Sunday, November 27, 2016

Periodic Table Revisited—Columns and Valences

Previously, much has been learned about the periodic table and what it tells about elements. Each listing in the table contains information about the number of protons (Atomic Number), its mass (Atomic Mass), and other information can be derived, such as number of protons and electrons.

It is worth repeating that the arrangement in the table is ascending from left to right, top to bottom by Atomic Number. Furthermore, each row of elements represents all of the elements filling a set of electron orbitals, and are called periods. Each column, therefore, represents all of the elements whose outer set of electrons are similarly filled, and are called groups or families.

Also, it should be remembered that the set of electrons being filled are called the valence electrons. The unfilled set of valence electrons result in the atoms of elements either being prone to give up or take in electrons. This tendency is expressed as a plus or minus, where the plus means the atom has more than it wants and will be happy to give it up, and where the minus means the atom wants to collect other electrons from other atoms. This tendency to either give up or take in electrons is called the valence charge.

In many of the element groups, all of the elements share the same number of valence electrons. This is true for columns 1, 2, 13, 14, 15, 16, 17, and 18. Columns 3 through 12 do not follow the pattern.

How the elements react with each other is based on their electron configuration. While it would be nice if each element in each family behaved exactly the same way, there is actually a great deal of variation, and elements can combine in many different ways. Another way to look at valence is a related concept called oxidation state, and this periodic table reveals that many of the groups/families can have a wide range of values:

However, to get a sense of what is going on in general, further discussion will focus on the simple, most common behaviors of each family.

Elements in column 1 tend to give up their single valence electron and have a valence charge of +1 (plus meaning they have the tendency to give it away).

Elements in column 2 have will give up both of their valence electrons and have a valence charge of + 2

Columns 3 through 12 reflect a great deal of variation with the number of valence electrons being 1 or 2 and valence charges ranging from +8 to -4

Elements in column 13 have 3 valence electrons and usually have a valance charge of +3.

Elements in column 14 have 4 valence electrons. They will either give up 4 or take 4 (usually) so they have a valence of ±4.

Elements in column 15 have 5 valence electrons. They wish to complete their orbital set, so they want 3 electrons giving them a usual valence charge of +3 (though they will behave differently in some cases, even giving away all five of their electrons).

Elements in column 16 have 6 valence electrons and need 2 more to complete their orbital set, so they have a usual valence charge of -2.

Elements in column 17 have 7 valence electrons and need one more to fill their orbital sets, so they have a usual valence charge of -1

Elements in column 18 have complete sets of electrons in all of their orbitals, wanting neither to take or give. Thus, they have a valence of 0.

In a general, over-simplified way, elements in columns 1-13 will give away electrons in order to empty their incomplete set of orbitals, while elements in columns 15 to 17 are trying to collect electrons to fill their incomplete set of orbitals. Thus, in an equally over-simplified way, valence charges can be thought of as:

  + ++ ??

+++ ++++
--- -- - 0
Usual Valence Charge
In many, many cases, these valence charges will allow the prediction of the ratio in which elements will combine. When combined, as a rule, the sum of the valence charges will be 0.

Thus, as a rule, and for example:

elements from column 1 and column 17 will combine in a 1:1 ratio (such as NaCl).
elements from column 1 and column 16 will combine in a 2:1 ratio (such as H2O).
elements from column 13 and column 17 will combine in a 1:3 ratio (such as AlCl3).

This is NOT all of the normal, set combinations. It is only three of the many, many examples showing how the column position determines the ratio in which elements will combine.

So, based on the electron configuration, 1 carbon will give up its 4 electrons to 2 oxygen so that both of the 2 oxygen can complete their sets of orbitals by adding 2 electrons each.

For many, many compounds, this over-simplified table will provide insight into why the elements are combined as they are. While it is not true for EVERY case, it is useful for gaining a basic understanding of how element combine.

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