Why does having 8 valence electrons make an element inert?

[Redriq1]

I was introduced to the Octet Rule recently and make me wonder, why does 8 valence electrons or a full p orbital always make an element inert?
What is so special with a full p orbital?
Like take Calcium for an example, its outer orbital is filled but its only the s orbital thats filled so its still reactive not so much as the Alkaline metals but still pretty reactive.
Can someone explain it to me?
Thanks!!

 

[Mayhem]

Strictly speaking, Ca does not have a filled outer shell in the sense you are referring to. It has 2 e in the 4s orbital. Elemental Ca is also very reactive since losing these two electrons yields a full octet shell (3s
² 3p⁶). In the same row. If you go a few elements back, the noble gas Argon (Ar) has the 3s² 3p⁶ octet in its elemental state, why you would consider it an inert element.

 

[mjc123]

You may find this interesting: https://www.chemicalforums.com/index.php?topic=77729.msg283639#msg283639

 

[TensorCalculus]

I was introduced to the Octet Rule recently and make me wonder, why does 8 valence electrons or a full p orbital always make an element inert?
What is so special with a full p orbital?
Like take Calcium for an example, its outer orbital is filled but its only the s orbital thats filled so its still reactive not so much as the Alkaline metals but still pretty reactive.
Can someone explain it to me?
Thanks!!

Don't know how good your Chemistry is so I'll give the simplest explanation possible (also, my chemistry isn't very good, but the fact that our teacher would not tell us why full valence shell = stable was irritating me a lot so I did look into it about a year ago)
When you have orbitals that are full, the electrons are arranged like this: $|\;\uparrow\downarrow|\;\uparrow\downarrow|\;\uparrow\downarrow\;|$
(The alternating up then down comes from the Pauli Exclusion Principle, which dictates how orbitals can be filled)
In this configuration all of the electrons are paired which means that the repulsion between them is at a minimum. Thus, so is the energy.
It would take a lot of energy to add a new electron to this, and getting rid of electrons will also push the atom into a higher energy state (and thus need to have added energy). Thus, the atom usually does not gain or lose electrons and we call it stable :)

 

[256bits]

why does 8 valence electrons or a full p orbital always make an element inert?

I don't know if this is true all the time, but a useful guide for the octet rule for the main group elements.
In bonding, the atom will 'seek' to have its valence shell that of a configuration of a noble element. One may have to go then on to ask and contemplate why the noble elements are considered inert.

Na loses an electron becoming Na+. Na+ has an electron configuration of Ne.
Cl accepts an electron becoming Cl-. The Cl- electron configuration is that of Argon.

Similar reasoning fo CaO bonding, except 2 electrons are lost or gained in the valence shell(s).

 

[Borek]

Once you know the basics of the quantum chemistry - how electrons behave in atom, how and why they are organized in "groups" (orbitals, shells) - answer becomes more or less obvious. Octet is one of these numbers that come naturally from the solution of Schroedinger's equation and relate to the very stable, low energy configuration. Other explanations are just conclusions, or echoes of this underlying scheme.

 

[Astronuc]

Helium (Z=2) is obviously an exception to the octet rule. Neon (Z=8) has a full p-shell and it is the first element with an octet. For noble gases, in addition to a full outer shell, the number of electrons equals the number of protons.

The group 1 (IA) elements prefer to give up an electron, or share 1 electron with group 17 (VIIA) elements, e.g., HF, HCl, NaCl, . . . . Group 2 (IIA) share 2 electrons with 2 group 17 elements, or complex anions with a valence of -1 (meaning the anion will attract 1 electron). Group 16 (VIA) prefer to attract 2 electrons, and one finds compounds such as Li₂O, or CaO, or CaF₂. Getting into other groups, it becomes more complicated with multiple valence states.

In some cases, the heaviest noble gases, e.g, Xe, can form compounds, e.g., XeF₆.
https://en.wikipedia.org/wiki/Xenon_hexafluoride

 

[Borek]

For noble gases, in addition to a full outer shell, the number of electrons equals the number of protons.

That's true for every atom.

 

[Astronuc]

That's true for every atom.

as opposed to ions, for which number of electrons exceeds number of protons for a negative ion, or number of electrons is less than the number of protons for positive ions. For example, Cl^-, the outer shell is filled like that of Ar, but Cl has 1 electron more than the number protons (17) for net atomic charge of -1 e.

 

[DrDu]

The point is that electrons in the same shell are not as efficient in screening other electrons from the nuclear charge as electrons in lower shells. This means that, despite the total atom being electrically neutral, the electrons in the valence shell see a the higher nuclear charge the further right in the period is the element. This leads to a continuous stabilization of the electrons in the valence shell in the course of going from the left (alkali metals) to the right (noble gasses). This is reflected in a continuous increase of ionization energy and electron affinity. A semiempirical method to find the effective nuclear charges are given by the so-called Slater rules: https://en.wikipedia.org/wiki/Slater's_rules
From this perspective it is advantageous for metals to give their electrons to non-metals as there they are more tightly bound. The process is limited by the atoms giving either away all their valence electrons or completing the valence shell, which gives naturally rise to the octet rule, at least in elements of the first period.

 

[dextercioby]

This "give electrons" idea is so 1916, yet we are in 2025. So, can one show me quantum chemistry calculations (valence bond, LCAO, orbital molecular, density functional, whatever) in which the natural interpretation of a NaCl molecule is with the "outer electron" from the Na atom being very probable to be found within the Cl orbital probability cloud? I always complain that the high-school idea of "ionic bonding" is only founded on pre-quantum ideas. It's like people never moved passed this book: Lewis, G. N. (1926). Valence and the Nature of the Chemical Bond. Chemical Catalog Company.

 

[DrDu]

I looked for bader's electron localisation function and NaCl and found the following paper which seems to support the classical picture:
https://www.lct.jussieu.fr/pagesperso/savin/papers/forbader-nacl/forbader-nacl.pdf

 

[dextercioby]

I looked for bader's electron localisation function and NaCl and found the following paper which seems to support the classical picture:
https://www.lct.jussieu.fr/pagesperso/savin/papers/forbader-nacl/forbader-nacl.pdf

Link is very slow or dead. Did you store the paper and can you upload it here? Thanks!

 

[DrDu]

For me it works fine. For legal reasons I find it problematic to upload papers I do not own.

 

[dextercioby]

Worked now, thank you. It was probably a lag back then.