![]() ![]() It loses two electrons to become a calcium ion with 20 protons and 18 electrons. ![]() Cations are given the same name as their element, followed by the word ion.Ī calcium atom has 20 protons and 20 electrons. Cations have a positive charge because they have more protons, which have a positive charge, than electrons, which have a negative charge. When atoms or molecules lose electrons, they are called cations. ![]() Therefore, the atomic mass of an ion is essentially the same as the neutral atom’s atomic mass. Electrons, being much smaller in mass than protons, do not contribute significantly to the atomic mass. When an atom or molecule gains or loses electrons, it is called an ion. An element is defined by the number of protons, which does not change however, many atoms can gain or lose one or more electrons to become a charged particle. Review the Periodic Table of the Elements in other formats in Appendix A (credit: Chemistry (OpenStax), CC BY 4.0).Atoms are neutral particles with equal numbers of protons and electrons. Figure 6.1b: Some elements exhibit a regular pattern of ionic charge when they form ions. 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 6.1b). (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.) 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 (credit: Chemistry (OpenStax), CC BY 4.0). Figure 6.1a: (a) A sodium atom (Na) has equal numbers of protons and electrons (11) and is uncharged. Identify the charges of the ions formed by main group elements on the periodic tableĪs a recap from Chapter 3, during the formation of some compounds, atoms gain or lose electrons, and form electrically charged particles called ions (Figure 6.1a). ![]() By the end of this section, you will be able to: ![]()
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