Which is generally more stable, a small nucleus or a large nucleus? Explain.?

the larger a nucleus becomes the more unstable it is. The protons repel each other due to the positive electrostatic force of each one. The neutrons help to neutralise this repulsive field effect and bind the protons to gether. However the greater the repusive forces the less able are the neutrons to bind them. Hence the more unstable are these nucei. The only atoms that can survive withiout any neutrons are hydrogen (one proton) and helium (two [protons). Some isotopes of hydrogen have neutrons ( one in deuterium and two in tritium)

A a small nucleus is more stable because the strong and weak nuclear forces can a smaller nucleus together. Even hydrogen has an in unstable isotope.Tritium is an hydrogen isotope that has TWO neutrons and one proton, but one neutron radioactively decays to produce stable He3 in 12.32 years. http://en.wikipedia.org/wiki/Tritium "...Nuclei are bound together by the residual strong force (nuclear force). The residual strong force is minor residuum of the strong interaction which binds quarks together to form protons and neutrons. This force is much weaker between neutrons and protons because it is mostly neutralized within them, in the same way that electromagnetic forces between neutral atoms (such as van der Waals forces that act between two inert gas atoms) are much weaker than the electromagnetic forces that hold the parts of the atoms internally together (for example, the forces that hold the electrons in an inert gas atom bound to its nucleus). The nuclear force is highly attractive at the distance of typical nucleon separation, and this overwhelms the repulsion between protons which is due to the electromagnetic force, thus allowing nuclei to exist. However, because the residual strong force has a limited range because it decays quickly with distance (see Yukawa potential), only nuclei smaller than a certain size can be completely stable. The largest known completely stable (e.g., stable to alpha, beta, and gamma decay) nucleus is lead-208 which contains a total of 208 nucleons (126 neutrons and 82 protons). Nuclei larger than this maximal size of 208 particles are unstable and (as a trend) become increasingly short-lived with larger size, as the number of neutrons and protons which compose them increases beyond this number. However, bismuth-209 is also stable to beta decay and has the longest half-life to alpha decay of any known isotope, estimated at a billion times longer than the age of the universe. The residual strong force is effective over a very short range (usually only a few fermis; roughly one or two nucleon diameters) and causes an attraction between any pair of nucleons. For example, between protons and neutrons to form [NP] deuteron, and also between protons and protons, and neutrons and neutrons. Halo nuclei and strong force range limits The absolute limit of the range of the strong force is represented by halo nuclei such as lithium-11 or boron-14, in which dineutrons, or other collections of neutrons, orbit at distances of about ten fermis (roughly similar to the 8 fermi radius of the nucleus of uranium-238). These nuclei are not maximally dense. Halo nuclei form at the extreme edges of the chart of the nuclides—the neutron drip line and proton drip line—and are all unstable with short half-lives, measured in milliseconds; for example, lithium-11 has a half-life of less than 8.6 milliseconds. Halos in effect represent an excited state with nucleons in an outer quantum shell which has unfilled energy levels "below" it (both in terms of radius and energy). The halo may be made of either neutrons [NN, NNN] or protons [PP, PPP]. Nuclei which have a single neutron halo include 11Be and 19C. A two-neutron halo is exhibited by 6He, 11Li, 17B, 19B and 22C. Two-neutron halo nuclei break into three fragments, never two, and are called Borromean because of this behavior (referring to a system of three interlocked rings in which breaking any ring frees both of the others). 8He and 14Be both exhibit a four-neutron halo. Nuclei which have a proton halo include 8B and 26P. A two-proton halo is exhibited by 17Ne and 27S. Proton halos are expected to be more rare and unstable than the neutron examples, because of the repulsive electromagnetic forces of the excess proton(s). ,,," http://en.wikipedia.org/wiki/Atomic_nucleus http://en.wikipedia.org/wiki/Atom

What substances do you know are radioactive? Which end of the scale are they on? Toward the top or the bottom of the periodic table? As for why, that I don't know. I don't think you are expected to get into a detailed explanation of nuclear physics. However, you might be expected to know that iron is the most stable nucleus of all. As stars fuse lighter nuclei into heavier ones, getting energy out, the chain ends at iron.

small, because... i dunno, maybe - this is a guess the forces keeping the neutrons and protons together are very short range so when you have a large nucleus it's less stable... that's a bit vague. why not google it?

Actually, a 'medium' one. Iron is the most stable nucleus around. You can get energy by fusion from light nuclei up toward that weight, and energy by fission of heavy ones down toward it