Why are elements with a low z have a higher electron density And why do these elements absorb neutrons better than lead?

Answer 1

You asked two widely disparate questions, but there is a connection. 1) See: http://www.chemistry.mcmaster.ca/esam/Chapter_5/section_3.html Light atomic weight elements have higher electron densities due to the nuclei's effective charge with regard to the orbital shells. This is a complicated geometric question of how the nucleons are arranged. The nucleus has a structure just as the electron orbital "shells" have one. 2) See: http://www.ncnr.nist.gov/resources/n-lengths/list.html. The neutron absorption of lead is not much compared to many other elements. And again this has to do with nucleus structure. Isotopic forms of the same element have wildly different absorption cross-sections. Neutron absorption is important to nuclear weapons design and nuclear energy. Here are some sites that do a great job at answering complicated questions such as this: http://csep10.phys.utk.edu/guidry/Math-9-94html/models.html http://forum.physorg.com