Formula to Calculate the Number of Valence Electrons

When you are looking at the Periodic Table, the horizontal rows (Periods) will tell you how many shells are required to house the electrons. For example, an element found in the 4th Period will require four shells for its electrons. The vertical columns, excluding the transition elements, (Groups) will tell how many electrons are in the valence shell. This only works using Groups 1A through 8A, not with the B columns. An element in Group 5A will have five electrons in its valence shell.

So, Carbon, being in Period 2 will require 2 electron shells. Since it is in Group 4A, there will be 4 electrons in the valence shell.

I'm in the same boat. I failed high school Chemistry. Here's what I have managed to grasp though. An atom can have 2 electrons in the first shell, and up to 8 electrons on each shell thereafter. The atom wants to fill up its shells to 8 so that it can be stable (happy!). Therefore, if an atom only has 6 electrons in its outer shell, it will try to gain 2 from another atom (which is what causes atoms to bond in various ways - either by taking electrons, giving electrons, or sharing electrons with another atom). The goal of the atom is to make its outer shell (the valence shell) full by having 8 electrons. Say for example an atom has 2 electrons on the inner shell, 3 on the outer. Then that atom will want to do one of two things.....it will want to either ditch those 3 on its outer shell (because its not stable therefore not happy). OR it will want to gain 5 electrons (to get to 8) so that it can be stable. Now which is easier for the atom to do? Lose 3 or gain 5? It's easier for it to lose the 3. So that atom will either form an ionic bond with another atom by giving that atom those 3 electrons (so that now the atom only has 2 in its only shell - so its stable). OR it will form a covalent bond with another atom, say another atom only has 5 electrons on its outer shell. It needs 3.....our atom needs 5....so they share and now both of them have 8. I hope this makes sense to you. My A&P instructor is great, he really makes it easy for me to understand. I hope I helped. If not, I would suggest you google atoms and see if you can find something that explains it better.

The valence layer is the outermost layer/shell of electrons. The first layer can hold only two electrons; in order for there to be another layer, this first layer must be filled. The second and third layers can each hold eight electrons. Again, in order for there to be a third layer, the second must be filled. However "full" the valence layer is helps to determine the reactivity of an atoms (stable vs. unstable). For example, Lithium has a only one electron in its second energy level, meaning that it is unfilled (remember that it's capable of holding eight), so it is very stable. In contrast, Neon has a full valence layer or shell---it has all eight electron spots filled, so it's not very reactive.

I hope this helps somewhat.

You got some great answers. I haven't really been here yet but someone recc'd these sites. Hope they help a bit! (check the bottom of the page for things like physics and biology)



http://www.chem4kids.com/

Are you talking about the thing like

1s 2s 2p 3s 3p 4s 3d........

I believe it goes: 2, 8, 18, 32 (can't remember what happens after 32 though)

That is a great explanation, Pezamistik!

A very easy formula is 2n^2. that is 2 times n to the second power, where n = the number of the principle energy level (PEL).


Example: PEL#- 1 2 3 4

max # of e- would be: one sq x 2 = (2), 2squared x 2 = (8), 3 sq x 2 = (18), 4 squared x 2 = (32)

hope it helps and good luck.