# Tag Info

## Hot answers tagged periodic-trends

Accepted

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

Here are the $\ce{H-X-H}$ bond angles and the $\ce{H-X}$ bond lengths: \begin{array}{lcc} \text{molecule} & \text{bond angle}/^\circ & \text{bond length}/\pu{pm}\\ \hline \ce{H2O} & 104.5 &...
• 82.5k
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### Why does basicity of group 15 hydrides decrease down the group?

Each of these molecules has a pair of electrons in an orbital - this is termed a "lone pair" of electrons. It is the lone pair of electrons that makes these molecules nucleophilic or basic. As you ...
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### 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 ...
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### Why does the solubility of group II hydroxides increase and the solubility of sulfates decrease down the group?

Okay, for this scenario, the two concepts that play a key role are the lattice enthalpy and the hydration enthalpy. Lattice enthalpy is the energy released when one mole of a compound is formed from ...
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### Why is lithium the most reducing alkali metal, and not caesium?

The trend in the reducing power of the alkali metals is not a simple linear trend, so it is a little disingenuous if I were to solely talk about $\ce{Li}$ and $\ce{Cs}$, implying that data for the ...
• 66.8k
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### "Middle row anomaly" of the periodic table

This is due to the transition metal contraction. Bromine has the electron configuration $\ce{[Ar] 4s^{2} 3d^{10} 4p^{5}}.$The 3d orbital has no radial nodes and is therefore quite contracted (close ...
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### Is there a function to approximate atomic mass from the atomic number?

The short answer is that you can find a power-law fit ($1.61Z^{1.1}$) with low average error. I'd never really thought about it much, but after downloading the IUPAC Atomic Weights, I decided to do ...
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### 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 ...
• 66.8k
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### 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 ...
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### 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 ...
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### 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 ...
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### 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 ...
• 32.6k
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### Why does screening effect decrease due to d-orbital?

The reason d-orbitals make a difference is that electrons in d-orbitals do not screen nuclear charge as effectively as those in s and p orbitals. This is because of something called penetration . ...
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### Why does tin form tin (II) compounds?

I feel very ashamed to answer my own question, but having found a possible answer, I don’t see why I shouldn’t share it with the community. I would like to start by bringing in this table of orbital ...
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