The mass number The mass number of an atom is the sum of the number of protons and the number of neutrons in the nucleus. These particles are designated interchangeably by the name of nucleons, therefore the mass number represents the quantity of them.
Let N be the number of neutrons present and Z the number of protons, if we call A as the mass number, then:
A = N + Z
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Here are some examples of mass numbers for well-known elements:
The most stable and abundant hydrogen atom is also the simplest: 1 proton and one electron. Since the hydrogen nucleus has no neutrons, it is true that A = Z = 1.
An oxygen nucleus has 8 neutrons and 8 protons, therefore A = 16.
Life on Earth is based on the chemistry of carbon, a light atom with 6 protons in its nucleus plus 6 neutrons, so A = 6 + 6 = 12.
This element, much heavier than the previous ones, is well known for its radioactive properties. The uranium nucleus has 92 protons and 146 neutrons. Then its mass number is A = 92 + 146 = 238.
As mentioned before, the mass number A of an element always corresponds to the sum of the number of protons and the number of neutrons that its nucleus contains. It is also a whole number, but ... is there any rule regarding the relationship between the two quantities??
Let's see: all the elements mentioned above are light, except uranium. The hydrogen atom is, as we said, the simplest. It has no neutrons, at least in its most abundant version, and in oxygen and carbon, there are equal numbers of protons and neutrons.
It also happens with other light elements, such as nitrogen, another very important gas for life, which has 7 protons and 7 neutrons. However, as the nucleus is more complex and the atoms become heavier, the number of neutrons increases at a different rate..
Unlike the light elements, uranium, with 92 protons, has approximately 1 ½ times that amount in neutrons: 1 ½ x 92 = 1.5 x 92 = 138.
As you can see, it is quite close to 146, the amount of neutrons it has.
All of this is evident in the curve in figure 2. It is a graph of N versus Z, known as nuclear stability curve. There you can see how light atoms have the same number of protons as neutrons, and how from Z = 20 the number of neutrons increases..
In this way, the large atom becomes more stable, since the excess of neutrons reduces the electrostatic repulsion between the protons..
A very useful notation that quickly describes the type of atom is the following: the symbol of the element and the respective atomic and mass numbers are written as shown below in this diagram:
In this notation, the atoms in the previous examples would be:
Sometimes another more comfortable notation is used, in which only the symbol of the element and the mass number are used to denote the atom, omitting the atomic number. In this way, the 12 6C is simply written as carbon-12, the 16 8Or it would be oxygen-16 and so on for any element.
The number of protons in a nucleus determines the nature of the element. For example, every atom whose nucleus contains 29 protons is a copper atom, no matter what..
Suppose a copper atom loses an electron for whatever reason, it is still copper. However now it is an ionized atom.
It is more difficult for an atomic nucleus to gain or lose a proton, but in nature it can occur. For example, inside stars, heavier elements are continuously formed from light elements, since the stellar nucleus behaves like a fusion reactor..
And right here on Earth there is the phenomenon of radioactive decay, in which some unstable atoms expel nucleons and emit energy, transforming into other elements.
Finally, there is the possibility that an atom of a certain element has a different mass number, in this case it is a isotope.
A good example is the well-known carbon-14 or radiocarbon, which is used to date archaeological objects and as a biochemical tracer. It is the same carbon, with identical chemical properties, but with two extra neutrons.
Carbon-14 is less abundant than carbon-12, the stable isotope, and it is also radioactive. This means that over time it decays emitting energy and particles until it becomes a stable element, which in its case is nitrogen..
Carbon exists in nature as a mixture of several isotopes, of which the most abundant is the already mentioned 12 6C or carbon-12. And in addition to carbon-14 there is 13 6C with an additional neutron.
This is common in nature, for example 10 stable isotopes are known of tin. On the other hand, of beryllium and sodium only a single isotope is known.
Each isotope, natural or artificial, has a different rate of transformation. In the same way, it is possible to create artificial isotopes in the laboratory, which are generally unstable and radioactively decay in a very short period of fractions of a second, while others take much longer, as long as the age of the Earth or more..
| Carbon isotopes | Atomic number Z | Mass number A | Abundance% |
|---|---|---|---|
| 12 6 C | 6 | 12 | 98.89 |
| 13 6 C | 6 | 13 | 1.11 |
| 14 6 C | 6 | 14 | Traces |
What is the difference between 13 7 N and 14 7 N?
Both are nitrogen atoms, since their atomic number is 7. However, one of the isotopes, the one with A = 13, has one less neutron, while 14 7 N is the most abundant isotope.
How many neutrons are in the nucleus of a mercury atom, denoted as 201 80 Hg?
Since A = 201 and Z = 80, and also knowing that:
A = Z + N
N = A - Z = 201 - 80 = 121
And it is concluded that the mercury atom has 121 neutrons.
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