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Nuclear Stability- Introduction, Stability Belt, Magic Number, Practice Problems and FAQs
Do you know how to determine whether you have an appropriate healthy weight or not? You must have learned that for a body to perform ideally (i.e. to perform all the activities perfectly) your weight should be equal to your height in cm minus100. For example, if your height is 172 cm then your appropriate weight should be approximately 72 kg. If it is more than the desired weight it leads to overweight and when it is less it leads to underweight. As both the cases are not good for the long run, doctors advise maintaining appropriate body weight.
But what is the overweight in chemistry? Like human beings heavy nuclei also are unstable and may break into smaller nuclei, a phenomenon called radioactivity. As we are able to determine if the body is overweight or underweight which affects the body’s functions, similarly it is important to determine the stability of an atom.
Let us learn more about nuclear stability in this article.
Table of content
- Introduction of Nuclear Stability
- Nuclear Stability and Stability Belt
- Practice Problems
- Frequently Asked Questions
Introduction of Nuclear Stability
Nuclei of atoms are made up of a number of protons and neutrons, except the hydrogen, which has only proton and not neutron in nuclei. Of course the isotopes of hydrogen, namely deuterium and tritium contain both neutrons and protons.In elements of low mass numbersthe number of neutrons and protons are same. But the ratio of neutron to proton increases with the increase in mass number. The maximum ratio is observed to be 1.3.
The number of protons is given by atomic number. The number of neutrons can be calculated by the difference between the mass number and atomic number. A single chemical element can have nuclei with varying numbers of neutrons. This difference in the composition of neutrons usually determines whether a nucleus is radioactive in nature or not.
Nuclear stability refers to an atom nucleus's natural tendency to decay or change into something else. If an element's isotope (called a nuclide) is unstable (not stable), the nuclide has a tendency to emit some kind of radiation and is referred to as an unstable radioactive nuclide. Radioactivity is linked to radioactive nuclides. For example, carbon 12(
) is a stable atom and it contains 6 protons and 6 neutrons respectively. Whereas carbon-14 is unstable, radioactive in nature and contains 6 protons and 8 neutrons respectively.
Nuclear Stability and Stability Belt
The neutron-proton ratio (n/p) helps in the determination of nuclear stability. For atoms with low atomic numbers (less than about 20 protons), this ratio is close to one.
As the atomic number increases fast, the n/p ratio steadily rises, from the element greater than the atomic number calcium to approximately element 84 which is polonium. Every element with a number greater than an atomic number 84 is unstable.
Neutrons help to keep the nucleus stable. If the attractive force between nucleons is less than the electrostatic repulsion, the nucleus becomes unstable and decay occurs.
The strong nuclear force acts as a magnet for nucleons. Higher the number of protons packed together more neutrons are required to hold it together.
Element till atomic number 20 have the nearly same number of proton and neutron but when the atomic number increases there is an increase in the number of neutrons to proton and therefore the number of neutrons needed to form stable nuclei increases more as compared with the number of protons.
The magic numbers for atoms are 2, 10, 18, 36, 54, and 86 which corresponds to the total number of electrons filled in the shell of an atom. It is generally seen that the element following the magic number is generally more stable and therefore doesn't participate in a nuclear reaction.
One of the most basic methods of forecasting nuclear stability is determined by whether a nucleus has an odd or even number of protons and neutrons.
| Neutrons | Protons | Nuclear stability |
| Odd | odd | Least stable |
| Even | Odd | Comparatively more stable as compared when both numbers of neutron and proton are odd. |
| Odd | Even | Comparatively more stable as compared when numbers of the neutron are even and protons are odd. |
| Even | Even | Most stable |
It can be concluded that Nuclides with an odd number of protons and neutrons are the least stable, and are generally radioactive in nature. Nuclides with an even number of protons and neutrons, or both, have higher nuclear stability.
The nuclear stability belt is also known as the line of stability and the elements lying along the line are stable and generally non-radioactive in nature. The straight black line represents the point at which proton and neutron numbers are equal. The jagged line is very close to the straight line for the elements till the atomic number 20. As the size of nuclides increases, they require more neurons than protons to function.
As long as the straight line remains stable, the jagged line begins to steepen. By lowering their 
ratio, nuclei above the belt of stability can move to the belt of stability. A neutron is converted to a proton by radioactive decay. This can happen if the nuclide contains too many protons. Protons are destroyed in this type of decay.
Case 1: When the nuclei are above the shaded area (i.e. there are too many neutrons), they emit 
radiation to achieve stability.
For example,
Case 2: When nuclei fall below the zone of stability, 
-emission, K-electron capture, or positron emission occurs.
