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Atoms are made up of a nucleus surrounded by electrons that rotate at high speed around it. The nucleus is made up of protons and neutrons and the number of protons is the part of the atom that gives it its identity. This is the reason why this number is called the atomic number.
The charge on each proton is equal in magnitude but opposite in sign to the charge on the electron. For this reason, it is understood that in the nucleus of a neutral atom there must be the same number of electrons as protons. In this sense, the number of electrons in a neutral atom is determined by the atomic number (Z).
This is very easy to understand, but what about ions? How is the number of electrons and protons in a cation or an anion determined? To understand it we must first understand what ions are and how they are formed.
What is an ion?
An ion is any chemical species that has a net electrical charge. This implies that there must be an imbalance between the number of positive charges in the nucleus and the number of negative charges in the surrounding electron cloud. In other words, an ion is a chemical species in which the number of protons and electrons is not equal.
Ions are formed in chemical reactions through the loss or gain of electrons, regardless of whether the ion is positive or negative. This is because chemical reactions never involve the nucleus, but only the valence electrons, which are the outermost electrons in an atom.
The loss or gain of electrons gives rise to two types of ions according to their charge: cations and anions.
cations
They are ions that have a net positive charge. These ions are formed when a neutral atom loses one or more electrons. The term cation comes from the fact that, during the electrolysis of a solution or a molten salt, these ions are the ones that are directed towards the cathode, or electrode in which the reduction reaction occurs.
How to calculate the number of protons and electrons in a cation?
The positive charge of cations is not because the nucleus has gained positively charged protons, but rather because the atom has lost negatively charged electrons, leaving an excess of positive charges. For this reason, the value of the positive charge represents the number of electrons that the neutral atom lost to become the cation.
In view of this, the number of electrons in a cation is no more than the number of electrons the neutral atom originally had, minus the number of positive charges on the cation. On the other hand, since the nucleus of the atom is not affected when forming the cation, the number of protons in both the neutral atom and the cation is the same, and is equal to the atomic number of the element in question.
If we call p the number of protons, n e – the number of electrons, and q the charge of the cation, then we can say that:
In summary, the number of protons in a cation is equal to the atomic number of the element, while the number of electrons is given by subtracting the atomic number and the charge of the ion:
anions
Anions are the opposite of cations. They are ions that have a net negative charge and that go towards the anode in the electrolysis cells, a fact from which they get their name. Anions are formed when a neutral atom gains electrons, creating an excess of negative charges that is not balanced by the positive charges of the nucleus.
How to calculate the number of protons and electrons in a cation?
Following the same logic as before, the number of protons in the nucleus of a monatomic anion is the same as in the neutral atom, so p = Z still holds . Instead, the number of electrons is, in this case, the sum of the number of electrons in the neutral atom, plus the number of negative charges on the anion. This can be written as:
In this case, the absolute value of q must be taken so that the value adds up, since anions have more electrons than neutral atoms. Therefore, to calculate the number of protons and electrons in an anion, the following formulas are used:
General way to calculate the number of protons and electrons in an ion
As we can see from the two previous expressions, to determine the number of electrons in an ion, it is enough to add or subtract the charge depending on whether it is an anion or a cation. However, this is equivalent to always subtracting the electrical charge of the ion, as long as we include the sign of the charge when substituting the value. In this way, we will be subtracting if it is a cation (because the charge is positive and when multiplying by the minus of the formula the result is negative) and adding if it is an anion (because the product of the negative of the formula with that of the charge results in a positive sign).
That is to say that, for any ion, be it positive or negative, the number of protons and electrons are given by:
Calculation of the total number of valence electrons in polyatomic ions
In addition to being distinguished by their charge, ions can also differ in the number of atoms that make them up. The previous sections correspond to cations and anions formed by a single atom. That is, the above equation applies only to monatomic ions (although it can be adapted for other types of ions).
However, there is also a wide variety of molecular cations and anions, composed of two or more atoms linked together by covalent bonds. In these cases, it is important to determine the number of electrons in this type of ions since this information allows us to build the Lewis structures of said ions, thus giving information about the types of bonds that are formed and the molecular geometry around the central atom. or central atoms, if there is more than one.
However, in these cases it is not so important to know the total number of electrons in all the atoms, but rather the total number of valence electrons, not including any of the inner electrons of the atoms that make up the ion.
In this case, the formula for the number of electrons is modified as follows:
Where n e-Valence is the total number of valence electrons in the ion, #eV i represents the number of valence electrons in element i , n i is the number of atoms of element i present in the ion, and q ion is, again, the charge of the ion with its respective sign. The number of valence electrons of an element can be determined either by means of the electronic configuration, or by looking at the group in which it is found in the periodic table (mainly in the case of representative elements).
Examples of the calculation of protons and electrons of monatomic ions
Example #1: Protons and electrons of the ferric cation
The ferric cation corresponds to the Fe 3+ ion , a monatomic ion with an iron nucleus. From the periodic table we get that the atomic number of iron is 26 (Z = 26) and, as we can see, the ion has three positive charges ( q = +3 ). Therefore, the number of protons and electrons in the ferric cation is:
Example #2: Protons and electrons of the sulfide anion
The sulfide anion corresponds to the S 2- ion , a monatomic ion with a sulfur nucleus and two negative electrical charges. The atomic number of sulfur is 16 (Z = 16) and, in this case, q = – 2 . Therefore, the number of protons and electrons in the sulfide anion is:
Examples of the calculation of valence electrons of polyatomic ions
Example #3: Valence electrons of the ammonium cation (NH 4 + )
The ammonium cation is made up of 4 hydrogen atoms and one nitrogen atom and has a positive charge of +1. The element hydrogen belongs to group 1A of the periodic table, so it has 1 valence electron, while nitrogen belongs to group 5A, so it has 5 valence electrons. Applying the formula for the number of valence electrons of polyelectron ions, we have:
Therefore, the ammonium ion has a total of 8 valence electrons distributed among its five atoms.
Example #4: Valence electrons of the nitrate anion (NO 3 – )
The nitrate anion is made up of 3 oxygen atoms and one nitrogen atom and has a negative charge of -1. The oxygen element belongs to group 6A of the periodic table, which is why it has 6 valence electrons, while it has 5 valence electrons, as we saw in the previous example. Applying the formula again, we have:
Therefore, the nitrate ion has a total of 24 valence electrons distributed among its four atoms.
References
Calculate the number of electrons and the number of neutrons in ions . (2019, September 4). Youtube. https://www.youtube.com/watch?v=nM7npEf27Do
Miralles, A. (2009, August 4). Protons, neutrons and electrons. isotopes. Ions 02 – The Wise Toad . The Wise Toad. https://www.elsaposabio.com/quimica/?p=780
REPRESENTATION OF IONS . (nd). 3° That. http://newton.cnice.mec.es/materiales_didacticos/el_atomo/iones.htm?4&1
wikiHow. (2019, January 5). How to find the number of protons, neutrons, and electrons . https://en.wikihow.com/find-the-n%C3%BAmer-of-protons,-neutrons-and-electrons
