Valence+electrons

Valence electrons are the number of electrons in the outermost shell of an atom. For the majority of atoms, the number of valence electrons will be between one and eight. Reactions between atoms and molecules take place in these valence shells; chemical bonding is the process of atoms or molecules coming to share valence electrons. That's why many use the number of valence electrons to judge the reactivity of an atom: atoms wish to obtain eight electrons for their valence shells (the most stable arrangement), or they wish to resemble their "noble gas." Noble gases are the atoms in the 8A group; they have eight electrons in their valence shell, and are very difficult to react with.







Return your attention to the periodic table displayed: the XA groups tell you how many electrons are in the outermost shell of any atom. Yes, lithium and sodium, though having different numbers of total electrons, have the same number of valence electrons. Lithium is closer to the noble gas helium, so it is looking to lose an electron and look more like helium, whereas chlorine is closer to the noble gas argon, so it looks to gain one electron and look like argon.