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Ok so each row on the periodic table actually tends to correspond to a particular shell of an atom. So the first row corresponds to the first shell (with only an s-orbital) so there are only 2 e- within that shell. So you know that H has 1 valence e- and He has 2 e- because those are the number of e- they have in their only shell. In the second row all elements have 3 e- or more, so you know that they have e- in the 2nd shell. They have 2 e- in the first shell, and the rest of the e- they have are in their valence shell. For example, O has 8e-, so the first 2e- are in the first shell, and the other 6e- are in the second shell, so O has 6 valence e-. So to find the number of e- in a valence shell for an element in row 2, subtract 2 e- from their total e- count. For an atom in row 3, subtract 10 e- (2e- from first shell, 8 e- from second shell) from their total number of e- to find their valence e- in the third shell .
A quick and easy way to find out how many valance electrons an element has is to count left to right across periods for the S orbital and the P orbital. For example H would have 1 valance electron, Be would have 2, B would have 3, and C would have 4. This would make the noble gases have 8 valence electrons and this would make them the most stable.
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