Thus the anode is negative.Īt the cathode, on the other hand, you have the reduction reaction which consumes electrons (leaving behind positive (metal) ions at the electrode) and thus leads to a build-up of positive charge in the course of the reaction until electrochemical equilibrium is reached. Since at the anode you have the oxidation reaction which produces electrons you get a build-up of negative charge in the course of the reaction until electrochemical equilibrium is reached. Now, in a galvanic cell the reaction proceeds without an external potential helping it along. That's how cathode and anode are defined. Takes place while the cathode is the electrode where the reduction reaction The barium cation is written Ba 2+, not Ba +2.The anode is the electrode where the oxidation reaction Note the convention of first writing the number and then the sign on a multiply charged ion. Figure 3.3 "Predicting Ionic Charges" shows how the charge on many ions can be predicted by the location of an element on the periodic table. On the other side of the periodic table, the next-to-last column, the halogens, form ions having a 1− charge. Ions made from alkaline earth metals, the second group on the periodic table, have a 2+ charge. For example, all ions made from alkali metals, the first column on the periodic table, have a 1+ charge. Thus, the periodic table becomes a tool for remembering the charges on many ions. In many cases, elements that belong to the same group (vertical column) on the periodic table form ions with the same charge because they have the same number of valence electrons. In macroscopic samples of sodium chloride, there are billions and billions of sodium and chloride ions, although there is always the same number of cations and anions. The number of electrons lost by the sodium atom (one) equals the number of electrons gained by the chlorine atom (one), so the compound is electrically neutral. Notice that there are no leftover electrons. The resulting combination is the compound sodium chloride. With two oppositely charged ions, there is an electrostatic attraction between them because opposite charges attract. On the right, the chloride ion has 18 electrons and has a 1− charge. On the left, the chlorine atom has 17 electrons. Most nonmetals become anions when they make ionic compounds.įigure 3.2 The Formation of a Chlorine Ion Negatively charged ions are called anions A negatively charged ion. When these atoms gain electrons, they acquire a negative charge because they now possess more electrons than protons. Some atoms have nearly eight electrons in their valence shell and can gain additional valence electrons until they have an octet. Most metals become cations when they make ionic compounds. Positively charged ions are called cations A positively charged ion. Atoms that lose electrons acquire a positive charge as a result because they are left with fewer negatively charged electrons to balance the positive charges of the protons in the nucleus. In cases where an atom has three or fewer valence electrons, the atom may lose those valence electrons quite easily until what remains is a lower shell that contains an octet. Some atoms have only a few electrons in their outer shell, while some atoms lack only one or two electrons to have an octet. Most atoms do not have eight electrons in their valence electron shell. Use Lewis diagrams to illustrate ion formation.
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