How to Calculate the Number of Valence Electrons

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In chemistry, valence electrons are electrons located in the outermost shell of an element’s electron shell. Determining the number of valence electrons in an element is an important skill in chemistry because this information helps determine the type of bonds the element can form. Determining the number of valence electrons can be easily done with the periodic table of the chemical elements.

Table of Contents

Find the number of valence electrons using the periodic table

With non-transition metals

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Have a periodic table of chemical elements ready. The periodic table of the chemical elements (referred to as the periodic table) is a table of many color-coded cells that lists all the elements known to man as well as some essential information about those elements. Based on the information available in the periodic table, we can determine the number of valence electrons of the element we are studying. The periodic table is often attached to textbooks. You can also refer to this readily available interactive periodic table. ^{[1] X Research Source}

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Number each column of the periodic table from 1 to 18. Usually in the periodic table, all elements in the same column will have the same number of valence electrons. If your periodic table doesn’t already number the columns, do it yourself by numbering 1 to 18 from left to right. Scientifically, each column of the periodic table is called a “group” . ^{[2] X Research Source}

For example, with the unnumbered periodic table, we would put the number 1 above the element Hydrogen (H), number 2 above the element Beryllium (Be) and do the same until the number 18 above Helium (He). ).

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Determine the position of the element in question. In this step, you determine the position of the element you are considering on the periodic table. You can find the location of an element based on its chemical symbol (the letter in each box), on its atomic number (the number in the upper left corner of each box), or on other information. information is available on the periodic table.

For example, we need to find the number of valence electrons of the element Carbon (C). The atomic number of this element is 6. Carbon is in the upper part of group 14 elements. In the next step we will determine the number of valence electrons of this element.
In this section, we will ignore the Transition Metals, i.e. the elements between group 3 and group 12. These transition metals are slightly different from the rest of the elements, hence the steps. mentioned in this section are not applicable to such metals. We will consider these groups of elements later in the article.

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Use the group number to determine the number of valence electrons. The group number of a non-transition metal can be used to calculate the number of valence electrons in the atom of that element. The digit “units of the group number” is the number of valence electrons present in the atoms of the elements of that group. In other words:

Group 1: 1 valence electron
Group 2: 2 valence electrons
Group 13: 3 valence electrons
Group 14: 4 valence electrons
Group 15: 5 valence electrons
Group 16: 6 valence electrons
Group 17: 7 valence electrons
Group 18: 8 valence electrons (except in the case of helium with 2 valence electrons)
In the example of carbon, since the carbon is in group 14, we can say that a carbon atom has four valence electrons .

With transition metal

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Identify an element between Groups 3 and 12. As mentioned above, elements in groups 3 through 12 are called “transition metals” and when it comes to valence electrons the valence electrons are referred to as “transition metals”. This element has properties that are different from the others. In this section, we will learn why it is generally not possible to assign valence electrons to atoms of transition metals.

In this section we take the element Tantan (Ta) with atomic number 73 as an example. The next steps will help determine the number of valence electrons of this element.
Note that the elements of group 3 lanthanum and actinium (also known as the “rare earth metals”) also belong to the group of transition metals — these two groups of elements are often listed below the periodic table, starting with head with lanthanum and actinium.

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The valence electron in a transition metal is not the same as the ‘regular’ valence electron. To understand why transition metals don’t really ‘behave’ like other elements on the periodic table, we need to know a little bit about how electrons behave in atoms, as explained below. , or you can skip this step. ^{[3] X Research Sources}

When electrons are introduced into an atom, they are arranged into different “orbitals” — that is, different regions around the nucleus. In short, valence electrons are electrons in the outermost orbital — in other words, the last electrons added to that atom.
To explain the orbitals in detail, which is perhaps a bit more complicated, when an electron is added to the outermost d subshell of the atomic shell of the transition metal (see below), the first of these electrons will be charged. like normal valence electrons, but then their properties can change, sometimes electrons from other orbitals can act as valence electrons. That is, an atom can have many valence electrons depending on the case.
You can learn more about this at Clackamas Community College’s valence electron website. ^{[4] X Research Sources}

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Determine the number of valence electrons based on the self-grouping number. As noted above for non-transition metals, group numbers on the periodic table can help determine the number of valence electrons. However, there is no definite formula to determine the exact number of valence electrons of a transition metal — in this case, the number of valence electrons of an element does not lie at a fixed value, the number The group itself can only give a relative estimate of the number of valence electrons. Specifically: ^{[5] X Research Sources}

Group 3: 3 valence electrons
Group 4: 2 to 4 valence electrons
Group 5: 2 to 5 valence electrons
Group 6: 2 to 6 valence electrons
Group 7: 2 to 7 valence electrons
Group 8: 2 to 3 valence electrons
Group 9: 2 to 3 valence electrons
Group 10: 2 to 3 valence electrons
Group 11: 1 to 2 valence electrons
Group 12: 2 valence electrons
Taking the example of the element Tanta (Ta) of group 5, we can say that this element has between 2 and 5 valence electrons , depending on the case.

