How Do You Know If An Isotope Is Heavier Or Lighter

What is the lightest element on Earth with no stable isotope? How light is it?

That title belongs to element 43, Technetium. It has no stable isotopes and isn't found naturally on Earth except for trace amounts of 99Tc found in Uranium ores produced from spontaneous fission of Uranium. All Technetium used in medicine and industry is synthetic. The next lightest is either Bismuth or Polonium, having atomic numbers 40 and 41 respectively higher than Technetium. Bismuth is technically radioactive, but it's Half-Life is so long that for all intents and purposes, it's stable. Polonium is generally accepted to be the lightest heavy metal to be radioactive.

Are isotopes heavier than the normal element?

An isotope is a set of nuclei with the same number of protons but different number of neutrons. Since some isotopes will have more neutrons and some isotopes will have fewer neutrons, the mass can be higher of lower than the “normal” isotope. Note that isotopes are not necessarily radioactive. It is just a atom with a different number of neutrons, but the same number of protons.Naturally occurring radioactive isotopes (which I suspect you mean) tend to have more neutrons. This is because they often come from processes that add neutrons, or from the decay of uranium and thorium, which are neutron rich. But some naturally occurring radioactive isotopes have fewer neutrons than the stable isotope.

WHY is it that heavier stable isotopes of elements form slightly stronger chemical bonds than lighter isotopes?

It has to do with the concept of zero-point energy, which comes out of quantum theory. Here’s the basic idea:Molecules are not static, they are constantly in internal motion; their bonds and bond angle oscillate around their equilibrium values. For instance, the H-O bonds in a water molecule oscillate around 97 pm and the H-O-H angle oscillates around 105˚.The blue curve in the diagram below shows how the energy of a diatomic molecule, e.g., H—F, varies as a function of internuclear distance. According to classical physics the molecule would “sit” at the bottom of the curve, with a fixed energy and equilibrium bond length. Quantum theory, however, says that rather than having any arbitrary energy (i.e., being anywhere on the blue line) it can exist only in specific energy states (the horizontal blue lines and in those states its bond will oscillate back and forth between a minimum bond length (the left end of the line) and a maximum one (the right side of the line). As the energy level increases, the stretching will get more and more extreme until at some point the bond will break!For a diatomic molecule the system can be approximated pretty well at low energies by the physics of a harmonic oscillator (the green curves). The lowest energy state (called the zero point energy) depends E=hω/2 and ω, the vibration frequency, is given by ω=(h/2π)√(k/µ) where k measures the strength of the bond and µ is the reduced mass:µ=(m1×m2)/(m1+m2), where m1 and m2 are the masses of the two atoms.To answer your question, we don’t have to do a detailed calculation; all we really need to look at are the reduced masses for two related molecules: H—F and D—F. Using atomic mass units for simplicity, here are their reduced masses:H—F µ =(1×19)/(1+19) = 0.95 and 1/µ = 1.05D—F µ =(2×19)/(2+19) = 1.81 and 1/µ = 0.55So, the zero-point energy of the D—F molecule will be lower than that of the H—F molecule, which means its dissociation energy (the distance between its zero-point energy and the dashed line at the top of the blue curve) will be greater than that of H—F, or the D—F bond will be stronger and shorter than the bond energy than the H—F bond.

Why does 82-lead have many stable isotopes and nothing heavier has any?

There are several “horizontal black bars” in this image, of elements with several stable isotopes:… and we are limited to making heavier elements with particle accelerators, which leaves us unable to add enough neutrons for stability. So “It isn’t that Lead is the heaviest stable isotope, it is the heaviest stable isotope we know of, perhaps because we are still largely stuck on the surface of this planet, and trapped in this solar system.” Note that Bismuth-209 is now known to be unstable.

How are isotopes of the same element alike and different?

Other answers have ably pointed out the rather small differences among the chemical properties of different isotopes of the same element. The nuclear properties can be very much different.One example is the difference betweenprotium = hydrogen with no neutron, hence atomic mass near 1anddeuterium = hydrogen with one neutron, hence atomic mass near 2.Since the protium nucleus has a mass near the mass of a neutron, a neutron that collides with a stationary protium nucleus may end up nearly stationary, having transferred almost all its momentum to the protium nucleus. In contrast, in collision with a deuterium nucleus, a neutron can lose at most around 1/9 of its kinetic energy.Another difference is that a protium nucleus has a fairly high probability of capturing an incident neutron, while a deuterium nucleus has a much lower probability of capturing an incident neutron with the same energy.Since most nuclear reactors depend on slowing high-energy neutrons without losing them, the difference between protium and deuterium is important.Another example is the difference betweenUranium 235 and uranium 238These are the two isotopes of uranium that are readily available. Uranium 235 readily absorbs a neutron that has been slowed down as described above, and usually responds by disintegrating into two smaller nuclei plus a few neutrons that are highly useful for continuing the process. In contrast, uranium 238 tends much less to absorb such slowed-down neutrons, and when it does, it not so cooperatively disintegrates.In generalwhenever the nuclear properties of an isotope are of practical importance, the other isotopes of that same element have no related importance.One exception:The Castle Bravo nuclear-explosion test was based on a fusion reaction using lithium 6. Since pure lithium 6, separated from the more common lithium 7, was not available, a mixture of the two was used, on the assumption that the lithium 7 would have no effect. In actuality, the lithium 7 participated in the explosion in much the same manner as the lithium 6. Consequently, the explosion was much more energetic than was planned, with disastrous results.