Question

Give 2 observation when lead nitrate is strongly heated in a hard glass test tube

Answer 1

le and other isotopes are not and undergo radioactive decay. Most elements found in nature are stable isotopes, but e.g. uranium is a radioactive element with only unstable isotopes. The reason why we still find uranium in nature is the fact that the main isotope U238 (99.27%) decays very slowly (half-life of 4.468*10^9 years), whereas U235 (0.72%), which is used in atomic reactors or nuclear bombs decays about 10 times faster.

This brings me to the second part of your question. As with uranium, most elements are a mixture of different isotopes, and because isotopes differ in the number of neutrons they also differ in the atomic mass. Therefore the atomic mass of an element in the periodic system always represents the mean atomic mass of the isotope mixture. In the case of chlorine, it is a mixture of 75.77% Cl35 and 24.23% Cl37, which both are stable isotopes of chlorine.