Periodic Table

Brief History and the development of periodic table

The periodic table is the systematic arrangement of elements in groups based on their chemical and physical similarities.  Elements belonging to the same group have the same number of electrons in the outermost energy. Since the beginning of the 19th century, many scientists have realized that elements could be grouped based on their virtue of the similarity of their physical and chemical properties.  During the 19th century,  70 elements have been discovered but until the work of Russian chemist Mendeleev, he made the first version of the periodic table of the known elements. He found that if all the elements are arranged in order of their increasing atomic mass, elements with similar chemical properties are placed recurred periodically. Mendeleev was successful in arranging the elements based on their similarities and he placed them in the same vertical column. He called the verticle column as a group of the periodic table. Mendeleev has left some blank space intentionally to place the unknown elements in the gaps. He was also able to predict beyond doubt the properties of these unknown elements by using the properties of existing elements in the table. Later on, British chemist, Henry Moseley came up with a technique of determining the atomic numbers of the known elements of the periodic table. He used the atomic number to organize the elements in their order of increasing atomic numbers. Henry didn't put into account the mass number of the atom, and up to now, the periodic table is arranged using this technique.




Dobereiner’s Periodic Table

Dobereiner’s triads were a group of three elements. The three elements had similar physical properties and also reacted with other elements in the same way.  Dobereiner computed the average weight of the smallest and largest element which he realized that they have the same atomic weight as that of the middle elements. For example:





Arithmetic  mean

Atomic weight









Arithmetic  mean

Atomic Weight








The modern periodic table

The periodic table shown above is a good example of the modern periodic table. The symbol of the elements in each square is used to identify each element, and its atomic number is placed above the symbol as shown in the diagram. Both the name and the mass number of the element are usually written below the symbols of the corresponding elements. From the periodic table above, the elements are arranged in order of their increasing atomic number from the LHS to the RHS and from top to bottom respectively. As you can see H is among the lightest element in the periodic table, it is placed at the far left corner with atomic number-1 while He is placed at the far right corner.  On the other hand, Li is placed left of the 2nd period with its atomic number three. The period refers to the horizontal arrangement of elements on a row. A complete periodic table consists of seven periods. Each period has a different number of elements. For instance, there are two elements in period one (H and He), eight elements in both periods two and three, and thirty-two elements in period six. The properties of the elements within a period change as you move across it from one element to another.


Periodic law

The periodic law states that the arrangement of elements in an ascending manner based on the atomic number leads to repetition in both the physical and chemical properties of elements. So, the organization of elements in their respective periods is essential. While organizing these elements, only those with similar physical and chemical properties should be placed in the same group. A group in the periodic table refers to the vertical column in which elements with similar chemical and physical properties are placed. Normally, the group is written using the number followed by either letter A or B, For example, Be, Mg, Ca, Sr, Ba, and Ra are members of group one written as "Group 1A." Group A elements are known as representative elements because they exhibit a wide range of both physical and chemical properties. These elements can be grouped further into 3 classes. Namely; the metals, which have a high electrical conductivity and a high lustre when fresh. They are ductile (can be drawn into wires) and malleable (can be stretched into sheets). All representative elements in the periodic table are metals except hydrogen. In chemistry, group 1A elements are referred to as Alkali metals while group 2A is known as Alkaline Earth metals.  Both the transition and inner transition elements are metals and form group B elements. Examples include; Cu, Ag, Au, and Fe. they are also known as the rare metals, inner transition elements appear at the lower part of the periodic table. On the other hand, non-metals in the periodic table are normally located at the RHS of the table.  Almost all non-metals are poor conductors of both heat and electricity and are nonlustrous. Halogens refer to salt producing nonmetals of group 7A and include F, Cl, and Br. Group O elements such as Ne, Ar, Kr, and Xe are given a special name” Noble or Inert gas” since they rarely undergo chemical changes.





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