The amount of shells in atoms?

I'm reading something here, and calcium has 4 shells and silicon only has 3 shells? What gives?

The amount of shells in atoms?
Electrons are arranged around an atom in what are called orbitals. These orbitals are denoted as S, P, D, F, starting with S and moving from there. The first orbital S can hold only two electrons, the second P can hold six, the third D can hold ten and F holds 14.

The first shell contains one S orbital and covers the elements H and He, the second shell contains an 2S and 2P orbital and covers the elements Li through Ne on the periodic table. The third shell contains an 3S and 3P orbital and covers the elements Na through Ar, this includes the Silicon you were wondering about, and the fourth shell has a 4S, 3D and 4P orbital and covers the elements K through Kr on the table, which includes Calcium as you were asking about.

In short, elements in each period (horizontal grouping of elements, reading from left to right) are contained in the same number "valence shell" (outermost shell of electrons) and as you go down a group (vertical grouping of elements) say from oxygen to sulfur you will find that oxygen has a valence shell of 2 while sulfur has a valence shell of 3.



I hope that wasn't too dense for you, it certainly was a lot of information, and I hope it helps answer your question.
Reply:Because Calcium has more electrons than Silicon. The 3rd shell can only hold 8, when you want to add more electrons, you need another shell.
Reply:Each atom actually has ALL the possible energy shells... they're just "there" in the sense that they are energy levels which electrons CAN occupy. Of course, atoms only have a defined number of electrons so each atom will only have so many shells fully or partially populated.



The rule for the number of electrons which each shell can contain is simple: Number = 2N^2, where N is the shell number.



So.. 1st shell (K-shell) can contain 2

2nd (L-shell) : 8

3rd (M-shell): 18

and so on.



So the difference between Calcium and Silicon will be the number of electrons and therefore how many shells are populated.



It actually gets a bit more complex than that for higher atomic numbers... Each shell has a number of sub-shells (which are labelled s, p, d, f, ..) and as the atomic number increases the energy levels of the subshells tend to overlap a bit. That means that sometimes the N+1th shell will start to fill before the Nth shell is completely full.



Try to look for "electron configuration" tables for the elements and follow how they fill as the atomic number increases and you'll see what I mean.
Reply:If you look at the periodic tables of elements youll see that Si is on the 3rd line making it have 3 shells, Ca is on the 4th making it have 4 shells.



I dont knwo how much chemistry you know but, it has to so with the number of electrons in each shell. Si has the the first two shells full and it has 4 electrons the 3rd shell, Ca has the first 3 shells full and 2 electrons in the 4th shell. if you want further explanation i can give it to you.
Reply:That refers to atoms' electron configurations. If you have not covered these yet in class, look them up on the Internet.



The shells are the "s" orbitals around an atom. They are sphere-shaped, and can hold up to two electrons per shell. They and "p" orbitals (propeller-shaped) compose the valence shell of the atom.



Different atoms have different numbers of shells due to their electron configurations and energy levels. Again, look this up if you haven't learned about it in chemistry class yet.