Why does an element's chemical properties rely only on its valence electrons and not on anything else?

Question

I understand that elements use their valence electrons for their reactions and whatnot, and that the whole idea of the electron-dot structure is all about valence electrons. But why can't the other electrons play a part in it?

Answer 1

Technically, other electrons could play a part in determining chemistry. However, there is a significant energy barrier between the p-orbitals of one period and the s-orbital of the following period.

Consider copper. The characteristic deep blue colour of $\ce{[Cu(NH3)4(H2O)2]^2+}$ derives from the absorption of photons whose energy corresponds to the difference of the $\mathrm{3d}_{z^2}$ and $\mathrm{3d}_{x^2 - y^2}$ orbitals. This is spin-allowed because copper(II) is a d⁹ system. The wavelength corresponds to approximately $610\:\mathrm{nm}$.

On the other hand, the energy between a $\ce{2p}$- and a $\ce{3s}$-orbital can be measured using x-ray transition — it corresponds to a photon of a wavelength of approximately $1\:\mathrm{nm}$. This is about six-hundred times the energy difference compared to inside the d-orbitals.

Of course, this is a slightly unfair comparison. The energies between $\ce{3d}$ and $\ce{4s}$ are noticeably larger than inside of the $\ce{3d}$ orbitals, and the energy difference between $\ce{4s}$ and $\ce{4p}$ should again be well larger — but they will remain significantly smaller than energy differences across shells. The former are typically visible or UV, the latter are typically x-ray.

Answer 2

As the valence electrons are the furthest from the nucleus, they are more loosely held by the nucleus. So, they can readily be lost, gained or shared to form ionic or covalent bonds. The inner electrons are tightly held by the nucleus. So, they need higher energy to participate in the formation of chemical bonds.