How to Calculate the Number of Valence Electrons

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.

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 6d10} 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.

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.

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.