# Why does the arrangement of the Periodic Table make no sense?

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• Bonkus Klunkit
In summary: For example, 3s orbit has 2 story and 4s orbit has 3 story. So 3s can crossover with 4s but not with 2s.This is the basic understanding about how the shells work.
Bonkus Klunkit
TL;DR Summary
Electron Shells and the periodic table
We're told that the electron shells give the relative reactivity/affinity to other atoms by their incompleteness, completeness or over-completeness, and arranged in the periodic table accordingly as columns. The shells are are concentrically arranged around the nucleus from smaller capacity to larger starting with the first a shell that fits two. Hence row 1 has two (H & He), one on the left and one on the right of the table. The one on the right (He) has a full shell and is stable and is placed with the column of "noble gasses". The next shells continue in their holding capacity to hold 8, 18 and 32 respectively and are placed into the "columns" they would fall into and Ne having a full 2 shell and a full 8 shell is stable and placed under He as another "noble gas". Shell 3 holds 18, but we see row 3 with only 8 elements and Ar placed under the "noble gasses" column as if it had a complete shell (18)--but it only has 8! Row 4 has 18, but what happened to row 3? And shouldn't row 4 have 32 (row 6 & 7 do)? I'm confused, this would make the concentric shells hold respectively 2, 8, 8, 18, 18, 32, 32. I know this BASIC physics, but I'm confused.

Bonkus Klunkit said:
...Shell 3 holds 18, but we see row 3 with only 8 elements and Ar placed under the "noble gasses" column as if it had a complete shell (18)--but it only has 8! Row 4 has 18, but what happened to row 3? And shouldn't row 4 have 32 (row 6 & 7 do)? ...

As a general rule, the periodic table is arranged in the order that electrons would fill orbitals at higher energy.

Here, orbitals are particular quantum states of the electron orbiting a hydrogen-like atom with principal quantum number $n={1,2,3,...}$, angular momentum quantum number $\ell={0,1,2,...,n-1}$, magnetic quantum number $m_{\ell}={-\ell,-\ell+1,...,\ell-1,\ell}$, and spin quantum number $m_{s}={-1/2,1/2}$. The models describing the valence electrons around different elements are more sophisticated, but these quantum numbers are useful enough to organize the elements.

Each orbital corresponds to a unique value of $n$, $\ell$, and $m_{\ell}$

Each orbital can be occupied by up to 2 electrons, due to $m_{s}$ having two possible values, and the Pauli exclusion principle preventing two electrons having the same set of all 4 quantum numbers at the same time.

In order of increasing energy, the orbitals are:
1s (row 1, 2 elements)
2s, 2p ( row 2, 2 + 6 = 8 elements)
3s,3p ( row 3, 2 + 6 = 8 elements)
4s,3d,3p ( row 4, 2 + 10 + 6 = 18 elements)
5s,4d,4p ( row 5, 2 + 10 + 6 = 18 elements)
6s, 4f, 5d, 5p ( row 6, 2 + 14 + 10 + 6 = 32 elements)
7s, 5f, 6d, 6p ( row 7, 2 + 14 + 10 + 6 = 32 elements)

Here as short-hand $\ell={0,1,2,3,4}\rightarrow{s,p,d,f,g}$, so that the 6d orbital corresponds to $n=6$ and $\ell=2$, which means $m_{\ell}={-2,-1,0,1,2}$, and so there are five, 6d orbitals, and it takes 10 electrons to fill it.

All electrons in an orbital with a particular value of $n$ is in that electron shell. Electron shells don't precisely relate to how the periodic table is organized.

I don't have a good explanation for why the 3d orbital has a lower energy than the 3p orbital and a higher energy than the 4s orbital, or why the noble gases below neon are noble in spite of having incompletely filled electron shells, but I hope this helps :)

jfizzix said:
I don't have a good explanation for why the 3d orbital has a lower energy than the 3p orbital and a higher energy than the 4s orbital, or why the noble gases below neon are noble in spite of having incompletely filled electron shells, but I hope this helps :)

Let me explain my understandings on this matter.

The two rules are sure;

As for different prime numbers and the same orbital number x,
nx < (n+1)x for x=s,p,d,.. where "nx" is energy level of nx orbit.
e.g.
1s < 2s < 3s < ...
1p < 2p < 3p < ...
1d < 2d < 3d < ...

As for different orbit number n and the same prime numbers x,
ns < np < nd < nf < ...
e.g.
1s < 1p < 1d <...
2s < 2p < 2d <...
3s < 3p < 3d <...

Satisfying these basic rules something like "crossover" could take place between
"nx" and "my". It is like that : a lower story of a building built on a higher land can become higher than the higher story of the building built on the lower land. For nx n is land height and x-th is the number of story.

Last edited:

## 1. Why are elements arranged in a seemingly random order in the Periodic Table?

The arrangement of elements in the Periodic Table is not random, but rather follows a specific pattern based on their atomic structure. Elements are arranged in order of increasing atomic number, which is the number of protons in the nucleus of an atom. This arrangement allows for similarities and trends among elements to be easily observed.

## 2. Why are some elements placed in groups that don't seem to have any similarities?

While some elements may appear to have little in common, they are still placed in the same group because they share similar chemical properties. For example, halogens such as chlorine and noble gases like neon may seem very different, but they both have a full outer electron shell, making them chemically stable.

## 3. Why are there gaps in the Periodic Table?

The gaps in the Periodic Table represent elements that have yet to be discovered or have only been recently discovered. These elements are predicted based on the patterns and trends of known elements, and their properties can be determined by their position in the table.

## 4. Why are some elements listed separately at the bottom of the Periodic Table?

The elements at the bottom of the Periodic Table are known as the "lanthanides" and "actinides." They are placed separately to conserve space and make the table easier to read, but they still follow the same pattern of increasing atomic number as the rest of the elements. These elements also have similar properties, which is why they are grouped together.

## 5. Why does the Periodic Table have a specific shape and layout?

The shape and layout of the Periodic Table is based on the properties of elements and their atomic structure. The table is divided into periods (rows) and groups (columns) to highlight similarities and trends among elements. This layout also allows for easy identification of elements and their properties.

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