The key difference between isotonic and isoelectronic species is that isotonic species have similar numbers of neutrons, whereas isoelectronic species have similar numbers of electrons.
The terms isotonic and isoelectronic refer to chemical species having something in common, e.g., same number of electrons, same number of neutrons, etc.
CONTENTS
1. Overview and Key Difference
2. What are Isotonic Species
3. What are Isoelectronic Species
4. Isotonic vs Isoelectronic Species in Tabular Form
5. Summary – Isotonic vs Isoelectronic Species
What are Isotonic Species?
Isotonic species are chemical species having identical numbers of neutrons. These are also known as isotones. Isotones are two or more nuclides with the same number of neutrons, but they have different numbers of protons. The neutron number is denoted by N, and the proton number is denoted by Z.
A common example would be boron -12 and carbon – 13 nuclei. Bothe these nuclides contain 7 neutrons in each atom. Therefore, we can name them as isotones. A similar group of isotonic species includes atoms with 20 neutrons per atom. This group includes S-36, Cl-37, Ar-38, K-39, and Ca-40. All these atoms have 20 neutrons but different numbers of protons. We can get the number of protons by deducting 20 from the mass number. E.g., for the sulfur atom, the number of protons per atom = 36 – 20 = 16.
The term isotonic comes from the Greek meaning “same stretching.” It was introduced by the German physicist K.Guggenheimer. There can be many atoms with the same number of neutrons when considering the isotopes of chemical elements. Typically, the largest number of observationally stable nuclides exits for two isotonic species 50 and 82.
What are Isoelectronic Species?
Isoelectronic species are chemical species having identical numbers of electrons. In other words, isoelectronic species have the same number of electrons or the same electronic structure. This phenomenon is known as isoelectronicity.
For example, carbon monoxide, NO+, and N2 are isoelectronic chemical species because these structures have the same number of electrons per compound. In contrast, CH3COOH and CH3N=NCH3 are not isoelectronic because they have a different number of electrons.
The importance of identifying the isoelectronic chemical species is the ability to study significantly related species as pairs or series. Moreover, we can expect this to be useful in the consistency and predictability of the properties of these chemical species. Therefore, it provides us clues about the possible properties and reactions.
For instance, N atom and O+ ion are isoelectronic with each other. This is because both these species have five valence electrons and the [He]2s22p3. Another common example is the series of cations with K+, Ca2+, and Sc3+. Similarly, Cl-, S2-, and P3- is an anion series with a similar number of electrons.
In diatomic molecules, we can use molecular orbital diagrams to illustrate the isoelectronicity in a diatomic molecule. This shows atomic orbitals that mix in isoelectronic species, indicating the identical orbital combination as well as the bonding.
There are some polyatomic compounds that can be isoelectronic with each other. A commonly known example would be amino acid series with serine, cysteine, and selenocysteine. These amino acids are different from each other according to the specific chalcogen that is present in a location in the side chain.
What is the Difference Between Isotonic and Isoelectronic Species?
Isotonic and isoelectronic chemical species are important in studying the chemical properties of related compounds. The key difference between isotonic and isoelectronic species is that in isotonic species, the number of neutrons is the same, whereas, in isoelectronic species, the number of electrons is the same.
The below infographic presents the differences between isotonic and isoelectronic species in tabular form for side-by-side comparison.
Summary – Isotonic vs Isoelectronic Species
Isotonic species are chemical species having identical numbers of neutrons. Isoelectronic species are chemical species having identical numbers of electrons. Therefore, the key difference between isotonic and isoelectronic species is that isotonic species have similar numbers of neutrons, whereas isoelectronic species have similar numbers of electrons.
Reference:
1. “Which of the Following Species Is Isotonic with 37 RB 86 ?” Tardigrade, 30 Oct. 2019.
Image Courtesy:
1. “Isotopes and half-life” By BenRG – Own work (Public Domain) via Commons Wikimedia
2. “Carbon monoxide simple” By Yikrazuul (talk) – Own work (Public Domain) via Commons Wikimedia
3. “Nitrosonium-2D-dimensions” (Public Domain) via Commons Wikimedia
