Carbon-14 and Uranium-238 have something we call a half live, which is basically a known time period for it to change half of its C-14 or U-238 radioactively decay. Since we know how long that is 5730 years for Carbon-14 for half a sample to deteriorate, than we can figure out how old it is.
Carbon dating C-14 is done to find the age of relatively younger fossils and rocks.Half life of C-14 is only 5700 years and can only be used to date fossils of about 50,000 years old. Where as Uranium-238 has half life of about 4.5 billion years. If 50% of U-238 is left in sample it means that the sample is 4.5 billion year old. It is to be noted that no fossils contain U-238 in it. It is contained only in the igneous and volcanic rocks. So no fossils can be dated using U-238.
Nonmetals which are located in the second row form pi bonds
more easily than the elements situated in the third row and below. Actually there
are no compounds or molecules known that forms covalent bonds to the noble gas
Ne and Ar. Hence the other second row element which is Carbon, is the element that
forms
pi bonds most readily.
