Valence electrons are the electrons in the outermost shell of an atom. Some elements only have 2, others have 7, and others have 4. Because of this, they do not have a full outermost shell and that is why they form different bonds to be able to have enough electrons to become full. Noble gases are the elements that already have 8 valence electrons, so they don't form bonds (no reaction). An ion is an atom or molecule which has lost or gained one or more valence electrons, giving it a positive or negative electrical charge. Anions are negatively charged ions that form when an atom gains electrons. Anions are negatively charged because there are more electrons than there are protons in their nuclei. Cations are positively charged ions that form when an atom loses electrons. Cations are positively charged because there are fewer electrons than protons. An element forms bonds with another element. One type of bond is an ionic bond. That is when one of the elements in the bond gives electrons to the other element and so loses electrons and the other element gains electrons. That is when an ion is formed. For instance, Chlorine has 7 valence electrons and Sodium has 1 valence electron. Since Sodium only has 1 electron, it is easier to give it away instead of having to take 7 more, so it gives it's 1 valence electron to Chlorine. Now they both have a full outer electron shell! Since Sodium gave 1 valence electron AWAY, it now has 1 less electron and therefore a charge of +1 (a cation). Chlorine RECIEVED 1 valence electron, so now it has one more electron and therefore a charge of -1 (an anion). Calcium has 20 electrons total. It has 2 valence electrons and when it gives these two away, it now has a charge of +2 (it is now an ion). Since it has 2 less electrons, it now only has 18 left. The element with this number (18) of electrons is Argon....
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