31
votes
Accepted
How come uranium's relative atomic mass is 238.03 when it only contains isotopes with a mass number of 238 or less?
Approximately 99.3% of uranium on Earth is the $\mathrm{^{238}U}$ isotope, and this specific isotope has an atomic mass of $\mathrm{238.05\ u}$, where $\mathrm{u}$ is the atomic mass unit, equivalent ...
- 27.6k
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 ...
- 27.6k
18
votes
Accepted
To which block of the periodic table does helium belong?
Blocks in the periodic table should not be mixed up with groups (like noble gases). The reason why Helium is considered a noble gas is because its outermost (and at the same time only) shell is fully ...
- 757
16
votes
How did Mendeleev know elements from compounds or mixtures?
Please do not underestimate the scientists of 19th century. They were as creative, intelligent and perhaps more genuinely dedicated to science than the scientists of the 21st century. Spectroscopy was ...
- 37.3k
14
votes
Accepted
Why can mercury(I) exist, but not zinc(I)?
On the contrary, zinc(I) compounds do exist, though they are rare, and relatively unstable. Most zinc(I) compounds contain a $\ce{[Zn2]^{2+}}$ core, which is analogous to the $\ce{[Hg2]^{2+}}$ cation. ...
- 23.8k
14
votes
Accepted
Spontaneity and nature of attack of fluorine gas on aluminum
First and foremost! Do not do this reaction unless you are properly trained and have the appropriate safety equipment. Fluorine is one of the most dangerous substances out there, and one of the ...
- 42.3k
13
votes
Accepted
Confusion with the Periodic Table
So if that is the case, shouldn't period 3 have more elements, since
it can hold up to 18 electrons, and therefore it can have up to 18
more protons from the largest atomic number element in ...
- 18.8k
13
votes
Accepted
Confused by notation of atomic number Z and mass number A on periodic table of elements
Periodic tables of elements (PTEs) are often abused by designers. Books are more trustworthy as long as they are written by scientists. Long story short, the second notation $(\ce{^{12}_{6}C})$ is the ...
- 37.2k
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 ...
- 38.6k
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 ...
- 136
12
votes
Calling Diamond an element?
It might be scientifically correct but it is linguistically misleading
The sentence "diamond is an element" can be seen to be misleading when compared to the sentence "diamond is an allotrope of the ...
- 34.6k
12
votes
Why are there only 7 periods in the periodic table?
There is no theoretical limit to the number of rows, but...
On one hand, as Tom Lehrer states, there may be many others but they haven't been discovard.
On the other hand, after all s, p, d, f ...
- 27.4k
11
votes
Accepted
Why is UuX used as a symbol for unnamed elements on the periodic table?
In 1978, the IUPAC Commission on the Nomenclature of Inorganic Chemistry decided that it would be necessary to have a systematic naming for the elements with atomic number greater than 100, even for ...
- 23.8k
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)...
- 27.4k
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: ...
- 70.2k
11
votes
Accepted
Is "pentel" a real chemistry term?
Use Google Scholar (scholar.google.com/) when needed. All I searched was: pentels pnictogen, and the third search result leads to an article in Angewandte Chemie which in turn leads to the original ...
- 37.3k
10
votes
In the periodic table, why doesn't the 2nd row have exactly 2 elements?
The pattern is better expressed this way:
...
- 551
10
votes
What organizational methods pre-date the Periodic Table?
One attempt to order chemical elements was Döbereiner's system of triades, published in Annalen der Physik und Chemie, back in 1829 (doi 10.1002/andp.18290910217 with Wiley); or (open access with ...
- 26.7k
8
votes
Accepted
Is there an abundance threshold of a 'synthetic' element for it to be considered natural?
Is there a natural abundance threshold of a 'synthetic' element for it
to be considered natural?
$\mathrm{> 0}$
In other words, any element that can be detected outside of the laboratory (e.g. ...
- 83.8k
8
votes
What is the usage of orbitals more complex than f orbitals?
One use of orbitals beyond f is in computational chemistry to construct basis sets. It's important to remember that orbitals are entirely a mathmatical construct that chemists and physicists have ...
- 11.5k
8
votes
Why is Promethium the only radioactive Lanthanide?
Lanthanoids are not generally supposed to be radioactive, with exception of mentioned very long lived radioisotopes.
The longest half-life radioactive nuclides of lanthanoids ( in years )
$\ce{^{...
- 36k
8
votes
Accepted
What organizational methods pre-date the Periodic Table?
Aaron J. Ihde's The Development of Modern Chemistry (DMC) has an entire chapter entitled "Classification of the Elements," which includes a very nice overview of the attempts before Mendeleev to bring ...
- 17.1k
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 ...
- 25.6k
7
votes
Accepted
Does helium have a 2s orbital?
Part 1: The elements on the periodic table are arranged according to their ground-state electron configurations. He 2s is an excited state of He 1s. There are even He 3s and 4s depending on whether ...
- 563
7
votes
Anomalous Electronic Configuration of Thorium
For first four actinide elements $\ce{Th}$, $\ce{Pa}$, $\ce{U}$ and $\ce{Np}$, the difference in energy between $\mathrm{5f}$ and $\mathrm{6d}$ orbitals is small. Thus in elements electrons may occupy ...
7
votes
Why are there only 7 periods in the periodic table?
On the other hand, it will be difficult to synthetize elements heavier than Oganesson ($\pu{Z = 118}$) for lack of neutrons. When the number of protons increases through the periodic table, the ratio ...
- 25.6k
7
votes
Is "pentel" a real chemistry term?
Group 13 and group 14 elements are trivially called triels and tetrels respectively (see previous posts- this and this). By same analogy, group 15 elements are called "pentels".
pentel (...
- 24.4k
6
votes
Why lanthanides and actinides are shown separate from standard Periodic Table layout?
There is no scientific reason not to put the f-blocks between s- and d-blocks!
However, doing so will create a very wide periodic table: The s-, p- and d-block are 18 elements wide, the f-block would ...
- 66.4k
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