Find the number of valence electrons based on electron configuration

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Learn to read electron configurations. Based on the electron configuration of an element we can also determine the number of valence electrons of that element. The electron configuration looks complicated, but it’s just a way of representing the orbitals of an element in alphanumeric form, once you know the rules, understanding the electron configuration is not difficult.

Consider an example of the electron configuration of Sodium (Na):

1s ² 2s ² 2p ⁶ 3s ¹
If you pay attention you will see that the electron configuration is just a string of characters repeating the form:

(number)(letter) ^(exponent) (number)(letter) ^(exponent) …
…and so on. The first (number)(letter) group is the name of the orbital and ^{the (exponent)} represents the number of electrons present in that orbital.
So, in the case we are looking at, we can say that Sodium has 2 electrons in the 1s orbital , 2 electrons in the 2s orbital , 6 electrons in the 2p orbital and 1 electron in the 3s orbital . A total of 11 electrons — sodium’s atomic number is also 11.

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Find the electron configuration of the element you are considering. Once you know the electron configuration of an element, finding the element’s electron configuration is not difficult (except in the case of transition metals). If in the question you need to solve the electron configuration already, you can skip this step. If you need to find the electron configuration, proceed with the following steps:

The complete electron configuration of the element ununoctium (Uuo), atomic number 118 is:

1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ 4s ² 3d ¹⁰ 4p ⁶ 5s ² 4d ¹⁰ 5p ⁶ 6s ² 4f ¹⁴ 5d ¹⁰ 6p ⁶ 7s ² 5f ^{14 6d}¹⁰ 7p ⁶
Once you have such a complete electron configuration, to find the electron configuration of another element, you just need to fill in the orbitals with electrons, starting from the first one, until you run out of electrons to fill. It sounds complicated, but when it comes to doing this is relatively easy. For example, if we wanted to write the complete electron configuration of chlorine (Cl), element number 17, that is, the atom of this element has 17 electrons, we would fill in the following:

1s ² 2s ² 2p ⁶ 3s ² 3p ⁵
Notice that the total number of electrons in the electron configuration is just right 17: 2 + 2 + 6 + 2 + 5 = 17. You only need to change the number in the last orbital — the rest stays the same because the penultimate orbital is full. electrons.
Learn more about writing the electron configuration of an element.

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Assign electrons to orbitals according to the Octave Rule. When electrons are added to an atom, they are placed into orbitals in the order outlined above — the first two electrons will be placed in the 1s orbital, the next two electrons into the 2s orbital, the next six electrons will be placed in the 2s orbital. 2p, do this until the electron is placed in the corresponding orbital. When we consider atoms of non-transition elements, we can say that these orbitals will form “layers” around the nucleus, where the latter will be further from the nucleus than the former. In addition to the first orbital layer which can hold only a maximum of two electrons, all subsequent orbital layers can hold up to eight electrons (except in the case of transition metals). This rule is known as the Octave Rule .

For example, consider the element Bo (B). The atomic number of this element is 5, so we have the electron configuration of this element’s atom as follows: 1s ² 2s ² 2p ¹ . Since the first orbital shell contains only 2 electrons, it can be determined that Bo has two orbital layers: a first subshell consisting of 2 electrons in the 1s orbital and a second with three electrons distributed in the 2s and 2p orbitals. .
As another example, an element similar to chlorine would have three shells: a shell consisting of two electrons in the 1s orbital, a shell consisting of two electrons in the 2s orbital and six electrons in the 2p orbital, and an outer shell consisting of two electrons in the 3s orbital. and five electrons in the 3p orbital.

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Find the number of electrons in the outermost shell. Once the electron configuration is determined, we know the shells of that element, finding the number of valence electrons can be done by determining the number of electrons in the outermost shell of the atom’s electron shell. If the outermost shell is full (i.e. has a total of eight electrons, or in the first shell 2 electrons) the element is called an inert element and hardly participates in chemical reactions. However, this rule does not apply to transition metals.

For example with the element Bo, since Bo has three electrons in the second, which is also the outermost shell, we can say that the element Bo has three valence electrons.

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Use the periodic table row number as a shorthand to determine the number of orbital layers. The horizontal row on the periodic table is called the “period” of the elements. Starting from the first row, each period corresponds to the number of “electron shells” of the elements in the same period. Therefore, you can use the period to quickly determine the number of valence electrons of an element — you just count the electrons in left-to-right order from the first element of that period. Note again that this does not work with transition metals.

For example, since selenium is in period 4, we can determine that the element has four electron shells in its atomic shell. Since it is, in order from left to right, the sixth element in period 4 (excluding transition metals), we can say that the fourth shell of selenium has six electrons, i.e. the element has six electrons . valence electrons .

Advice

Note that the electron configuration can be written briefly using noble gases (elements of group 18) instead of the orbitals at the beginning of the configuration. For example, the electron configuration of Sodium can be written as [Ne]3s1 — that is, the electron configuration of Sodium is the same as that of Neon but with an extra electron in the 3s orbital.
Transition metals can have incomplete valence subclasses. To accurately determine the valence electron number of a transition metal, it is necessary to apply complex quantum principles that are not within the scope of this article.
You should also note that the periodic table of the chemical elements may be different in different countries. So, make sure you’re using the common periodic table where you live to avoid confusion.

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