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Tuesday, January 7

Periodic Table

Periodic Table

Purpose Of Periodic Table:

  1. It is tabular arrangement of elements in groups or periods
  2. Periodic table provides basic framework to study the periodic behavior of physical and chemical properties of elements as well as their compounds 
  3. It organizes elements according to trends so that you can see the relationships between them.
Historical Background
  1. Al-Razi:
    His contributions were based on the physical and the chemical properties 
  2. Dobereiner:
    A German chemist, arranged known elements in groups called Triads.
    Elements can be classified in triads where the atomic weight of middle element of each triad is almost equal to the arithmetic mean of atomic weights of other elements 
  3. Newland:
    An English chemist, he gave Law of octaves , if elements are arranged in order of their increasing atomic weights, every 8th element starting from a given one has similar properties as that of first one 
  4. Mendeleev:
    A Russian chemist, Dmitri Mendeleev, he said that physical and chemical properties of elements are periodic functions of their atomic weights 
  5. Mosley:
    Mosley said that the physical and chemical properties of elements are periodic functions of their atomic numbers. 
Contribution Of Mendleev to the periodic table

Mendeleev gave a more useful and comprehensive scheme for the classification of elements. He presented the formal presentation to Russian Chemical Society. This presentation stated that:

  • The elements, if arranged according to their atomic weight, exhibit an apparent periodicity of properties.
  • Elements which are similar regarding their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g. K, Rb, Cs).
  • The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F.
  • The elements which are the most widely diffused have small atomic weights.
  • The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
  • We must expect the discovery of many yet unknown elements–for example, two elements, analogous to aluminium and silicon, whose atomic weights would be between 65 and 75.
  • The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of tellurium must lie between 123 and 126, and cannot be 128. Here Mendeleev seems to be wrong as the "atomic mass" of tellurium (127.6) remains higher than that of iodine (126.9) as displayed on modern periodic tables, but this is due to the way atomic masses are calculated, based on a weighted average of all of an element's common isotopes, not just the one-to-one proton/neutron-ratio version of the element to which Mendeleev was referring.

Certain characteristic properties of elements can be foretold from their atomic weights.

Important points of Mendeleev's periodic table

  • Arrangement of elements in ascending order of atomic numbers to rectify four misfit pair of elements in the Mendeleev’s Periodi Table i.e. K & Ar, Ni & Co, I & Te.
  • Addition of group VIII to properly place the newly discovered noble gases.
  • Introduction of two types of vertical groups A and B to eradicate the misplacement of Zn Cd & Hg and Be, Mg, Ca, Sr & Ba in same vertical group and so many others in same manner.

Draw backs of Mendeleev's periodic table 

  1. No concepts of isotopes and position of isotopes
  2. No position of Hydrogen
  3. No concept of atomic number
  4. No concepts of electronic configuration 
  5. No concepts of normal and transition elements 
  6. No concepts of lanthanides and actinides 
  7. No concept of atomic size or volume 

Modern Periodic table 

If elements are arranged in ascending order of their ATOMIC NUMBERS , their chemical properties repeat in periodic manner 

Features of Modern Periodic Table

Groups and Periods
Elements with similar properties are placed in vertical columns called Groups. There are total 8 groups.
Each group is divided into two subgroups A and B.
  • A Subgroups:
    The subgroups containing the representative or normal elements are labelled as A subgroups 
  • B Subgroups:
    B subgroups contains less typical elements, called Transition elements and are arranged in the center of the periodic table.
The horizontal rows of the periodic table are called Periods. All elements in a period have same no. of
shells. There are seven periods in the periodic table that are known by Arab numbers 1 to 7.

The Families:
Some groups or families are given special names and have certain properties that should be addressed. But first you must understand why elements are put into the same group. Think about a family you know, not a chemical family, but a human family. Children look like their parents. They learn to do things from their parents and do them in the same way. The same holds true for the elements in the families of the periodic table; they react the same way (for the most part). As you learned in the last chapter, each element has a certain number of valence electrons. As you will learn in the next chapter, it is the number of valence electrons of an atom that determines its chemical reactivity. Because the elements in a family have the same number of valence electrons, they will have a similar chemical reactivity. For example, Na and K can be compared in electron configuration and ions formed:
Na — 1s2, 2s2, 2p6, 3s1
— 1s2, 2s2, 2p6, 3s2, 3p6, 4s1
Both atoms have 1 valence electron and will lose this one electron to form ions with charges of 1+. This similar charge will mean that both elements have a similar chemical reactivity.The important families and groups are listed below followed by their important characteristics. These characteristics will become more familiar to you as you study the chapter on bonding.

Alkali Metals(Group 1)
Important Points:
All group 1 metals have one valence electron. When they form ions, they will have a charge of 1+. Group 1 alkali metals are highly reactive and will react vigorously with water.
Alkaline Earth Metals(Group 2)
Important Points:
All group 2 metals have two valence electrons. When they form ions, they will have a charge of 2+. Group 2 alkaline earth metals are highly reactive and will react with water.
Transition Metals(Group 3-10) D block 
Important Points:
Transition metals are famous for the colored salts and colored solutions they form. Many gems contain numerous transition metals. It is hard to predict the charge of a transition metal ion because the transition metals have multiple oxidation states. One transition metal, Hg, exists as a liquid at  room temperature

Entry # 7
By Ekra Malik


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