Showing posts with label isomers. Show all posts
Showing posts with label isomers. Show all posts


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It is that part of atmosphere which is ionized by solar radiation.  The region exists at 85km to 600km altitude, and contains plasma.  It influences radiation propagation to distant places on earth.


Process of exposing a substance to radiation beam of selective dose/energy/type is called as irradiation.

Irrational Number

Real number which cannot be written as a simple fraction.

Irreversible Change

The change which cannot be reversed or the change which on reversing the process, magnitudes of ∆Q, ∆U & ∆W are not same as in direct process is called irreversible change.

Irreversible Process

An irreversible process does not involve restriction of adjusting the external conditions by infitesimal quantities. The system need not remain in equilibrium during the process. Irreversible processes are real processes and can be carried out experimentally. All spontaneous transformations which occur in nature are real processes and hence also irreversible.

Isobaric Process

A process taking place at constant pressure.


Nuclides of different elements having same number of nucleons are called isobars.

Isochoric Process

A process taking place at constant volume.

Isolated System

The type of system which has no interaction with its surroundings. Neither energy nor matter can be transferred to or from it.


Atoms having same atomic number and mass number but differ from one another in their nuclear energy states and exhibit differences in internal structure are called Isomers. These nuclei are distinguished by their different life times.



Neutrons and Protons are similar in all respects except charge. On this basis, Heisenberg suggested that these particles are just different manifestations of same inherent particle, the nucleon. To describe their quantum state, quantum number used was termed as isotopic spin quantum number by wigner. It is generally named as isospin or T-spin. The isospin is a vector in a three dimensional space called isospin space which has no relation to physical space.    

Isothermal Process

It is a physical change in which temperature remains constant and there is exchange of heat between system and surroundings and Boyle’s law holds good for this process.


Nuclides of elements having same number of Neutrons but different number of Protons are called as isotones.


The atoms of an element, which have same atomic number but different mass numbers are called as isotopes. Number of protons inside the nucleus and number of electrons outside the nucleus of such atoms is same but number of neutrons inside the nucleus is different.

Isotopic Abundance

Refers to the existence of fraction of given isotope for a given element in its normal state.

Isotropic Medium

The medium in which measured properties are independent of direction of measurement.


Substance in which a physical property have identical values in all crystallographic directions. 

What are Isomers?

In earlier days, if two half lives were observed in a given sample, it was assumed that two different isotopes were present, each decaying with particular half life.

One interesting example is isotope of Protactinium, Z=91, A=234 which is formed in beta decay of its parent, Thorium Z = 90, A=234[UX1]. The isotope of Protactinium was found to decay by the emission of beta particles of two distinct half lives, one of 1.18 min, the other of 6.6hr. It was assumed that these two half lives were due to two different isotopes and these were given separate names UX2 and UZ respectively.

In 1921, "Hahn" showed that these  two substances form a pair of Nuclear Isomers; i.e they are different energy states of same nucleus.

"Feather" and "Bretscher" later showed that these nuclear isomers are genetically realted; i.e. one type of nucleus is formed from other.

The nucleus called UX2 is an isomeric state of Pa-234 at an energy of 0.394MeV above ground state called UZ.

The nucleus may decay by beta particle emission directly from isomeric state of higher energy with a half life of 1.18 min or it may first emit a gamma ray photon of 0.394MeV, going to ground state of Pa, and then undergo beta decay to U-234 with a half life of 6.66 hours.

An isomeric state differs from ordinary excited state of a nucleus in that it lasts for measurable time.


" Isomers are atoms which have same atomic number and mass number but differ from one another in their nuclear energy states and exhibit differences in their internal structure. These nuclei are distinguished by their different life times".