In 1829 Johan Dobereimer first classified elements into a system of so-called triads. Previously, there were so few discovered atoms that this was impossible to group atoms in any way. Johan's system was a pioneer in a series of scientific attempts to accurately place like elements, discovered and undiscovered into categories with similar properties. Although Johan's system was far from accurate, it laid important foundations for John Newland's and Dimitri Mendeleev, other pioneers of our modern day periodic table. While parts of Newland and Mendeleev's ideas were also inaccurate, they did contribute important discoveries to our table. Newland discovered the relationship between atomic mass and chemical properties while Mendeleev arranged the elements by their atomic mass. Although Mendeleev is credited with the development of the periodic table, he made on critical error in his arrangement of the elements. In 1912 Henry Mosley discovered that if atoms were arranged by atomic numbers (as opposed to Mendeleev's arrangement by atomic mass) they would line up perfectly.
The modern periodic table contains a host of information concerning the elements within. Each element has a block stating its name, symbol, atomic mass, electron structure and atomic number. Elements in vertical rows are grouped into families of similar elements, which I will discuss later. Row headings indicate how many valence electrons exist within elements of each row. The periodic table also divides the elements into periods of metals, metalloids, nonmetals, and transition metals.
By looking at the periodic table, one can predict the number of electrons in the element's atoms. In general, the numbers of electrons increase from left to right and from top to bottom. This trend is because of the fact that as the elements progress across, more energy levels and consequently electrons are added. Besides this noticeable progression, the elements in the periodic table are observed to follow several other general trends as they progress form left to right and from top to bottom on the periodic table.