Ions
Ions are charged particles that can be either atoms or groups of atoms with an unequal number of protons and electrons, resulting in a net electrical charge. They come in two types: cations, which carry a positive charge due to a deficit of electrons, and anions, which carry a negative charge due to an excess of electrons. Ions play a critical role in various natural and biochemical processes, including the formation of ionic compounds, which occur when oppositely charged ions attract each other and create electrovalent bonds. These compounds often dissolve in polar solvents, such as water, with their solubility influenced by factors like charge density and crystal lattice structure. The process of ion formation typically involves the loss or gain of electrons in an atom's outermost valence shell, allowing atoms to achieve a stable electronic configuration. Additionally, certain elements can form multiple types of ions, particularly transition metals, by losing differing numbers of electrons. Understanding ions is fundamental to chemistry and biology, as their interactions underpin much of the molecular behavior observed in living systems.
Ions
FIELDS OF STUDY: Inorganic Chemistry; Organic Chemistry; Physical Chemistry
ABSTRACT
The basic structure of ions is defined, and the modern theory of atomic structure is elaborated. Ionic materials are not only ubiquitous in nature but also essential to the biochemical processes that support life.
The Nature of Ions
Ions are simply atoms or chemically bonded groups of atoms that have a net electrical charge. Ions carrying a positive charge do not have the full complement of electrons necessary to balance the positive charge of their nuclei, while ions carrying a negative charge have more electrons than they have protons in their nuclei. Positive and negative ions are termed cations and anions, respectively. Oppositely charged ions typically interact to form crystalline compounds through electrovalent bonds, or ionic bonds. Many such compounds are highly soluble in water and other highly polar solvents; dissociation of these compounds occurs when they are dissolved by the solvent molecules, causing them to split into their component ions. The compound can typically be re-formed by evaporating the solvent.
The strength of the electrovalent bonds in an ionic compound, and thus its ability to dissolve in polar solvents, is highly dependent on the charge density of the ions. "Charge density" refers to the net electrical charge on the ion relative to its physical size. Generally, the higher the charge density of the two opposing ions, the less soluble the material. Other factors, such as the structure of the material’s crystal lattice, also affect its solubility. The ionic compound sodium chloride (NaCl), for example, is highly soluble in water, while sodium fluoride (NaF) is much less so due to the smaller size and higher charge density of the fluoride ion compared to the chloride ion. Magnesium chloride (MgCl2) will dissolve 54.6 grams in 100 milliliters of cold water, but magnesium fluoride (MgF2) is barely soluble and dissolves only slightly fewer than 8 milligrams in the same amount of cold water. The more soluble an ionic compound is, the more easily the solvent molecules can surround each ion in solution. "Solvation energy" is the term used to describe the difference in energy between dissolved ions and the undissociated ions in the compound.
Atomic Structure and the Formation of Ions
All ions are formed by the loss or addition of electrons in the outermost valence shell of an atom. The modern theory of atomic structure describes atoms as being composed of a small, dense nucleus containing a fixed number of protons and a variable number of neutrons. The number of protons in the nucleus defines the identity of the atom as an element. The neutrons that are present act as a sort of nuclear glue that counteracts the force of electrostatic repulsion between the positively charged protons. Together, the protons and neutrons make up almost all of the mass of an atom.
Surrounding the nucleus is a large, diffuse cloud of negatively charged electrons. This cloud is approximately one hundred thousand times larger in diameter than the atomic nucleus and contains the same number of electrons as there are protons in the nucleus, rendering the atom electrically neutral. According to quantum mechanical theory, these electrons may possess only very specific energies within the electron cloud. The limits on these energy levels are determined by the structure of the nucleus, and each allowed energy level defines a shell of electrons about the nucleus and the number of electrons each shell can contain. Within each shell are one or more subshells, designated either s, p, d, or f and containing either one, three, five, or seven mathematically defined regions of space called orbitals, each of which is allowed to contain no more than two electrons at any time. Thus, the s subshell can contain up to two electrons, the p subshell up to six, the d subshell up to ten, and the f subshell up to fourteen. The outermost shell is called the valence shell, and in most atoms, this shell does not contain the full complement of electrons that is allowed.
The electrons in the valence shell are the ones that are involved in interactions between atoms, such as chemical bond formation, and in the formation of ions. Atoms and ions are in a state of maximum stability when their outermost electron shell contains its full complement of electrons allowed, and attaining this state is the driving force for atoms with unfilled valence shells to either donate electrons to or accept electrons from other atoms. This ionization process has an energy cost, however, and the amount of energy required to remove an electron from its stable location in an atomic or molecular orbital is called ionization energy. Each electron in a particular orbital has its own associated ionization energy, which increases dramatically as the shells get closer to the nucleus.
Occurrence of Ions
Atoms are described as "electronegative" if they accept electrons to form anions and as "electropositive" if they give up their valence electrons to form cations. The electronegativity of atoms increases across the periodic table, with hydrogen and the alkali metals being the least electronegative and the halogens being the most electronegative. At the extreme right-hand side of the table are the noble gases, also called inert gases because they have very stable, completely filled valence shells and are thus very resistant to ionization.
Ions are readily formed as the result of reduction-oxidation (redox) reactions, in which one reactant gives up electrons and another reactant gains those electrons. For example, the reaction between chlorine gas (Cl2) and sodium metal (Na) produces sodium chloride (NaCl), composed of sodium ions (Na+) and chloride ions (Cl−). Each sodium atom has ejected its lone valence shell electron, leaving a filled outermost electron shell. Each chlorine atom has accepted one electron into its valence shell, adding to the seven that are normally there to make a full complement of eight electrons. In the reaction, a great deal of heat and light energy are given off, indicating the difference in stability between the reactants and the product.
Multiple Possibilities
Many elements, particularly the transition metals, are able to form more than one ion. As the atoms of the elements increase in size, overlapping electron-shell energies result in different orbitals being of the same or very similar energy. Orbital interactions between different atoms also become easier, allowing the formation of different types of bonds, such as coordination bonds. These factors provide atoms with a number of ways to achieve filled outermost electron shells, usually by releasing different numbers of valence shell electrons. Copper, for example, can lose either one or two electrons to form either the Cu+ or the Cu2+ ion, while iron atoms typically eject either two or three electrons, forming either the Fe2+ or the Fe3+ ion.
PRINCIPAL TERMS
- anion: any chemical species bearing a net negative electrical charge, which causes it to be drawn toward the positive pole, or anode, of an electrochemical cell.
- cation: any chemical species bearing a net positive electrical charge, which causes it to be drawn toward the negative pole, or cathode, of an electrochemical cell.
- dissociation: the separation of a compound into simpler components.
- electrovalent bond: an alternate term for an ionic bond, which is a type of chemical bond formed by mutual attraction between two ions of opposite charges.
- ionization: the process by which an atom or molecule loses or gains one or more electrons to acquire a net positive or negative electrical charge.
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