I. History

The periodic table of the chemical elements (also known as Mendeleev’s table, periodic table of the elements or just periodic table) is a tabular display of the chemical elements. Although precursors to this table exist, its invention is generally credited to Russian chemist Dmitri Mendeleev in 1869, who intended the table to illustrate recurring (“periodic”) trends in the propertiesof the elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior. [1] [2]

II. The Periodic Table of Elements

You can see the Periodic Table of Elements in the picture below.

There are also some other forms of the table but this is the most common. Here elements are listed in order of increasing atomic number (i.e., the number of protons in the atomic nucleus). Rows are arranged so that elements with similar properties fall into the same columns.

III. Analysis: How to read the table

As mentioned above, the Periodic Table is organized in rows and columns called Periods and Groups respectively. You can see those coloured in the following picture. What is important is to know how to read the table and understand the meaning behind that categorization. [3] [4]

Periodic Table of Elements with Periods and Groups coloured [1]

The organization of the elements in this ways is directly linked to the way our chemistry views the inner structure of the elements: as atoms having a nucleus and electrons moving around it. It all starts with the Hydrogen, which has only one proton in its nucleus and one electron spinning around that core. Then the table moves over to Helium with an atomic number equal to 2 etc.

1. Scientific meaning of Periods

According to quantum mechanical theories of electron configuration within atoms, each row (period) in the table corresponds to the filling of a quantum shell of electrons. You can imagine a “shell” as an area around the nucleus able to have a specific maximum number of electrons. There is a standard notation for these areas around the atom nucleus: the inner shell (the orbital closer to the atom’s nucleus / core) is called “1s” and then we move on to shells like {2s, 2p}, {3s, 3p, 3d} etc.

Electronic orbitals [7]

Electrons in an atoms fill in the first (inner) orbital first, which is 1s. Each orbital can have up to 2n² electrones, where “n” is a number starting from 1 for 1s orbital and increasing as we go on further away from the core. That means that 1s orbital can contain up to 2 electones. When that area if filled with the maximum number of electrones, we move on to the next one which is 2s and so on.  In each period of the table a different “team” of orbitals is “completed” with electrons, as the following table illustrates.

2. Scientific meaning of Groups

The modern explanation of the pattern of the table is that the elements in a group have similar configurations of the outermost electron shells of their atoms (most chemical properties are dominated by the orbital location of the outermost electron). That can be seen in the following table.

The electon configuration of elements related to the grouping in the Periodic Table of Elements [5]

In the above table one can see that the atoms in the first column have one (1) electon in their outermost orbital. Similarly atoms in the second group have two (2) electons in the outermost orbital.

Z	Element	No. of electrons/shell
4	Beryllium	2, 2
12	Magnesium	2, 8, 2
20	Calcium	2, 8, 8, 2
38	Strontium	2, 8, 18, 8, 2
56	Barium	2, 8, 18, 18, 8, 2
88	Radium	2, 8, 18, 32, 18, 8, 2

Example of similar number of elentrons in the outermost orbital for the 2nd Group atoms

The above table clearly indicates that all those elements have 2 electrons in their external electron orbital. That is something of great importance as one can see from the next section of the article.

3. The importance of Groups for Chemistry

Each atom has the inherent tendency to get into a more stable condition. That is the way things go around in nature, that is the reason behind any spontaneous chemical reaction: everything want to become as stable as it can get. How can atoms get stable? By filling in their outermost orbital with 8 electons (or 2 if the outermost orbital is the 1s). That can happen via chemical bonds with other elements, which have the necessary electrons for something like that to happen. So oxygene (which has 6 electrons in its outermost shell) needs to connect with two hydrogen atoms (everyone of which has 1 electron in the outermost orbital) in a bond called “covalent bond”, so as to form a single molecule of water. A simple look at the table of elements can help us predict the possible reactions an element can participate in.

The water molecule: Oxygene takes the 2 electrons it needs to fill in its outermost orbital from the two hydrogen atoms with which it bonds

IV. The official Table of Elements from IUPAC

The Table of Elements constantly changes as new elements are discovered (or should we say… “discovered” ?)

One can find the IUPAC (International Union of Pure and Applied Chemistry) official table of elements at http://old.iupac.org/reports/periodic_table/index.html. At that site you can even download for free a printable PDF version of the Periodic Table of Elements. [6]

References

1. http://en.wikipedia.org/wiki/Periodic_table 
2. http://www.wou.edu/las/physci/ch412/perhist.htm 
3. MIT University – Periodic Table of elements 
4. Cambridge University – Periodic Table of Elements 
5. http://en.wikipedia.org/wiki/Electron_configuration 
6. IUPAC official Periodic Table of Elements