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1500 questions
20
votes
3 answers
Superscript open circle notation in formula
I may be only a physicist but I'm familiar with the superscripted dot to indicate a radical. In a paper I'm reading the authors have also used a superscripted open circle. I can't find any reference online or in my physical chemistry textbook…
Chris H
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20
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1 answer
Why is an ionic bond a chemical and not a physical bond?
Ionic bonds seem to be intermolecular but are classified as chemical bonds.
"Ionic bonding is a type of chemical bond that involves the electrostatic attraction between oppositely charged ions." - Wikipedia, definition of an ionic bond
"The…
Dylan Spano
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20
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1 answer
When simulating spectral line broadening, which convolution is preferred?
Many computational chemistry packages permit the calculation of vibrational and electronic spectra. These spectra are obtained as a set of discrete eigenvalues however they are often convolved with some distribution to give a continuous spectrum…
Richard Terrett
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20
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1 answer
Relative acidities of alkanes, alkenes, and alkynes
How does one explain the trend in bond lengths and acidity of the following hydrocarbons?
$$\begin{array}{ccc}
\hline
\text{Species} & \ce{C-H}\text{ bond length / Å} & \mathrm{p}K_\mathrm{a} \\
\hline
\text{Ethane, }\ce{C2H6} & 1.102 & 50…
tkhanna42
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20
votes
2 answers
Is there a function to approximate atomic mass from the atomic number?
The product of $2Z$ generally approximates the value of $m_\mathrm{a}$ of atoms that comprise relatively few protons. Whereas, the product of $2.5Z$ generally approximates the value of $m_\mathrm{a}$ of atoms that comprise relatively many protons.…
Hal
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20
votes
3 answers
Why doesn't EDTA complex with alkali metal ions?
EDTA complexes with all the other metal ions in the periodic table except those from the group 1. Why is this so ? What is the coordination chemistry behind this ?
user16096
20
votes
2 answers
What does it mean when it is said that an sp³ orbital has 25% s character?
My textbook frequently mentions:
$\mathrm{sp^3}$ hybrid orbital has 25% $\mathrm{s}$-character and 75% $\mathrm{p}$-character.
What are these "characters"? And how do these characters influence the bond and the hybrid orbital?
user16347
20
votes
6 answers
Why does HCN boil at a higher temperature than NH3?
The boiling point of ammonia is −33 °C while that of $\ce{HCN}$ is 25 °C. In a recent AP (Advanced Placement) Chemistry test, a free response question asked why this is the case. Can someone shine light on this?
Based on Jan's answer:
Although…
Huy Nguyen
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20
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3 answers
How to calculate molecular dipole moment from a known wavefunction?
Say I have a molecular wavefunction as a set of molecular orbitals and want to calculate the molecule's dipole moment, but don't know how! I searched a lot but couldn't find any practical example.
$$\psi _i=\sum ^N_{i=1}C_i\mathrm e^{-\alpha…
Aug
- 2,141
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20
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0 answers
Manuals for a Cahn TG-171 Thermogravimetric Analyzer
Recently a Cahn TG-171 Thermogravimetric Analyzer was donated to my school. We did not get a service or user's manual with it, and some of it still needs to be put back together.
I have tried to contact Thermo Fisher who bought Cahn Instruments,…
Cohen_the_Librarian
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20
votes
1 answer
Why can't helium be solidified at 'ordinary' pressures?
According to the UC Davis ChemWiki Chemistry of Helium, helium has a comparatively unusual property, specifically:
Helium is the only element that cannot be solidified by lowering the temperature at ordinary pressures.
'Ordinary' referring to…
user15489
20
votes
5 answers
Where can I find a downloadable spreadsheet of element properties?
Sorry for the FAQ, but I really don't know where to find a full spreadsheet for element properties. I mean a table that has one row for each element, and the columns are its properties, such as name, atomic number, density, melting point, etc. I…
Mostafa Farzán
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20
votes
1 answer
What are the g-block's predicted properties?
I have been researching for a Physics/Chemistry exam and thought: what will the future periods in the periodic table (periods 8 and above) entail? Each block contains its own properties that allow for accurate predictions of elements in these…
Bored915
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20
votes
2 answers
Salts that are more hydrophilic than NaCl
What other, rather easy to obtain, salts are more hydrophilic than $\ce{NaCl}$? Is there a hydrophilic scale for substances like the Mohs scale of mineral hardness?
Liviu-Aurelian Rau-Neacsu
- 395
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20
votes
2 answers
Formula of entropy change
In my 12th standard book, the formula for entropy change is given as $\Delta S = \frac{q_\text{reversible}}{T}$. What is the importance of absorbing heat reversibly and not irreversibly? What does absorbing heat reversibly mean? Does it mean that…
user47024
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