number but different atomic mass number are known as Isotopes . In other words (according to equation . ), it is evident that difference between the isotopes is due to the presence of different number of neutrons present in the nucleus. For example, considering of hydrogen atom again, .
% of hydrogen atoms contain only one proton. This isotope is called protium ( H) . Rest of the percentage of hydrogen atom contains two other isotopes, the one containing proton and neutron is called deuterium ( D , . %) and the other one possessing proton and neutrons is called tritium ( T ) .
The latter isotope is found in trace amounts on the earth. Other examples of commonly occuring isotopes are: carbon atoms containing , and neutrons besides protons ( C, C, C ); chlorine atoms containing and neutrons besides protons ( Cl, Cl ). Lastly an important point to mention regarding isotopes is that chemical properties of atoms are controlled by the number of electrons, which are determined by the number of protons in the nucleus. Number of neutrons present in the nucleus have very little effect on the chemical properties of an element.
Therefore, all the isotopes of a given element show same chemical behaviour. Problem . Calculate the number of protons, neutrons and electrons in Br . In this case, Br , Z = , A = , species is neutral Number of protons = number of electrons = Z = Number of neutrons = – = , (equation .
) Problem . The number of electrons, protons and neutrons in a species are equal to , and respectively. Assign the proper symbol to the species. The atomic number is equal to number of protons = .
The element is sulphur (S). Atomic mass number = number of protons + number of neutrons = + = Species is not neutral