Atomic Structure and Chemical Reactions Do all elements lose their top electrons in chemical reactions? No; sometimes the opposite happens. Take a look at the second column from the right in the periodic table--the one that starts with fluorine (F). These elements have their highest electrons in p orbitals--five at the same energy. p has room for six, doesn't it? That's right. These elements are called the halogens; they're highly reactive, too, but in a different way than the alkali metals. What sets them off is not losing one of their own, but picking up a stray electron, which fits perfectly into that "empty space" in the p sublevel. Wow, it sounds like the halogens and the alkali metals would be perfect for each other. You're absolutely right; those two groups love to form compounds together. For example, I'm sure you're familiar with sodium chloride, NaCl. What would the electrons look like in an element that wasn't very reactive? Look at the far right group, with helium (He) at the top. Except for helium, all of these have a "full" p sublevel at the top, with six electrons. Yes. Helium, of course, has a "full" s sublevel--you may recall that the first primary level has only s orbitals. These guys are called the noble gases, and they're perfectly happy with themselves as they are--no desire to give up or take in electrons. As a consequence, they hardly ever form compounds with other elements. In general, the arrangement of the outermost electrons, called valence electrons, tells you all about an element's chemical behavior.

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