Ions

Elements share a number of important properties with other elements found in the same group. The chemical behavior of a given element is largely dictated by the configuration of its valence electrons. Many elements have a tendency to gain or lose electrons in order to achieve a more stable configuration. When a neutral atom gains or loses electrons, it becomes an ion.

 Octet Rule  The noble gases are unreactive because of their electron configurations. American chemist Gilbert Lewis ( ) used this observation to explain the types of ions and molecules that are formed by other elements. He called his explanation the octet rule. The octet rule states that elements tend to form compounds in ways that give each atom eight valence electrons. An exception to this rule is the elements in the first period, which are particularly stable when they have two valence electrons. A broader statement that encompasses both the octet rule and this exception is that atoms react in order to achieve the same valence electron configuration as that of the nearest noble gas. Most noble gases have eight valence electrons, but because the first principal energy level can hold a maximum of two electrons, the first noble gas (helium) needs only two valence electrons to fill its outermost energy level. As a result, the nearby elements hydrogen, lithium, and beryllium tend to form stable compounds by achieving a total of two valence electrons.

 There are two ways in which atoms can satisfy the octet rule. One way is by sharing their valence electrons with other atoms, which will be covered in the next chapter. The second way is by transferring valence electrons from one atom to another. Atoms of metallic elements tend to lose all of their valence electrons, which leaves them with an octet from the next lowest principal energy level. Atoms of nonmetallic elements tend to gain electrons in order to fill their outermost principal energy level with an octet.

 Electron Dot Diagrams  A common way to keep track of valence electrons is with Lewis electron dot structures. In an electron dot structure, each atom is represented by its chemical symbol, and each valence electron is represented by a single dot. Note that only valence electrons are shown explicitly in these diagrams. For the main group elements, the number of valence electrons for a neutral atom can be determined by looking at which group the element belongs to. In the s block,  Group 1 elements have one valence electron, while Group 2 elements have two valence electrons. In the p block, the number of valence electrons is equal to the group number minus ten. Group 13 elements have three valence electrons, Group 14 elements have four, and so on. The noble gases in Group 18 have eight valence electrons, and the full outer s and p sublevels are what give these elements their special stability.

 Representative dot diagrams are shown:

 Cations and Anions  Metals will typically lose electrons to achieve stability, while non- metals typically gain electrons to achieve stability.  Two atoms or ions with the same number of electrons are referred to as isoelectronic.

 Cations  A positively charged ion is called a cation. Main group metals will typically form ions by losing enough electrons to become isoelectronic with the nearest noble gas. For example, lithium, whose configuration is [1s 2 2s 1 ], will typically lose one electron to become isoelectronic with helium, which has a configuration of [1s 2 ].

 Similarly, beryllium has 4 electrons (with the configuration [1s 2 2s 2 ]), so it prefers to lose two electrons, in order to become isoelectronic with helium (again, [1s 2 ]).

 Transition Metal Cations  The valence electrons for transition metals are variable, and electrons in the highest occupied d orbitals (which are not part of the valence shell) may or may not be lost in the formation of a transition metal cation. As a result, many transition metals commonly form more than one type of cation, depending on how many d electrons are lost. This picture depicts some of the typical electron arrangements for the transition elements.

 Anions  A negatively charged ion is called an anion. Nonmetals will typically form ions by gaining enough electrons to become isoelectronic with the nearest noble gas. For example, fluorine has 7 valence electrons and is one electron away from being isoelectronic with neon, which has a stable noble gas electron configuration. F

 Oxygen has 6 valence electrons in its ground state. Remember that ground state refers to the neutral atom in which the electrons occupy the lowest possible energy positions. Oxygen is two electrons away from being isoelectronic with the nearest noble gas. Oxygen will therefore form ions by gaining two electrons to become isoelectronic with neon, as shown below:

 Similarly, nitrogen has five valence electrons in it ground state, which is three electrons away from the nearest noble gas. Nitrogen can gain three electrons to become isoelectronic with neon:

 Here’s an example:  Write the ground state configuration for the nonmetal sulfur, and predict the ion it must form to be isoelectronic with the nearest noble gas.  Answer:  The ground state configuration for the nonmetal sulfur is written as: 1s 2 2s 2 2p 6 3s 2 3p 4. Sulfur has 16 electrons. The nearest noble gas to sulfur is argon, which has an electron configuration of: 1s 2 2s 2 2p 6 3s 2 3p 6. To be isoelectronic with argon, which has 18 electrons, sulfur must gain two electrons. Therefore sulfur will form a 2- ion, becoming S 2 −.

 Lesson Summary  Atoms or groups of atoms that carry an overall electrical charge are referred to as ions. Cations can be formed when a neutral species loses electrons, while anions are formed when a neutral species gains electrons.  Particularly for main group elements, the number of electrons a given element has in its outer (valence) shell largely determines the chemical behavior of that element.  The octet rule states that atoms will lose, gain, or share electrons to achieve the electron configuration of the nearest noble gas (8 valence electrons, except for helium, which has 2).  Electron dot diagrams are used to help us visualize the arrangement of valence electrons in a given chemical species.  When an element loses one or more electrons, a cation is formed. Metals typically become cations when they interact with other chemical species.  Some transition metals can produce ions with multiple different charges due to the optional participation of d electrons.  When an element gains one or more electrons, an anion is formed. Nonmetals typically become anions when they interact with other chemical species.