It is important to realise that radioactive nuclei disintegrate:. This means that the process of radioactive decay can not be speeded up or slowed down by artificial means spontaneous decay. It also means that we cannot tell when a particular unstable nucleus will decay random decay. Key points. Stable and unstable nuclei The balance of protons and neutrons in a nucleus determines whether a nucleus will be stable or unstable.
There are only certain combinations of neutrons and protons, which forms stable nuclei. Neutrons stabilize the nucleus , because they attract each other and protons , which helps offset the electrical repulsion between protons.
As a result, as the number of protons increases, an increasing ratio of neutrons to protons is needed to form a stable nucleus. If there are too many neutrons also obey the Pauli exclusion principle or too few neutrons for a given number of protons, the resulting nucleus is not stable and it undergoes radioactive decay. Unstable isotopes decay through various radioactive decay pathways, most commonly alpha decay, beta decay, or electron capture.
Many other rare types of decay, such as spontaneous fission or neutron emission are known. The Pauli exclusion principle also influences the critical energy of fissile and fissionable nuclei. For example, actinides with odd neutron number are usually fissile fissionable with slow neutrons while actinides with even neutron number are usually not fissile but are fissionable with fast neutrons.
On the other hand, nuclei with an odd number of protons and neutrons are mostly unstable. A magic number is a number of nucleons in a nucleus , which corresponds to complete shells within the atomic nucleus. Atomic nuclei consisting of such a magic number of nucleons have a higher average binding energy per nucleon than one would expect based upon predictions such as the mass formula of von Weizsaecker also called the semi-empirical mass formula — SEMF and are hence more stable against nuclear decay.
Magic numbers are predicted by the nuclear shell model and are proved by observations that have shown that there are sudden discontinuities in the proton and neutron separation energies at specific values of Z and N. These correspond to the closing of shells or sub-shells. Nuclei with closed shells are more tightly bound than the next higher number.
It is found that nuclei with even numbers of protons and neutrons are more stable than those with odd numbers. There are further special propertis of nuclei, which have a magic number of nucleons:. Nuclear decay Radioactive decay occurs when an unstable atom loses energy by emitting ionizing radiation.
Radioactive decay is a random process at the level of single atoms, in that, according to quantum theory, it is impossible to predict when a particular atom will decay. During radioactive decay an unstable nucleus spontaneosly and randomly decomposes to form a different nucleus or a different energy state — gamma decay , giving off radiation in the form of atomic partices or high energy rays. This decay occurs at a constant, predictable rate that is referred to as half-life. A stable nucleus will not undergo this kind of decay and is thus non-radioactive.
There are many modes of radioactive decay:. The rate of nuclear decay is also measured in terms of half-lives. The half-life is the amount of time it takes for a given isotope to lose half of its radioactivity. Half lives range from millionths of a second for highly radioactive fission products to billions of years for long-lived materials such as naturally occurring uranium. Notice that short half lives go with large decay constants. Radioactive material with a short half life is much more radioactive at the time of production but will obviously lose its radioactivity rapidly.
No matter how long or short the half life is, after seven half lives have passed, there is less than 1 percent of the initial activity remaining.
Atomic Nucleus. If the isotope is located above the band of stability what type of radioactivity is it? Introduction A isotope is an element that has same atomic number but different atomic mass compared to the periodic table.
Isotope Stability The graph of stable elements is commonly referred to as the Band or Belt of Stability. Magic Numbers The Octet Rule was formulated from the observation that atoms with eight valence electrons were especially stable and common. The magic numbers are: proton: 2, 8, 20, 28, 50, 82, neutron: 2, 8, 20, 28, 50, 82, , Also, there is the concept that isotopes consisting a combination of even-even, even-odd, odd-even, and odd-odd are all stable.
Unstable or Stable Here is a simple chart that can help you decide is an element is likely stable. Calculate the total number of nucleons protons and neutrons in the nuclide. If the number of nucleons is even , there is a good chance it is stable. Are there a magic number of protons or neutrons? Do not delete this text first. Chemistry Fourth Edition. Petrucci, Ralph H. Harwood, F.
Geoffrey Herring, Jeffry D Madura.
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