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Valence electrons are the outer electrons that are involved in bonding. Only electrons in the s and p orbitals are valance electrons, so a given atom can have between 0 and 7 valance electrons. Atoms with 0 valence electrons are called noble gases and don t like form bonds. An atom s valence electrons can be depicted pictorially using Lewis Dot Diagrams.
Alright, so we're going to talk about valence electron. We know electrons are the negatively charged particles in an atom they are the ones that are surrounding the nuclears. But we're going to talk about more specially valence electrons and valence electrons are electrons in the outermost orbitals the highest principle energy level. So they are the ones that actually responsible for bonding, the chemical properties of each element and so on and so forth. So they're the ones we actually going to be using much more often as we go on. So let's talk about them, lithium we know has a total of 3 electrons using this electron configuration, so has 3 electrons but it only has 1 valence electron and the highest energy level is 2 and there's only 1 electron in the second principle energy level. So we're going to say it has 1 valence electron that ve is going to indicate valence electron.
Going onto fluorine, fluorine has a total of 2 for 10 electrons total however only 8 of them, the ones in the atom or shell the s and the p. The second principle in energy level are going to be our valence electrons, the ones that are going to really make a difference. So in this case we have 8 valence electrons, okay fair enough. Looking at gallium, gallium has 31 electrons total but in the fourth principle energy level it has a total of 3 valence electrons. Notice only have 3 to the d does not count because it's only the third principle energy level. The fourth is the highest but we're only going to take electrons in the fourth principle energy level in this case there's 3 of them.
Okay so there's actually a pattern that goes along in the periodic table that tells us about the valence electrons and how we can determine them without havig to go through the electron configuration. Alright so if you look at things in group 1 hydrogen, lithium, sodium and so on these guys all have one valence electron. If you go to group 2 these guys all have 2 valence electrons. We're going to skip the transition metals because they are all dealing with inner shell electron and we'll deal with those another time.
But going on to group 3 has 3, carbon's group which is group 4 has 4 so on and so forth all the way up to noble gas which have 8. So I can just look at a periodic table and say "okay sulphur is in group 1, 2, 3, 4, 5, 6 it must have 6 valence electrons." And that is how you calculate valence electrons.