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In order to understand how to determine the protons, electrons, and neutrons in an atom, we must first know what the characteristics of these subatomic particles are. An atom is the smallest entity into which an element can be divided without losing its chemical properties . Atoms are made up of even smaller particles, subatomic particles, and there are basically three of them: electrons, protons, and neutrons.
Electrons are negatively charged and are the lightest subatomic particles that make up atoms. The charge on protons is positive, and protons weigh about 1,836 times more than electrons. The only subatomic particles that do not have an electrical charge are neutrons, which weigh about the same as protons.
The protons and neutrons are grouped in the center of the atom forming the atomic nucleus, while the electrons move describing various orbits around the nucleus of the atom.
What are the steps to follow to find out the number of subatomic particles in an atom
1. Get information about the item of interest
We can find the basic information about an element in the Periodic Table , including the number of protons and electrons. The number of protons is equal to the atomic number of the element, represented by the letter Z, and the number of electrons is equal to the number of protons. Some versions of the Periodic Table also include the isotopic composition of each element, that is, those atoms that have the same number of protons but a different number of neutrons.
2. How to find out the number of protons
Each element is defined by the number of protons found in each of its atoms. No matter how many electrons or neutrons an atom has, the element is always defined by the number of protons. In particular, it is possible to have an atom consisting of only one proton: ionized hydrogen. The Periodic Table is organized according to the increasing atomic number of the elements, so the number of protons is the number of the element in the table; For example, for hydrogen the number of protons is 1, for zinc the number of protons is 30.
If you have the atomic mass of an isotope, you can find the number of protons by subtracting the number of neutrons from the atomic mass. But if you have the atomic weight,which is the weighted average with the isotopic abundance of the atomic mass of the different isotopes that make up an element, different situations can arise. Let’s see some examples. If you have an element with an atomic weight close to 2, the only possibility is that it is composed mostly of an isotope of hydrogen, deuterium, which has a neutron in its nucleus, since the next element on the Periodic Table, helium , they do not have any isotope that has only protons, without any neutrons. If, on the other hand, the atomic weight is around 4, it is helium, whose most abundant isotope has 2 protons and 2 neutrons in its nucleus (although it also has a stable isotope with only one neutron and atomic mass 3). But what could we say if the atomic weight is around 3.
3. How to find out the number of electrons
In general, the number of electrons in an atom is equal to the number of protons, and the atom then has a net zero or neutral charge, but sometimes the number of protons and electrons in an atom are not the same, so the atom has a net positive or negative charge and is called an ion or ionized atom. If we know the net charge of the atom, it is possible to determine the number of electrons by subtracting the charge, considering the sign of the charge, from the number of protons in the nucleus. An atom with a net positive charge and has more protons than electrons is called a cation, while an anion has a net negative charge and has more electrons than protons. Neutrons have no net electrical charge, so the number of neutrons in the nucleus is not relevant in this calculation.
It is important to note that chemical reactions do not change the number of protons in an atom, the number of which determines the atom’s net charge by losing or gaining electrons in the chemical reactions that occur.
examples
If an ion has a net positive charge of two units, like Zn 2+ , this means that the number of protons exceeds the number of electrons by two units. The atomic number of zinc (Zn) is 30 so, applying the previous rule, the number of electrons in this atom is 28: 30 – 2 = 28 electrons.
If an ion has a net negative charge of one unit, such as F – , then the number of electrons exceeds the number of protons by one unit. The atomic number of fluorine (F) is 9 so, applying the previous rule, the number of electrons in this atom is 10: 9 – (-1) = 10.
4. How to find out the number of neutrons
The number of neutrons in an isotope is calculated as its mass number minus the number of protons. To find the number of neutrons in an atom, it is necessary to know its mass number. In the Periodic Table we can find the atomic weight of each element which, as we have already said, is the weighted average with the isotopic abundance of the atomic mass of the isotopes that compose it (that is why weights can be found in the Periodic Table atomic numbers with decimal numbers while the atomic mass is an integer, to be defined as the sum of the number of neutrons and protons of the isotope). It should be noted that the number of neutrons depends on the isotope of an element that we consider, so it is not a property of the element itself. For example, The most abundant isotope of hydrogen is the one with only one proton and no neutron, but a small fraction of the hydrogen atoms found in nature correspond to the isotope that has one neutron, deuterium. Most versions of the Periodic Table do not include the isotopic composition of the elements, so we cannot obtain in this case the information necessary to determine the number of neutrons of the isotopes of an element, and we must specifically look for the information of the isotope we study. In practice, an isotope is defined by the number of protons, corresponding to the element to which it belongs, and its number of neutrons. Most versions of the Periodic Table do not include the isotopic composition of the elements, so we cannot obtain in this case the information necessary to determine the number of neutrons of the isotopes of an element, and we must specifically look for the information of the isotope we study. In practice, an isotope is defined by the number of protons, corresponding to the element to which it belongs, and its number of neutrons. Most versions of the Periodic Table do not include the isotopic composition of the elements, so we cannot obtain in this case the information necessary to determine the number of neutrons of the isotopes of an element, and we must specifically look for the information of the isotope we study. In practice, an isotope is defined by the number of protons, corresponding to the element to which it belongs, and its number of neutrons.
Fountain
WN Cottingham, DA Greenwood, DA An introduction to nuclear physics . Cambridge University Press, 2004.
