Valence Electrons and Chemical Properties

You have learned that atoms, in general, are not very stable. In fact, there are only six known chemically stable atoms: the noble gases. The noble gas atoms are chemically stable because of their electron configurations. Each of them has a “full” set of electrons in their outermost shell.

Bohr diagram of an He atom
Bohr diagram of an Ne atom
Bohr diagram of an Ar atom
Helium atom (4He)
Neon atom (20Ne)
Argon atom (40Ar)

But wait a minute. The helium and neon atoms both have full outer electron shells… the helium atom has two electrons in its first (outer) shell and the neon atom has eight electrons in its second (outer) shell… but the argon atom only has eight electrons in its third (outer) shell when the third shell can hold up to eighteen electrons. How is the argon atom’s outer shell anywhere close to full?

Bohr diagram of an K atom
Bohr diagram of an Ca atom
Bohr diagram of an Sc atom
nineteenth electron in the potassium atom (40K)
twentieth electron in the calcium atom (20Ca)
twenty-first electron in the scandium atom (45Sc)

The reason why is because the Bohr model of an atom, based on electron shells, is not accurate. Electrons do not occupy electron shells; they occupy atomic orbitals. The next element after argon is potassium (K). The potassium atom has one more proton and one more electron than the argon atom. The Bohr model predicts that this nineteenth electron will go into the third electron shell. But in reality, the nineteenth electron goes into the s-orbital in the fourth electron shell, even though the third shell is not completely filled yet. The remaining orbitals in the third shell only get filled once the s-orbital in the fourth shell has been filled first. (Bohr diagrams are only accurate up to Argon.)

Although the third electron shell can hold 18 electrons, the fourth electron shell can hold 32 electrons, the fifth electron shell can hold 50 electrons, and the sixth electron shell can hold 72 electrons, once each of those electron shells gets filled with 8 electrons, the next two electrons will go into the next higher electron shell. This means that the outermost shell of an atom can only have between 1 and 8 electrons in it before a new outer shell gets started, and this shell becomes an inner shell instead. The electrons in the outermost shell of an atom are called valence electrons.

VThe Geometry of Atomic Orbitals and Electron Placement
The octet rule and Lewis structures

Because most atoms are not very stable, they tend to chemically bond with other atoms to form stable molecules. The way that many atoms bond can be predicted by the octet rule. The octet rule states that atoms tend to lose, accept, or share electrons until they have an electron configuration with a “full” outer shell (full, in this case, meaning two electrons if the outermost shell is the first shell, and eight electrons if the outermost shell is a second or higher shell). This is the same electron configuration that noble gas atoms have, which are the only chemically stable atoms in the periodic table. The octet rule does not always work since there are so many variables that can affect an atom’s stability. In general, the octet rule does not apply to transition or inner transition metals.

As you have seen, Bohr diagrams can be a little misleading. They are also not terribly useful for predicting the chemical properties of atoms. Chemical properties describe how an atom or molecule reacts in the presence of other atoms or molecules. Atoms react by losing, accepting, or sharing electrons, and this almost always involves an atom’s outermost (valence) electrons, not the electrons in the inner shells. A Lewis structure is a diagram that highlights an atom’s valence electrons. It can be very useful for predicting how atoms will react to each other.

 
Br
 
To draw an atom’s Lewis structure, find the atom’s symbol and position in the periodic table.
 
Br
 
Bromine’s symbol is Br. It has 35 protons and 35 electrons, and it is in the fourth row and seventeenth column of the periodic table.
 
Br
 
Start filling in bromine’s valence electrons. Because bromine is in the fourth row of the periodic table, its valence electrons will be in the fourth electron shell.
 
Br
electron
Begin at the first element in the fourth row of the periodic table (K), and start counting over. Bromine’s nineteenth electron goes into the 4s-orbital. Draw one dot to the right of the bromine symbol.
 
Br
electrons
The twentieth electron also goes into the 4s-orbital. Draw a second dot to the right of the bromine symbol.
 
Br
electrons
The 4s-orbital is now filled, and the next ten electrons go into the 3d-orbitals. Because these electrons are going into the third electron shell, they are inner electrons and not valence electrons. Do not add any dots to the bromine symbol.
 
electron
Br
electrons
The thirty-first electron goes into the 4px-orbital. Draw one dot above the bromine symbol.
electron
electron
Br
electrons
The thirty-second electron goes into the 4py-orbital. Draw one dot to the left of the bromine symbol.
electron
electron
electron
Br
electrons
The thirty-third electron goes into the 4pz-orbital. Draw one dot below the bromine symbol. Each 4p-orbital now has one electron.
electron
electrons
electron
Br
electrons
The thirty-fourth electron goes into the 4px-orbital. Draw a second dot above the bromine symbol. The 4px-orbital is now filled.
electrons
electrons
electron
Br
electrons
The thirty-fifth (and final) electron goes into the 4py-orbital. Draw a second dot to the left of the bromine symbol. The 4py-orbital is now filled.
electrons
electrons
electron
Br
electrons
The Lewis structure of the bromine atom is now complete. It has seven valence electrons in its outermost shell. Because the outermost shell is not “full” (it is short one electron), the bromine atom is not chemically stable and will react with other atoms or molecules until its outermost shell is full.

Some people fill in the dots (valence electrons) of a Lewis structure in different orders. There does not seem to be one right way to do it, but this is the way that I prefer. Start by filling in the s-orbital electrons to the right of the atomic symbol, and then fill in the p-orbital electrons one-by-one, going in the counterclockwise direction around the symbol.

Element groups in the periodic table

If you were to draw Lewis structures for all of the elements in the periodic table, you would find an interesting pattern. All of the elements in the seventeenth column of the periodic table have the same Lewis structures.

electrons
electrons
electron
F
electrons
electrons
electrons
electron
Cl
electrons
electrons
electrons
electron
Br
electrons
electrons
electrons
electron
I
electrons
electrons
electrons
electron
At
electrons

These elements are collectively known as the halogens or the fluorine group. Because they have seven valence electrons in the same electron configuration, they have similar chemical properties. The halogens are highly reactive, and will bond with other atoms by accepting or sharing one electron to fill their valence shells.

 
H
electron
 
Li
electron
 
Na
electron
 
K
electron
 
Rb
electron
 
Cs
electron
 
Fr
electron

At the other end of the periodic table are the alkali metals or the lithium group. Because they have one valence electron in the same electron configuration, the alkali metals also have similar chemical properties. They are highly reactive, and will bond with other atoms by losing their one valence electron. This makes the next lower (and full) electron shell the outermost, valence shell. While hydrogen is technically part of this group, it has some different chemical properties. This is because, in addition to losing its electron, a hydrogen atom can also accept or share an electron to fill its valence shell.

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