence point. However, I do not understand why … I mean I am adding the same drops of acid to the alkali but just as I near the ...
70
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2answers
24k views

Bonding in diatomic C2, a carbon-carbon quadruple bond?

Carbon is well known to form single, double, and triple $\ce{C-C}$ bonds in compounds. There is a recent report (2012) that carbon forms a quadruple bond in diatomic carbon, $\ce{C2}$. The excerpt ...
34
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3answers
2k views

Why is it wrong to use the concept of “hybridization” while studying complexes?

I have asked a lot of questions on coordination chemistry here before and I have gone through a lot others here as well. Students, including me, attempt to answer those questions using the concept of ...
25
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3answers
8k views

What is the reason for the exceptional stability of the cyclopropylmethyl carbocation?

Can someone explain this to me by drawing resonance structures for the cyclopropylmethyl carbocation please? Also one more question, is the tricyclopropylmethyl carbocation more stable than tropylium ...
10
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2answers
3k views

What is the perfect definition for chirality?

Why is chirality defined differently for organic and inorganic compounds? Why are inorganic compounds deemed to be optically active if they have more than one of the same ligands attached to the ...
28
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3answers
6k views

IUPAC nomenclature: “Smallest sum of locants”?

While naming any compound, the numbering should be done such that position of substituted groups gives smallest sum. I'm struggling with the IUPAC naming for cyclic compounds. How exactly does this ...
27
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2answers
24k views

What exactly is hydrogen bonding (and does it really need fluorine, oxygen, or nitrogen)?

I'm not satisfied with the rationale for the intermolecular attraction known as hydrogen bonding. In my book, it states that Hydrogen bonding is a special type of intermolecular attraction between ...
14
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1answer
3k views

Utility of Bent's Rule - What can Bent's rule explain that other qualitative considerations cannot?

Does Bent's rule have any utility? I got a vehement earful for "NO." Points raised by my professor: Coulombic considerations can be used to rationalize bond angles, strengths, and lengths without ...
8
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3answers
25k views

How to get the valencies of elements?

How to find the valencies of elements by using its distribution of electrons? Please explain the method in simple words. Do you have to study the valencies or is there a simple way of remembering? PS:...
69
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7answers
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Why is the 2s orbital lower in energy than the 2p orbital when the electrons in 2s are usually farther from the nucleus?

My chemistry book explains that even though electrons in the 2p orbital are closer to the nucleus on average, electrons from the 2s orbital spend a very short time very close to the nucleus (...
26
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4answers
11k views

What is the difference between ∆G and ∆G°?

In Brady's Molecular Nature of Matter, I read that $\Delta_\mathrm{r} G^{\circ}$ is $\Delta_\mathrm{r} G$ at $25~^\circ\mathrm{C}$. But later, it gives a value for $\Delta_\mathrm{r} G^\circ$ at ...
41
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3answers
242k views

Why is it important to use a salt bridge in a voltaic cell? Can a wire be used?

I was learning about voltaic cells and came across salt bridges. If the purpose of the salt bridge is only to move electrons from an electrolyte solution to the other, then why can I not use a wire? ...
51
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4answers
32k views

How can one explain niobium’s weird electronic configuration?

As cited in an answer to this question, the ground state electronic configuration of niobium is: $\ce{Nb: [Kr] 5s^1 4d^4}$ Why is that so? What factors stabilize this configuration, compared to ...
49
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5answers
7k views

The last element's atomic number

I was just thinking what can be the last atomic number that can exist within the range of permissible radioactivity limit and considering all other factors in quantum physics and chemical factors.
36
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3answers
82k views

What is the inert pair effect?

I was reading about the p-block elements and found that the inert pair effect is mentioned everywhere in this topic. However, the book does not explain it very well. So, what is the inert pair effect? ...
23
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3answers
3k views

Why can there not be more than one sigma bond in a set of bonds?

A question on an exam asked why there is exactly one sigma bond in double and triple covalent bonds. I looked in my text and online after the exam, but couldn't find an anawer to the question. Why ...
21
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3answers
4k views

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

According to Hund's first rule, a set of degenerate orbitals are singly occupied first, before the second slot in any of the orbitals are populated. This is quite intuitive because electron-electron ...
16
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4answers
16k views

Why does the energy gap for π - π* transitions shrink with the size of the pi-conjugated system?

Quoting from this site: As conjugated pi systems become larger, the energy gap for a π - π* transition becomes increasingly narrow, and the wavelength of light absorbed correspondingly becomes ...
27
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3answers
21k views

Why do salts such as NaCl dissolve?

If we look at solubility of salts in water, we are told that they disassociate, because the positive $\ce{Na}$ ion is attracted to the partially negative oxygen in water and the negative $\ce{Cl}$ is ...
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2answers
10k views

What do the prefixes meta, ortho, pyro mean in inorganic chemistry?

In inorganic chemistry, when is the prefix meta used? (as in metaborate and metasulphite) What about the terms pyro and ortho (as in orthophosphorous acid)? For example, I recently came to know about ...
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2answers
940 views

Why does the reaction quotient use the products (multiplications) of reactants and products, rather than their respective sums?

As you may know, the reaction quotient $Q_c$ is defined by the equation $$ Q_c = \frac{[C]^\gamma [D]^\delta}{[A]^\alpha [B]^\beta} $$ for the chemical reaction $$ \alpha A + \beta B \rightarrow \...
14
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5answers
14k views

Metal Compounds that bond covalently [closed]

I would like to ask if anyone has a list or knows which covalent compounds have metals in them. For example, beryllium and aluminium are both metals but they bond covalently with chlorine to form ...
12
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2answers
3k views

Electronegativity Considerations in Assigning Oxidation States

I have never seen anything other than a set of rules like these when textbooks present how to assign oxidation numbers. Such as these: However, if we keep in mind that oxidation numbers are simply ...
11
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2answers
589 views

Which equilibrium constant is appropriate to use?

I have learnt that the standard free energy change is related to the equilibrium constant of a reaction by, $$\Delta G^\circ = -RT \ln K$$ Here, does $K$ refer to $K_p$ or $K_c$? Also, please give ...

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