Zinc (Zn) is a metal located in the group 12 of the d-block on the periodic table. The atomic number of zinc is 30 meaning that its nucleus contains 30 protons. Zinc most commonly forms positively charged cations with a charge of +2.
Zinc will rarely form ions with a +1 charge but it will never form ions with a negative charge. As zinc is a metal, it generally forms metallic compounds with other metals. Because it has an ion charge of +2, zinc ions are strong reducing agents and readily form ionic bonds.
Zinc ions form a number of compounds that have practical applications. Zinc oxide (ZnO) for example, is mass-produced as a semiconductor for use in electronics, while zinc sulfide (ZnS) is used as an optical material in infrared devices. Zinc is also an essential trace mineral required for signal transduction and transcription in eukaryotic cells. Compounds made out of zinc ions tend to adopt a symmetric crystalline structure and have a colorless luster like iron.
What Is An Ion?
Let’s take a step back from zinc and first review what, exactly, an ion is. Normally, atoms contain an equal number of protons and electrons. Since protons and electrons have equal and opposite charges (+1 and −1 respectively) atoms with equal numbers of protons and electrons will be electrically neutral as each positive charge is canceled out by a corresponding negative charge and vice versa.
Atoms can gain or lose electrons. When this happens, the atom no longer has equal amounts of opposite electric charges, so the atom will take on an overall charge and become an ion If the atom gains electrons, it will have an abundance of negative charges, and will take on an overall negative charge. If the atom loses electrons, it will have an abundance of positive charges and will take on an overall positive charge. Negatively charged ions are called anions and positively charged ions are called cations.
“Life is not found in atoms or molecules or genes as such, but in organization; not in symbiosis but in synthesis.” — Edwin Grant Conklin
Whether or not an atom will form an anion depends on its electronegativity. Electronegativity is a measure of how much an atom pulls on electrons. More electronegative elements, like oxygen or fluorine, pull very hard on electrons, so they are more likely to form anions by stealing electrons from other atoms.
Whether an atom will form a cation depends on the atom’s ionization energy. Ionization energy is a measure of how much energy input is required to remove an electron from the atom. In general, atoms that have low ionization energies tend to form cations. Atoms with low ionization energies can more easily have their electrons removed, and so can more easily form cations. As a general rule of thumb, elements on the right of the periodic table tend to make cations, and elements on the left of the periodic table tend to make anions. This tendency across the periodic table reflects trends in the electronegativities and ionization energies of the elements that are built into the organization of the periodic table.
Ions made from a single atom are called monatomic ions. Molecules that have an ionic character are called polyatomic ions. Polyatomic ions are not to be confused with polar molecules. Polyatomic ions have integer charges while polar molecules have partial electric charges.
Ions engage in a unique form of bonding called ionic bonding. During ionic bonding, positively charged cations will attract nearby anions and arrange themselves in a tightly packed crystalline lattice structure. As such, ionic compounds tend to be brittle and have high melting points.
Ions made from metals and semi-metals undergo a special type of ionic bonding called metallic bonding. Metallic bonds consist of a positively charged lattice of metal cations surrounded by a delocalized “sea” of electrons. In metallic bonds, valence electrons are loosened from their orbitals and are shared freely across the molecular structure. The delocalization of electrons in metallic bonds explains many of the unique properties of metals, like their ductility, conductive properties, luster, and malleability.
Zinc As An Ion
Zinc is an element in the d-block of elements. It has an electron configuration of [Ar]3d104s2. Zinc is generally not considered a transition metal as it has a full d-shell, though it is considered a transition metal in some texts.
The chemistry of zinc is almost entirely governed by its +2 ion charge. A Zn2+ cation has shed the two electrons in its 4s subshell, leaving only the filled 3d subshell. Zn2+ cations will dissolve in water to form octahedral complexes of the form [Zn(H2O)6]2+. Very rarely, two Zn1+ ions will bond to form diatomic zinc ion Zn22+, similar to the mercury diatomic ion Hg22+. This is the only known instance of a compound formed with Zn+1 ions. All other zinc compounds are formed with Zn2+ ions.
