1. Ions

2. Atoms are electrically neutral, that is, they have the same number of protons (+) as electrons (-). However, atoms are most stable when their valence shell is full. To obtain a full valence shell, atoms will lose or gain electrons, whichever takes less energy. This upsets the neutral charge and creates atoms with either a positive or negative charge. These are called ions.

3. Anions (“an-eye-on”) When a neutral atom gains an electron, the resulting negatively charged ion is called an anion. We know from our definition of non-metals that they will gain electrons, and thus usually form anions.

4. Consider Cl Cl atom has 17 protons and 17 electrons. It has 7 valence electrons. If it gains one more, then it’s outermost shell will be full. Therefore, Cl becomes more stable by gaining an e - 17+

5. Consider Cl Cl now has 17 protons and 18 electrons, producing a charge of 1–. The resulting anion is written as Cl – – 17+

6. This gain of e- can be expressed in an equation: Cl + e -  Cl - Oxygen atoms will gain 2 e- to obtain a full outer shell. The equation is: O + 2e -  O 2- Anions are named by dropping the ending and adding –ide to the end of the element name Example, Cl - is chloride ion, O 2- is oxide ion.

7. Cations (“cat-eye-on”) When a neutral atom loses an electron, the resulting positively charged ion is called a cation. We know from our definition of metals that they will lose electrons, and thus usually form cations.

8. Consider Na Na atom has 11 protons and 11 electrons. It has only 1 valence electron. If it loses it, then it’s outermost shell will be the 2 nd shell, which is full. Therefore, Na becomes more stable by losing an e - 11+

9. Consider Na But with 11 protons and 10 electrons, Na will have a charge of 1+. The resulting cation is written as Na + + 11+

10. This loss of e- can be expressed in an equation: Na  Na + + e - Magnesium atoms will lose 2 e- to obtain a full outer shell. The equation is: Mg  Mg 2+ + 2e - Aluminum forms a 3+ cation by losing 3 e-. The equation is: Al  Al 3+ + 3e -

11. Important Note Ions are always formed when electrons are either lost or gained. Ions are never formed by changing the number of protons. An atom by itself will not gain or lose e - to form an ion. It has to come in contact with another atom so that a transfer of e - may occur.

12. Making Predictions Will e- be lost or gained? How many? –K–K – Rb – Ca – Be –F–F – Br –S–S