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What are valence electrons and their importance (A level Chemistry)

What are valence electrons (A level Chemistry)

What are valence electrons? Welcome, and in this post we are going to be discussing about:

  • what valency electrons are.
  • how valency electrons determined the chemical properties of an atom.
  • the relationship between the number of valence electrons of an element and its position on the periodic table.
  • and many other interesting things about valency electrons.

Valence electrons are outer shell electrons of an atom. They participate in the formation of a chemical bond, if the outer shell of the element is not full.

Valency electrons of the first 20 elements

Electrons of the first 20 elements are arranged around the nucleus in different energy levels as follows:

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  • first shell holds a maximum of 2 electrons
  • second shell holds a maximum of 8 electrons
  • third shell holds a maximum of 8 electrons
ElementNumber of electronsElectronic ConfigurationNumber of valence electronsGroup number
Hydrogen1111
Helium2228
Lithium32.111
Beryllium42.222
Boron52.333
Carbon62.444
Nitrogen72.555
Oxygen82.666
Fluorine92.777
Neon102.888
Sodium112.8.111
Magnesium122.8.222
Aluminium132.8.333
Silicon142.8.444
Phosphorus152.8.555
Sulphur162.8.666
Chlorine172.8.777
Argon182.8.888
Potassium192.8.8.111
Calcium202.8.8.222

By arranging the first 20 electrons in as in the table above we can see that the number of valence electrons is equal to the group number of the element on the periodic table. That is true except for helium which has 2 valency electrons but is in group 8 of the periodic table. This is due to the fact that helium has 1 shell which holds a maximum of 2 electrons. So its outer shell is full just like all other group 8 elements. Group 8 is also known as group 0.

Valence electrons in relation to periodic table blocks

Valence electrons of s-block and p-block elements

The electronic configuration of an atom can be denoted according to the number of electrons in the subshells. Using the sub-shell notation, the electron fill up in the following sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.

We are not going to go into much detail about how to fill sub-shells because that is the subject of another chapter which I assume you went through before reading this chapter.

ElementNumber of electronsElectronic Configuration (showing sub-shells)Periodic Table BlockValence
Hydrogen11s1s-block1
Helium21s2s-block2
Lithium31s2 2s1s-block1
Beryllium41s2 2s2s-block2
Boron51s2 2s2 2p1p-block3
Carbon61s2 2s2 2p2p-block4
Nitrogen71s2 2s2 2p3p-block5
Oxygen81s2 2s2 2p4p-block6
Fluorine91s2 2s2 2p5p-block7
Neon101s2 2s2 2p6p-block8
Sodium111s2 2s2 2p6 3s1s-block1
Magnesium121s2 2s2 2p6 3s2s-block2
Aluminium131s2 2s2 2p6 3s2 3p1p-block3
Silicon141s2 2s2 2p6 3s2 3p2p-block4
Phosphorus151s2 2s2 2p6 3s2 3p3p-block5
Sulphur161s2 2s2 2p6 3s2 3p4p-block6
Chlorine171s2 2s2 2p6 3s2 3p5p-block7
Argon181s2 2s2 2p6 3s2 3p6p-block8
Potassium191s2 2s2 2p6 3s2 3p6 4s1s-block1
Calcium201s2 2s2 2p6 3s2 3p6 4s2s-block2

As we can see from the table above:

  • p-block elements have less than 3 valency electrons
  • s-block elements have 3 or more valency electrons

For s-block elements, the outer sub-shell is the s orbital which is made up of 1 sub-orbital. Since a single sub-orbital holds a maximum of 2 electrons, s-block elements are limited to a maximum of only 2 electrons.

However for p-block elements, the outer sub-shell is the p orbital which is made up of 3 sub-orbitals. 3 sub-orbitals can hold a maximum of 6 electrons. Since every p orbital has a corresponding s orbital in the same shell with it, the number of the valency electrons of a p-block element is the sum of the p orbital and corresponding s orbital electrons. For example, Oxygen has 6 valence electrons from 2 electrons in the s orbital (2s2) and 4 electrons in the p orbital (2p4). All of them in the second shell.

Valence electrons of d-block elements

For s-block and p-block elements valence electrons exist only in the outermost electron shell but however for d-block elements (transition metals), valence electrons can also exist in an inner shell.

Lets start by looking at the electronic configurations of the first 10 d-block elements.

ElementNumber of electronsElectronic Configuration
Sc21[Ar] 4s2 3d1
Ti22[Ar] 4s2 3d2
V23[Ar] 4s2 3d3
Cr24[Ar] 4s1 3d5
Mn25[Ar] 4s2 3d5
Fe26[Ar] 4s2 3d6
Co27[Ar] 4s2 3d7
Ni28[Ar] 4s2 3d8
Cu29[Ar] 4s1 3d10
Zn30[Ar] 4s2 3d10

Transition elements have a partially filled (n − 1)d energy level (shell) adjacent to the ns energy level. As you can see on the table above, those elements have a partially filled 3d energy level next to a full 4s energy level (except in the cases of Cu and Cr which also have partially filled 4s energy level).

On the table, we used the electronic configuration of Argon for the shorthand notation. For example, manganese (Mn) which has an electronic configuration of 1s2 2s2 2p6 3s2 3p6 4s2 3d5 is abbreviated as [Ar] 4s2 3d5, where [Ar] represents the configuration of argon (1s2 2s2 2p6 3s2 3p6).

Another important thing to note about the short hand we used in the table, all the electrons outside the "Argon configuration" can behave as valence electrons.

Going back again to the configuration of manganese (Mn) → [Ar] 4s2 3d5, each 3d electron has the same energy level as a 4s electron, and a much higher energy level than that of 3s or 3p electrons. This leads to manganese having up to seven valence electrons (4s2 3d5), hence its oxidation state being as high as +7 (such as in the permanganate ion, MnO−4).

The effect of valence electrons in chemical reactions

The number of valence electrons that an atom has determines its chemical properties.

For example:

  • alkali metals (group 1) are generally the most reactive metals because each atom has only 1 valence electron. During the formation of an ionic bond 1 valence electron is easier to lose when forming a positive ion.
  • alkaline earth metals (group 2) are a bit less reactive, because each atom must lose 2 valence electrons (which requires a bit more energy) to form a positive ion.
  • in both group 1 and group 2 the reactivity of the elements increases down the group. The more the electron shells an element has the further the valence electrons are from the attraction of the nucleus and the easier they are to remove.
  • a nonmetal atom reacts reacts in two ways to attain a full valence shell. It can either share electrons with another atom (to form a covalent bond), or it can pull electrons from another atom (to form an ionic bond). The closer the the valence electrons are to the nucleus the greater the sharing or pulling power and more reactive the atom is.
  • halogens (group 7) are the most reactive nonmetal elements because each atom only needs to pull or share one electron to form a full valence shell.
  • however in non metal groups the reactivity decreases down the group because the valency shells are further away from nucleus and therefore the "pulling power" of the nucleus is weakened.
  • oxygen (group 6), though it requires 2 electron to complete its valence shell, is the most reactive nonmetal after fluorine. This is because its valence shell is closer to the nucleus than most halogens.
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