35
votes
Why is BCl3 a monomer whereas AlCl3 exists as a dimer?
Introduction
The bonding situation in $\ce{(AlCl3)2}$ and $\ce{(BCl3)2}$ is nothing trivial and the reason why aluminium chloride forms dimers, while boron trichloride does not, cannot only be ...
- 44.3k
25
votes
Accepted
Why doesn't ionization energy decrease from O to F or F to Ne?
It's a good question, and I agree that it can be quite confusing, but Chemguide is (at least partly*) correct about this.
I'd phrase it this way, which I hope is clearer. First, think about what would ...
- 71.6k
22
votes
Accepted
Thermal stability of alkali metal hydrides and carbonates
Down the group, atoms of the alkali metals increase in both atomic and ionic radii, due to the addition of electron shells. This results in the charge density of their corresponding cations decreasing ...
- 10.7k
21
votes
Why does bond angle decrease in the order H2O, H2S, H2Se?
The question asks why water has a larger angle than other hydrides of the form $\ce{XH2}$ in particular $\ce{H2S}$ and $\ce{H2Se}$. There have been other similar questions, so an attempt at a general ...
- 30.8k
21
votes
Accepted
Why is the boiling point of stibane higher than that of ammonia?
Though it does go against your intuition, you've actually mentioned the answer in your question. Stibane has a higher boiling point than ammonia/azane on account of van der Waals interactions (owing ...
- 18.8k
18
votes
Which atom is the smallest atom?
The volumes of the nuclei are negligible compared to the atomic radii (like a "pea in a soccer/football/cricket stadium").
The higher the nuclear charge, the closer the electrons to the ...
- 41.2k
17
votes
Why is BCl3 a monomer whereas AlCl3 exists as a dimer?
TL;DR
$\ce{BCl3}$ does not dimerize to $\ce{B2Cl6}$ due to a conflict between the short $\ce{B-Cl}$ bond length in $\ce{BCl3}$ $(1.74~\mathring{\mathrm{A}}$ in my calculations$)$ and the long $\ce{B-...
- 17.4k
17
votes
Exchange energy of d6 configuration
As @orthocresol points out, the key is that you need to compare the exchange energy before vs after the ionization process. Anything that is unchanged by ionization cannot affect ionization energy. ...
- 837
13
votes
Accepted
Why is Aluminium oxide amphoteric?
I will be using an approach which has been enlisted in the following book for answering this question: Arrow Pushing in Inorganic Chemistry ;A Logical Approach to the Chemistry of the Main-Group ...
- 3,206
13
votes
Why is platinum denser than gold?
Since OP is still in the high school, I'll try to explain it simply as possible using mathematical manipulation (hoping OP is more familiar with mathematics than chemistry).
Both gold and platinum ...
11
votes
Accepted
Why is fluorine more reactive than iodine despite the weaker I-I bond?
In comparing the reactivity of fluorine or iodine, there are a couple of factors to consider. Part of it is the strength of the $\ce{X–X}$ bond, which is a barrier that tends to discourage them from ...
- 71.6k
11
votes
Accepted
On the periodic table: Why are groups of elements organized by 'letter'
The letters are related to the electron orbitals, which were originally observed through spectroscopy. The lines shown in the spectroscope were named sharp, principal, diffuse and fine (or fundamental)...
- 34.3k
11
votes
Accepted
Do organopolonium compounds exist?
As one might expect for an element with no long-lived isotopes, little is known about organopolonium compounds. What is known may be broadly divided into two categories [1,2; cited by Wikipedia]:
...
- 60k
11
votes
Accepted
Is there a canonical variable for period and group?
There isn't a standard symbol. However, if you choose these current symbols, please note that "table" should be placed in upright font, not italics:
correct: ...
- 71.6k
10
votes
Which has more metallic character: aluminium or magnesium?
Two types of metallic character
In fact, there are two type of metallic character if you look at the metal from the chemical point of view or if you look at the metal from the physical point of view:
...
