Molecular Weights – Different Atomic Species

In 1803, John Dalton put forward the theory that every matter was composed of tiny indivisible particles called atoms. It was later discovered that an atom is made up of three subatomic particles – electrons, protons, and neutrons. The atomic number or mass number of an atom is based on the number of electrons, protons, and neutrons present in them. Based on this, there are different atomic species (isotopes, isobars, isotones, isoelectronic).

Atomic Number and Mass Number

The number of protons in an atom represents its atomic number. It is denoted by the letter Z. The number of protons and neutrons in an atom together represents its mass number. It is represented by the letter A.

Depending on these numbers, we get different atomic species. They are as follows:

1.     Isotopes

2.     Isobars

3.     Isotone

4.     Isoelectronic species

Isotopes 

Isotopes are two or more species of atoms of a chemical element with the same atomic number. In other words, they have the same number of protons and the same position in the periodic table. The chemical properties of isotopes of the same element are fairly similar, but their nuclear properties differ. More than 90 naturally occurring elements in the periodic table form approximately 250 isotopes. There are many isotope forms for each and every element on the periodic table. We can also find stable and radioactive isotopes. Isotopes possess different physical properties since physical properties are the result of their mass. These differences can be exploited to separate isotopes using various techniques like fractional distillation or diffusion. The term Isotope was first coined by English radiochemist Frederick Soddy. They were separated for the first time by British chemist Francis William Aston using a mass spectrometer.

Examples for isotopes

• Hydrogen

Hydrogen has three isotopes – protium, deuterium, and tritium. They are represented by 1H1,2H1,3H1 respectively. Among these, protium contains zero neutrons, deuterium has one neutron, and tritium has two neutrons. Tritium is radioactive in nature.

• Carbon

12C6,14C6are isotopes of carbon. 12C6has 6 neutrons and 14C6has 8 neutrons.14C6 is radioactive and 12C6 is stable.

Isobars

Isotopes are also called nuclides. They are chemical species having the same mass number. That means isobars contain the same number of nucleons. A nucleon is the addition of the number of protons and neutrons. Unlike isotopes, they are atoms of different chemical elements. 

The term isobar originated from two Greek words ‘isis’ and ‘baros’ meaning equal and weight respectively. This term was suggested by Alfred Walter Stewart in 1918. Although the number of protons and neutrons will vary, the number of nucleons, or the sum of protons and neutrons in isobars, will remain constant. Due to the difference in atomic numbers, isobars always have different atomic structures. As a result, isobar exhibits distinct chemical characteristics, but their physical properties are almost the same. Isobars also possess equal atomic weights.

Examples of isobars

•  40Ar18 ,40K19 ,40Ca20

•  76Ce32 ,76Se34

•  24Na11 ,24Mg12

•   58Fe26 ,58Ni27

Isotones

Isotones are species with the same number of neutrons but different number of protons. This is the reason why they have different chemical elements. Despite its resemblance to the Greek word for “same stretching,” German physicist K. Guggenheimer coined the term by converting the “p” in “isotope” from “p” for “proton” to “n” for “neutron.”

Examples for isotones

• 13C6 , 12B5

Both boron-12 and carbon-13 contain 7 neutrons. So they are isotones.

• 36S16,37Cl17,38Ar18   

These three atoms have 20 neutrons in them. Hence, they can be called isotones

Isoelectronic Species

Atoms or ions with the same number of electrons are known as isoelectronic species. The number of electrons in isoelectronic species would be the same, but the elements would be different. It is formed when two or more molecules have the same structure (atom positions and connections, and electronic configurations), but differ in which specific elements are present at certain points in the structure.

Examples of isoelectronic species

• Na+ , F–

Both fluorine ions and sodium ions possess 10 electrons. Since the number of electrons in them is the same, they are called isoelectronic species.

•  N-3,Al3+,Mg+2     

All these species possess 10 electrons each. Hence, they are examples of isoelectronic species.

Conclusion

Different Atomic Species are classified and categorised in different ways. One of the categorisations is based on the number of protons, electrons, and neutrons they contain.  Isotopes, isobars, isotones, and isoelectronic species fall under this category. Isotopes are chemical species with the same number of protons or the same atomic number. They may differ in the number of electrons and neutrons. Isobars are chemical species with the same mass number. In other words, the same number of nucleons. Isotones are species possessing the same number of neutrons. Isotones differ in the number of electrons and protons. Isoelectronic species are chemical species having the same number of electrons.