A magnesium atom must lose two electrons to have the same number electrons as an atom of the previous noble gas, neon. Magnesium’s position in the periodic table (group 2) tells us that it is a metal. Write the symbol for each ion and name them. Predict which forms an anion, which forms a cation, and the charges of each ion. Magnesium and nitrogen react to form an ionic compound. For example, copper can form ions with a 1+ or 2+ charge, and iron can form ions with a 2+ or 3+ charge. In fact, transition metals and some other metals often exhibit variable charges that are not predictable by their location in the table. This trend can be used as a guide in many cases, but its predictive value decreases when moving toward the center of the periodic table. For example, group 17 elements (one group left of the noble gases) form 1− ions group 16 elements (two groups left) form 2− ions, and so on. Moving from the far right to the left on the periodic table, elements often form anions with a negative charge equal to the number of groups moved left from the noble gases. That is, group 1 elements form 1+ ions group 2 elements form 2+ ions, and so on. Moving from the far left to the right on the periodic table, main-group elements tend to form cations with a charge equal to the group number. Note the usefulness of the periodic table in predicting likely ion formation and charge ( Figure 2.29). (A discussion of the theory supporting the favored status of noble gas electron numbers reflected in these predictive rules for ion formation is provided in a later chapter of this text.) It has the same number of electrons as atoms of the next noble gas, krypton, and is symbolized Br −. This results in an anion with 35 protons, 36 electrons, and a 1− charge. For example, the neutral bromine atom, with 35 protons and 35 electrons, can gain one electron to provide it with 36 electrons. Atoms of group 17 gain one electron and form anions with a 1− charge atoms of group 16 gain two electrons and form ions with a 2− charge, and so on. When atoms of nonmetal elements form ions, they generally gain enough electrons to give them the same number of electrons as an atom of the next noble gas in the periodic table. The name of a metal ion is the same as the name of the metal atom from which it forms, so Ca 2+ is called a calcium ion. It has the same number of electrons as atoms of the preceding noble gas, argon, and is symbolized Ca 2+. This results in a cation with 20 protons, 18 electrons, and a 2+ charge. For example, a neutral calcium atom, with 20 protons and 20 electrons, readily loses two electrons. To illustrate, an atom of an alkali metal (group 1) loses one electron and forms a cation with a 1+ charge an alkaline earth metal (group 2) loses two electrons and forms a cation with a 2+ charge, and so on. Atoms of many main-group metals lose enough electrons to leave them with the same number of electrons as an atom of the preceding noble gas. You can use the periodic table to predict whether an atom will form an anion or a cation, and you can often predict the charge of the resulting ion. (b) A sodium cation (Na +) has lost an electron, so it has one more proton (11) than electrons (10), giving it an overall positive charge, signified by a superscripted plus sign. Figure 2.28 (a) A sodium atom (Na) has equal numbers of protons and electrons (11) and is uncharged.

Strontium sulfide charge