Chapter 1 - BASIC CONCEPTS - ISOTOPES

ISOTOPES AND THEIR RELATIVE ABUNDANCE

Atoms of same element having same atomic number but different atomic weights are called isotopes. This phenomenon is called isotropy. It was discovered by Soddy. Isotopes have same number of elections, protons and same electronic configuration. They differ in number of neutrons present in nucleus. The isotopes have same chemical properties. Consider example of hydrogen, it has three isotopes, i.e.

Thus we can define isotopes as atoms of same element which differ in number of neutrons in nucleus. Carbon has three isotopes, ₆C¹², ₆C¹³, ₆C¹⁴. Each one of them have 6 electrons and 6 protons but they have 6, 7, 8 neutrons respectively. The number of isotopes of some other elements are O = 3, Ni = 5, Ca = 6, Pd = 6, Cd = 9, Sn = 11.

RELATIVE ABUNDANCE OF ISOTOPES

The isotopes of all elements have their own natural abundance. This relative abundance is determined from mass spectrometry. The properties of element which are written in books are that of most abundant isotope of an element.

There are 280 stable naturally occurring isotopes in nature. There are 40 radioactive isotopes. Besides 300 unstable radioactive isotopes have been produced by artificial radioactivity.

The number of isotopes of an element is a complex property. Some general information is given over hare.

MONOISOTOPIC ELEMENTS: These elements only single isotope for example Gold, Iodine, Fluorine, Arsenic.

ODD ATOMIC NUMBER: Elements having odd atomic number never possess more than two stable isotopes.

EVEN ATOMIC NUMBER: Elements with even atomic no have large number of isotopes.

MASS NUMBER MULTIPLE OF FOUR: The isotopes with mass number which is multiple of four are quite abundant. For example ₈O¹⁶, Mg²⁴, Si³², Ca⁴⁰, Fe⁵⁶ are almost 50% of earth crust.

EVEN MASS NO AND ATOMIC NO: Out of 280 naturally occurring isotopes, 154 have even mass number and even atomic number.

MASS SPECTROMETRY: Determination of relative atomic masses of isotopes.

Mass spectrometer is an instrument which is used to measure the exact mass of different isotopes of an element.

BASIC PRINCIPLE: The substance whose mass is to be determined is first charged into vapours. These vapours are then ionized in an ionization chamber, with the beam of high energy electrons. The positive ions are separated on the basis of their m/e ratio in a magnetic analyzer. These ions are detected by ion collector. The results are represented in the form of a spectrum. m/e is plotted on x-axis and relative no. of ions along y-axis.