“The same few dozen organic molecules are used over and over again in biology for the widest variety of functions.” — Carl Sagan
Zinc ions tend to behave similarly to copper (Cu) and Nickel (Ni) ions. Zinc will rapidly oxidize in the presence of oxygen, forming zinc oxide (ZnO). Treating pure zinc with acid will form Zn2+ and hydrogen gas (H2). Compounds made from Zn2+ ions tend to arrange in a tetrahedral or hexagonal lattice structure.
Examples Of Compounds With Zinc Ions
Zinc Oxide
Zinc oxide (ZnO) is the most widely used zinc compound. Zinc oxide is an inorganic insoluble white powder at room temperature. While a naturally occurring substance, most zinc oxide is synthetically produced. Zinc oxide comes in two main structure, a hexagonal lattice and a cubic lattice. Hexagonal zinc oxide is the most stable form of zinc oxide so it is the most common. The hexagonal structure consists of a complex of tetrahedral subunits, each consisting of a central zinc ion surrounded by 4 oxygen ions (O2−).
The symmetrical crystalline structure of zinc oxide allows it to deform without breaking. This property makes zinc oxide a highly piezoelectric material. Piezoelectric materials are crystalline materials that can convert a mechanical force into an electrical signal. When a piezoelectric material is deformed, the ion structure is displaced so that the electric charges are no longer evenly distributed through the material. Since there is now a net difference in electric charge, an electric current is generated across the material. Zinc oxide, in particular, has a very high piezoelectric tensor, so it very readily converts mechanical stress into electric potential.
The geometric structure of zinc oxide gives it a high heat capacity and low thermal expansion. This makes zinc oxide useful for application in ceramics to make glazes and enamels. Coating treated with zinc oxide is less likely to crack under the high temperatures of the firing process. This same property also makes zinc oxide useful in the vulcanization of rubber and as a coating that absorbs UV light. It is also used as a pigment for white paints.
Zinc oxide has shown some hygiene and medicinal applications. Historically, zinc oxide is a primary constituent of calamine, a lotion used to combat itchy dry skin. Modern-day skin products like baby powder and anti-dandruff shampoos still utilize the anti-itch properties of zinc oxide. In dentistry, zinc oxide is mixed with eugenol and applied to teeth to serve as a restorative. Lastly, zinc oxide is a common ingredient in sunscreen products. Nanoparticles of zinc oxide in sunscreen absorb light effectively and protect the skin from UV radiation.
Zinc Sulfide
Zinc sulfide is an inorganic compound with a chemical formula (ZnS). Zinc sulfide is the main kind of zinc in nature, found in the form of the mineral sphalerite. Zinc sulfide exhibits phosphorescence so it is used in products like cathode rays, X-ray screens, and glow in the dark objects.
It is also used as a material in optical devices. Solids composed of zinc sulfide can transmit visible to infrared wavelengths, so it is a useful compound for devices that use infrared sensors. The properties that give its optical character also makes zinc sulfide a good semiconductor.
“Molecular biology is essentially the practice of biochemistry without the license.” — Erwin Chargaff
Zinc Chloride
Zinc chloride (ZnCl2) is an ionic salt formed from the union of one Zn2+ cation and 2 Cl− anions. One of the main uses of zinc chloride is in metal soldering. Zinc chloride reacts with various metal oxides, dissolving them and exposing the clear metal surface. Most smoke grenades produce zinc chloride smoke through the reaction of zinc oxide with hexachloroethane. Zinc chloride has also historically found use as an antiseptic, particularly in mouthwash.
Zinc Acetate
Zinc acetate (Zn(CH3CO2)2) is an organometallic salt Zinc acetate is formed by a Zn2+ ion forming a bond with 2 acetate ions that are derived from acetic acid. It is commonly used in throat lozenges for colds and as a basic dietary supplement.
To summarize, zinc is a d-block element with an atomic number of 30. Zinc most commonly forms positively charged ions with a charge of 2+. Very rarely, zinc will form ions with a +1 charge. Zinc ions are a constituent of many commonly used compounds. Zn2+ ions are an effective reducing agent and easily form ionic bonds. Most inorganic compounds of zinc are malleable, have a high thermal capacity, and are piezoelectric. Zinc compounds have a wide range of applications, including ceramics, optical technologies, paints, and medicine.