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Nuclear Stability
A major concept to remember: “Nature seeks the lowest energy
state”. In the lowest energy state, things are most stable…less
likely to change. The following information that talks about
stability is all based on the nucleus tending towards the lowest
energy state. Stable atoms have low energy states.
All nuclei are composed of two basic particles, neutrons and protons. Neutrons and protons are almost the same size but differ in their electrical charge. Neutrons have no electrical charge and contribute only mass to the nucleus. Each proton has a positive charge equal in strength to the negative charge carried by an electron.
The number of protons in a nucleus is the atomic number (Z) and establishes the chemical identity of the atom. Each atomic number corresponds to a different chemical element; there are now approximately 106 known chemical elements that correspond to nuclei containing from 1 to 106 protons.
Because of their very small size it is not convenient to express the mass of nuclei and atomic particles in the conventional unit of kilograms. A more appropriate unit is the atomic mass unit (amu), the reference for which is a carbon atom with a mass number of 12, which is assigned a mass of 12.000 amu. The relationship between the atomic mass unit and kilogram is
1 amu = 1.66 x 10-27 kg.
The difference in mass between a neutron and proton is quite small: approximately 0. %. The larger difference is between the mass of these two particles and the mass of an electron. More than 1,800 electrons are required to equal the mass of a proton or neutron.
The total number of particles (neutrons and protons) in a nucleus is the mass number (A, would have been nice if it were called nucleon number). Since neutrons and protons have
approximately the same mass, the total mass or weight of a nucleus is, within certain limits, proportional to the mass number. However, the nuclear mass is not precisely proportional to the mass number because neutrons and protons do not have the same mass, and some of the mass is converted into energy when the nucleus is formed (E=MC^2 ). The relationship between mass and energy is considered in more detail later.
There is a standard method for labeling different nuclear compositions: The mass number is designated by either a superscript preceding the chemical symbol, such as 14 C or 131 I, or by a number following the symbol, such as C-14, I-131, etc. The atomic number is added as a subscript preceding the chemical symbol. Adding the atomic number to the symbol is somewhat redundant since only one atomic number is associated with each chemical symbol or element.
With the exception of the most common isotope of hydrogen, all nuclei contain neutrons and protons. The lighter elements (with low atomic and mass numbers) contain almost equal numbers of neutrons and protons. As the size of the nucleus is increased, the ratio of neutrons to protons increases to a maximum of about 1.3 neutrons per proton for materials with very high atomic numbers. The number of neutrons in a specific nucleus can be obtained by subtracting the atomic number from the mass number. One chemical element may have nuclei containing different numbers of neutrons. This variation in neutron composition usually determines if a nucleus is radioactive.
Nuclear stability refers to the tendency of a nucleus of an atom to decay, which means to change into something else. If the isotope of an element (called a nuclide) is unstable (not stable), the nuclide has the tendency of emitting some kind of radiation, and is called radioactive. Radioactivity is associated with unstable nuclides. Carbon-12 is a carbon atom with a total atomic mass of 12. Since carbon can only have 6 protons, carbon- must have 6 neutrons (mass of 12 – 6 = 6). Carbon-12 is stable. Carbon-14 is unstable and has 8 neutrons (mass of 14 – 6 = 8).
Stable nucleus – non-radioactive Unstable nucleus – radioactive Also-- less stable means more radioactive and more stable means less radioactive.
What makes a nucleus stable?
The major underlying reason is: “Nature seeks the lowest energy state”. In the
lowest energy state, things are most stable…less likely to change. One way to view
this is that energy makes things happen. If an atom is at its lowest energy state, it
has no energy to spare to make a change occur. Think of yourself when you are
tired and ready for sleep. In this case you will most likely just stay put and not do
anything. The following information that talks about stability is all based on the
nucleus tending towards the lowest energy state. Stable atoms have low energy
Odd Odd 4 least stable Odd Even 50 more stable Even Odd 57 even more stable Even Even 168 most stable
Stability
- Nuclides containing odd numbers of both protons and neutrons are the least stable and this means more radioactive.
- Nuclides containing even numbers of both protons and neutrons are most stable and this means less radioactive.
- Nuclides contain odd numbers of protons and even numbers of neutrons are less stable than nuclides containing even numbers of protons and odd numbers of neutrons. In general, nuclear stability is greater for nuclides containing even numbers of protons and neutrons or both.
This is a diagram of what is sometimes called the “belt of stability” or “line of stability”. The black jagged line is the most stable region. The straight black line is where proton numbers equal neutron numbers. For the first 20 or so nuclides, the jagged line is very close to the straight line. As nuclides get larger they need more neurons than protons to remain stable, so the jagged line starts getting steeper than the straight line.
Nuclei above the belt of stability can lower their ratio and move to the belt of stability by radioactive decay, which converts a neutron to a proton. This increases the number of protons and decreases neutrons and gets the nuclide on the jagged line. The opposite also can happen when the nuclide has too many protons. In this sort of decay, protons are converted to neutrons.
Answers
(a) The 16 O 8 contains 8 protons and 8 neutrons (even-even) and the 17 O 8 contains 8 protons and 9 neutrons (even-odd). Therefore, 17 O 8 is radioactive. (b) The 35 Cl 17 has 17 protons and 18 neutrons (odd-even) and the 36 Cl 17 has 17 protons and 19 neutrons (odd-odd). Hence, 36 Cl 17 is radioactive. (c) The 20 Ne 10 contains 10 protons and 10 neutrons (even-even) and the 17 Ne 10 contains 10 protons and 7 neutrons (even-odd). Therefore, 17 Ne 10 is radioactive. (d) The 40 Ca 20 has even-even situation and 45 Ca 20 has even-odd situation. Thus, 45 Ca 20 is radioactive.