Note: When the nuclei of naturally occurring elements are present, -emission generally occurs. In the case of an artificially prepared radioactive element, K-electron capture or positron emission occurs.
Practice Problems
Q1. In which of the following pairs of atoms will have the maximum stability on the basis of odd and even rule.
and 
and 
and - andCa-40
Answer: (D)
Solution: One of the most basic methods of forecasting nuclear stability is determined by whether a nucleus has an odd or even number of protons and neutrons.
| Neutrons | Protons | Nuclear stability |
| Odd | odd | Least stable |
| Even | Odd | Comparatively more stable as compared when both numbers of neutron and proton are odd. |
| Odd | Even | Comparatively more stable as compared when numbers of the neutron are even and protons are odd. |
| Even | Even | Most stable |
It can be concluded that Nuclides with an odd number of protons and neutrons are the least stable, and are generally radioactive in nature. Nuclides with an even number of protons and neutrons, or both, have higher nuclear stability.
| Element | Neutron | Proton |
|
7 | 10 |
|
|
10 | 10 |
|
9 | 8 |
|
|
8 | 8 |
|
25 | 20 |
| Ca-40 | 20 | 20 |
In the case of and Ca-40 the number of neutron and proton is (even, even) and will have maximum stability.
Q2. Select the correct option for the element which has value greater than 1.
Answer: (D)
Solution: value signifies neutron to proton ratio.
For an oxygen atom,
Atomic number = 8
Number of neutron = Mass number - atomic number = 16-8 = 8
Therefore value = 1
Similarly,
value for neon = 0.7
value for calcium = 1
value for lead = 1.53
Therefore in case of lead value greater than 1.
Q4. Which of the following emission take place to increase the stability of an atom when the element lies above the stability belt?
Answer: (D)
Solution: When the nuclei are above the shaded area (i.e. there are too many neutrons), they emit radiation to achieve stability but when nuclei fall below the zone of stability, -emission, K-electron capture, or positron emission occurs. Therefore option (D) is correct.
Q. Which of the following emission take place to increase the stability of an atom when the element lies below the stability belt?
- Both A and C
Answer: (D)
Solution: When nuclei fall below the zone of stability, -emission, K-electron capture, or positron emission occurs but when the nuclei are above the shaded area (i.e. there are too many neutrons), they emit radiation to achieve stability. Therefore answer is (D)
Frequently Asked Questions-FAQs
Q1.What is radioactive decay?
Answer: The process by which an unstable atomic nucleus disintegrated to release energy (radiation) with or without additional particles is described as radioactive decay. The process is also referred as nuclear decay, or radioactivity, or nuclear disintegration. Radioactive material is one that contains unstable nuclei. Alpha decay (-decay), beta decay (-decay), and gamma decay (-decay) are three of the most common types of decay, all of which involve the emission of one or more particles.
Q2. What is the difference between the alpha particle and beta particles?
Answer: The main difference between the alpha particles and beta particles on the basis of its properties are:
Comparison in the properties of different types of Radiation
| Properties of radiation | Alpha rays | Beta rays |
| Nature of radiation |
It consists of a stream of
|
It consists of a stream of particles. They are identical to electrons with the charge of  and represented as |
| Ionisation power | Due to higher kinetic energy it has the highest ionisation power and can easily ionise a gas through which it passes. | Due to comparatively lower kinetic energy, it has lower ionisation power as compared with the particles. |
| Penetration Power | It has minimum penetration power due to its larger size and can be stopped by a sheet of paper,  thick aluminum foil. |
It has moderate penetration power due to high velocity and negligible mass and can be stopped by  thick aluminum foil. |
| Velocity | It travels with a velocity approx one-tenth of the speed of light, but the actual velocity depends upon the nucleus from which it is emitted. | It travels about  times faster than particles |
and represented as 
Q3. What is artificial radioactivity?
Answer: It was discovered in 1934 that when boron and aluminium were bombarded with particles, neutrons, protons, and positrons were emitted. When the bombardment was stopped, neutron and proton emissions ceased, but positron emissions decreased exponentially. Artificial radioactivity is defined as the process of converting a stable isotope into radioactive elements
Q4. What is the magic number and what is the relation with nuclear stability?
Answer: The magic numbers for atoms are 2, 10, 18, 36, 54, and 86 which corresponds to the total number of electrons filled in the shell of an atom. It is generally seen that the element following the magic number is generally more stable and therefore doesn't participate in a nuclear reaction.
Related Topics
| Nuclear reaction | Rate of radioactive decay | Radiocarbon dating |
| Positron | Nuclear reactor |
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