About Lewis Structure

Actually this is not always the case. This is only a general rule (the octet rule)derived from quantum physics. Valence electrons means the number of outermost shell electrons that are present in an atom. For example, there are 2 electrons in the first shell, 2 in the second, then we say the number of valence electrons is 2.

From calculations of quantum physicsm, we found there exists different energy levels in the atoms' electrons. And as those energy levels are discrete, they generally differ from each other much unless the atoms get excited.

So, we come across a generalized energy level picture as 2,8,18,32.... they represent the highest number of electrons which could be hold in the 1st, 2nd, 3rd and 4th electron shells.

For the 1st shell, it can only hold 2 electrons, so elements on the first period can only hold 2 electrons at most. Hence hydrogen exists as a diatomic molecule. Recieving extra electron would involve antibondig orbitals which pull the hydrogen atoms apart.

For the 2nd shell, it can hold 8 electrons at most. Like the former case, extra electrons would go into antibonding orbitals, or else high lying orbitals, so these extra electrons destabilise the molecule and restores the 8 valence electron configuration. And actually nitrogen can often exist as stable radicals having only 7 valence electrons (nitrogen monoxide, NO)

But when talking about the third period elements, the octet rule can be relaxed. Examples like sulphuric acid, perchloric acid can be easily found. It is because they have d orbital of rather low energy comparing to their p orbitals, so electrons can fill into them after the p orbital is full. But the eight valence electron configuration is still more stable relative to the higher valence forms.

For transition metals, we also talk about the number of valence electrons, that is the eighteen electron rule which is similar to the octet rule.

for most of the atom have 8 valence electrons (in period 2,3), because those atom have 4 obital, so have 8 valence electrons. but for period 1, the maxium is 2 electron, and peirod 4 have 18 valence electrons, because have 9 obital ( if you are under F.6 you only need to know the first 3 period)
if you want to define the numbers of valence electrons, you must know the atomic number of the atom, and define the electronic configuration, then you also can define the number of valence electrons. ( valence electrons means the outermost electrons)

Most compound want to obtain the most stable form for themselves! So the octet structure 8 valence electrons is the most stable form for them.

Valence electrons are regarded as the outermost shell electrons. So you can find our how many electrons are there in the outermost leyer.