- 9,089
10
votes
Accepted
Increasing Covalent character increases Colour intensity
In molecular orbital theory, as the covalent character increases the homo-lumo gap decreases, and the excitation happens more easily. Thus the colour intensity increases.
- 116
10
votes
Boiling point trend in group 13
London forces are the forces that you should always consider in the end. They're like the slow pawns on the chessboard, their effect is negligible when the fast rooks and the industrious queens are ...
- 9,881
10
votes
Accepted
Why does magnesium have a greater ionization energy than lithium?
The book has omited the other factors.
The short answer is: In spite of the bigger atomic radius, the valence electrons of magnesium are attracted by the greater net electrostatic force compared to ...
- 42.7k
8
votes
Accepted
Which has the largest bond angle between water, oxygen difluoride and dichlorine oxide?
There is practically only one effect that determines the angle, but there is an underlying mechanism that you also should understand.
Ideally, a central atom such as oxygen would want to use its p-...
- 68.4k
8
votes
Why is the melting point of magnesium oxide higher than aluminium oxide?
Two ideas to consider:
Alumina is not fully ionic. Neither is magnesia, but magnesia has more ionic character and we may see the greatest amount of ionic attraction in magnesia versus alumina which ...
- 60k
8
votes
Confusion between electronegativity and electron affinity
I wouldn't shoot this down so quickly - consider, e.g. Mulliken electronegativity, defined as the average of the electron affinity and ionisation energy. If you think about it, electronegativity ...
- 71.6k
8
votes
Accepted
Covalent atomic radii: oxygen vs nitrogen
The van der Waals radius of nitrogen is larger than that of oxygen, and has been calculated as such for quite a long time.
$$
\begin{array}{lll}
\hline
\text{Reference} & R_\ce{O} & R_\ce{N} \\...
- 6,278
8
votes
Lower melting points of zinc, cadmium and mercury
Mercury is a rather special case, which cannot be compared to Zn and Cd. Mercury (Z = $80$) belongs to the heaviest elements of the periodic table. Pekka Pyykkö's calculations have shown that nuclei ...
- 29.2k
7
votes
Accepted
On the oxyacids of +III pnictogens and +IV chalcogens
Disclaimer: This isn't really the answer Jan is looking for.
Holleman, Wiberg "Lehrbuch der Anorganischen Chemie", de Gruyter, Vol. 101, notes that:
Selenium
Selenium forms a weaker, but more ...
- 6,941
7
votes
Accepted
Why does cobalt have a higher atomic mass than nickel?
Protons in each nucleus repel each other electrostatically.
As atomic number increases, there is more repulsion and an increasing advantage of spreading out the protons with neutrons.
Calcium-40 is ...
- 40.9k
7
votes
Accepted
Why does hydration of ions depend on their charge to area ratio?
Many people have done experimental, theoretical, statistical, molecular dynamics, etc. studies of solvation free energies of ions. I encourage you to just search solvation free energies of ions on ...
- 13.1k
7
votes
Accepted
The melting and boiling point decreases down the group up to group 14 (not including transition metals), but the trend reverses from group 15. Why?
The problem is, we want to see a trend in everything. There are various factors that govern melting point and boiling point which is the reason of perceived anomalies. "The equation which has ...
- 1,280
7
votes
Why an atom is more stable when only sublevels s and p are full?
Supposedly when explaining electronegativity and stability of an element, they tell you that it is more stable if the last level is full.
No "supposedly" about it. That concept is ...
- 190
6
votes
Order of positive mesomeric effect of halides
$\ce{2p-2p}$ orbitals overlaps better than $\ce{2p-3p}$, $\ce{2p-4p}$, $\ce{2p-5p}$. Although $\ce{F}$ is more electronegative, but it is also smaller in size. Both $\ce{F}$ and $\ce{C}$ (next to it) ...
6
votes
Why does basicity of group 15 hydrides decrease down the group?
Realise that there is a notable drop in basicity from nitrogen to phosphorus and then a slow and continuous further diminishing. The notable drop is due to the difference of the molecular structures ...
- 68.4k
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