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Periodic Table of the Elements. In 1869, the Russian chemist Dmitri Mendeleev first proposed that the chemical elements exhibited a "periodicity of properties." 

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periodic table of the elements
Periodic Table of the Elements

In 1869, the Russian chemist Dmitri Mendeleev first proposed that the chemical elements exhibited a "periodicity of properties." 

Mendeleev had tried to organize the chemical elements according to their atomic weights, assuming that the properties of the elements would gradually change as atomic weight increased. 


What he found, however, was that the chemical and physical properties of the elements increased gradually and then suddenly changed at distinct steps, or periods. To account for these repeating trends, Mendeleev grouped the elements in a table that had both rows and columns. 


The modern periodic table of elements is based on Mendeleev's observations; however, instead of being organized by atomic weight, the modern table is arranged by atomic number (z). As one moves from left to right in a row of the periodic table, the properties of the elements gradually change. At the end of each row, a drastic shift occurs in chemical properties. The next element in order of atomic number is more similar (chemically speaking) to the first element in the row above it; thus a new row begins on the table.  


For example, oxygen (O), fluorine (F) and neon (Ne) (z = 8, 9 and 10, respectively) all are stable, non-metals that are gases at room temperature.  Sodium (Na, z = 11) however, is a silver metal that is solid at room temperature, much like the element lithium (z = 3).  Thus sodium begins a new row in the periodic table and is placed directly beneath lithium, highlighting their chemical similarities. 


Rows in the periodic table are called periods. As one moves from left to right in a given period, the chemical properties of the elements slowly change. 

Columns in the periodic table are called groups or families. Elements in a given group or family in the periodic table share many similar chemical and physical properties. 

electron configuration and the table
Electron Configuration and the Table

The 'periodic' nature of chemical properties that Mendeleev had discovered is related to the electron configuration of the atoms of the elements. In other words, the way in which an atom's electrons are arranged around its nucleus affects the properties of the atom.

Bohr's theory of the atom tells us that electrons are not located randomly around an atom's nucleus, but they occur in specific electron shells. Each shell has a limited capacity for electrons. As lower shells are filled, additional electrons reside in more-distant shells.


The capacity of the first electron shell (or energy shell) is two electrons and for the second shell the capacity is eight. So, oxygen, with 8 protons and 8 electrons, carries 2 electrons in its first shell and 6 in its second shell. Fluorine, with 9 electrons, carries 2 in its first shell and 7 in the second. Neon, with 10 electrons carries 2 in the first and 8 in the second. As the number of electrons in the second shell increases, one can see why the chemical properties gradually change moving from oxygen to fluorine to neon.


Sodium has 11 electrons. Two fit in its first shell, but remember that the second shell can only carry 8 electrons. Sodium's 11th electron cannot fit into either its first or its second shell. This electron takes up another orbit, a 3rd electron shell in sodium. The reason that there is a dramatic shift in chemical properties when moving from neon to sodium is because there is a dramatic shift in electron configuration between the two elements. But why is sodium similar to lithium? Look at the electron configurations.


While sodium has 3 electron shells and lithium 2, the characteristic they share in common is that they both have only 1 electron in their outermost electron shell. 

These outer shell electrons (called valence electrons) are important in determining the chemical properties of the elements.  


An element's chemical properties are determined by the way in which its atoms interact with other atoms.  

If we picture the outer (valence) electron shell of an atom as a sphere surrounding everything inside, then it is only the valence shell that can interact with other atoms - much the same way as it is only the paint on the exterior of your house that 'interacts' with, and gets wet by, rain water.



The valence shell electrons in an atom determine the way it will interact with neighboring atoms and determine its chemical properties. Since both sodium and lithium have 1 valence electron (and are in the same Group), they share similar chemical properties.


For elements in Groups 1, 2, 13, 14, 15, 16, 17, 18, the number of valence electrons corresponds to the group number.  Thus Li, Na and other elements in group 1 have one valence electron. 

Be, Mg and other group 2 elements have two valence electrons.  B, Al and other group 3 elements have three valence electrons, and so on. 


The row, or period, number that an element resides in on the table is equal to the number of total energy shells that contain electrons in the atom. 

H and He in the first period normally have electrons in only the first energy shell; Li, Be, B and other period two elements have two shells occupied; etc.