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The Periodic Table

Written by tutor Heather G.


History of the Periodic Table

The periodic table is a way to organize the elements based on their similarities. The first version was constructed by Dmitri Mendeleev in 1869. He put the elements into a grid with increasing atomic weights and noticed the elements in each column had similar chemical properties. Mendeleev was able to predict the existence of elements before they were discovered or synthesized by looking at gaps in his table. However, he also found inconsistencies. For example, according to Mendeleev’s table, Argon should have been placed before Potassium. The solution to this problem came in 1913 when henry Moseley organized the elements by atomic number (the number of protons an atom has). He did this by shooting electrons at atoms and analyzing emission x rays. This new organization fixed all of the contradictions created by the the atomic weight organization. Below is a picture of the final result.

Structure of the Periodic Table

The periodic table is structured in a way that is very useful for chemists. The horizontal rows are called periods. Typically the element’s symbol, atomic number, and atomic weight are shown. An element’s period tells us the highest unexcited energy level for an electron. We can tell the number of valence (outer) electrons as well as other useful properties an element has from its group number. This will be discussed more in the next section. The periodic table is also organized into four blocks according to the subshell in which the last electron resides. The s block contains the first two groups, the p block contains the last six groups, the d block contains groups 3 to 12, and the f block has the lanthanides and actinides usually located in two rows at the bottom of the period table. Blocks help when writing out the electron configuration of an element.

Groups

There are ten named groups in the periodic table. All elements in a group have similar properties. It is also possible to tell how many valence electrons elements in certain groups have. The common group names and valance electrons are outlined below:

Group Number    Group Name Valence Electrons
1 Alkali Metals 1
2 Alkaline Earth Metals      2
3-12 Transition Metals varies
13   3
14   4
15   5
16   6
17 Halogens 7
18 Noble Gases 8

A few points should be noted. Although helium is in group 18, it only has two valence electrons. Also, hydrogen is a nonmetal. In order to predict the number of valence electrons that transition metals have, other methods must be used.

Trends of the Periodic Table

There are also specific trends that the periodic table follows. These deal with the size, ionization energy, electronegativity, and electron affinity of an element. The size (atomic radius) of an electron decreases from left to right. This is because each time a proton is added the electrons are drawn closer in towards the center. The atomic radius increases from top to bottom. The ionization energy of an element, or the amount of energy required to remove an electron increases from left to right and decreases from top to bottom. Electronegativity, the tendency of an element to attract electrons, follows the same pattern as ionization energy. Electron affinity also shows trends, but they are less consistent than the others. Electron affinity is the amount of energy released when an electron is added to a neutral atom to form a negative ion. It generally increases left to right and decreases top to bottom. One last major trend of the period table deals with the metallic character of an element. Elements with lower ionization energies, electron affinities, and electronegativity have stronger metallic characteristics. This means it decreases left to right and increases top to bottom which helps explain the staircase shape in many periodic tables that metalloids create.

Future Changes to the Periodic Table

The periodic table is not set in stone. New elements will be discovered, and scientists are not sure whether or not the periodic table will continue with the current trends. Adjustments may need to be made. Also, the number of possible elements is debated. Current predictions range from 126 to 155 elements.

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