43 votes
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 &...
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36 votes
Accepted

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 ...
  • 82.5k
34 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 ...
32 votes

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 ...
30 votes
Accepted

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 ...
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29 votes
Accepted

"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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28 votes
Accepted

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 ...
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 ...
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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 ...
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21 votes
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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 ...
19 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 ...
  • 27.6k
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 ...
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17 votes
Accepted

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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17 votes
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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 ...
  • 730
15 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-...
  • 16.9k
14 votes
Accepted

Why is osmium the densest known element?

The density of an element is related to how many atoms can be placed in a given volume and the weight of the nuclei. Therefore, the smaller the atomic radius of an atom and the higher the atomic ...
  • 82.5k
14 votes

Memorizing the periodic table

There isn't really much sense in memorizing the periodic table. The elements you often use you will know them by heart after a while. And you can always use a table when you need it for the others. ...
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14 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. ...
13 votes
Accepted

Bond angles for the hydrides

The simplest way to look at this trend is through VSEPR theory, which produces a very good qualitative understanding of molecular geometry even if it is not compatible with modern molecular orbital ...
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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 ...
12 votes
Accepted

Memorizing the periodic table

You can memorize the periodic table in one night, simply by emulating best-practice memorization techniques and doing what memory experts do. Common sense, right? Memory experts and world champion ...
12 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 ...
  • 3,056
11 votes
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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 ...
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11 votes
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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]: ...
  • 42.4k
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: ...
  • 66.8k
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: ...
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10 votes

In the periodic table, why doesn't the 2nd row have exactly 2 elements?

The pattern is better expressed this way: ...
10 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)...
9 votes

Why does the melting point get lower going down the Alkali Metal Group with increase in atomic number?

As you go down any group, there are extra electron shells (for example Lithium has 2 shells, Francium has 7) which cause electron shielding. Because the force of attraction must extend further and ...
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9 votes
Accepted

What is the reason for the different solubility of silver halides in ammonia?

Silver chloride is very thermodynamically stable in ammonia solution. The $K$ for the dissolution of silver chloride in ammonia solution is only $\pu{2.9E-3}.$ suggesting that the product — silver ...
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