CHEMICAL PERIODICITY
Periodic table is an arrangement of elements with similar properties placed together. The periodic table obtained largely as a result of experimental observations.
The former attempt to classify elements
(1) Dobereiner's law of triads (in the year1829) : It was the classification of the elements into groups of three elements each with similar properties such that the atomic weight of the middle element was the arithmetic mean of the other two such as Br, Sr, Ca, Ba, Cl, I etc.
(2) Telluric screw or Helix (in the 1862): It was given by Chancourtois.
(3) Newlands law of the octaves (in the year 1864) : It was an arrangement of elements in order of the increasing atomic weights in which it was observed that every eighth element possessed properties similar to those of the first just like the eighth node of an octave of music.
(4) Mendeleef's period law (in the year1869) : The first significant classification was given by Mendeleeff in the form of periodic table, generally known as Mendeleeff's periodic table. The periodic table given by him was based on the chemical, physical properties and periodic law, of elements are periodic functions of their atomic weights.'
In Mendeleef's periodic table elements are arranged in order of their increasing atomic weights in such a way that elements with similar properties are placed in the same group. It comprises of the seven horizontal rows termed as periods. These are numbered from 1 to 7.
Mendeleef's original table comprises of 8 vertical columns called groups. They are numbered as I, II III..... VIII. However, 9th vertical column called Zero group was added with the discovery of inert gases. Apart from for group VIII and zero, each of the group is further splitted into two sub-groups designated as A and B. Group 8th comprises of 9 elements arranged in three sets each containing three elements.
(5) Modern Periodic Law : The recent work has established that the fundamental property of an atom is atomic number and not atomic weight. Thus, atomic number is taken as the basis of the classification of the elements. The modern periodic law was produced by Moseley, it can be stated as : "Properties of elements are periodic functions of their atomic number".
When atomic number is taken as the basis for classification of elements, number of anomalies of Mendeleef's table disappear, such as the,
(i) Position of hydrogen : Dual behaviour of hydrogen is explained on the fact that it has one electron in its outermost orbit. When it loses its electron it gives H+ and behaves like alkali metals and when it gains an electron it gives H- and behaves like halogens. Therefore, it resembles with both the alkali metals and the halogens.
(ii) Dissimilar elements placed together : The lengths of periods are determined by the arrangement of electrons in different orbits. The period lasts on the completion of last orbits (last members always being the inert gas). Different periods comprise 2, 8, 18 or 32 elements. Now amongst the two elements which every long period adds to the group, one appears like the typical elements while the other does not. This gives mount to formation of subgroups. This illustrates the inclusion of dissimilar elements in the same group but different subgroups.
(iii) Position of rare earth elements : The electronic arrangement of rare earths can be written as 2, 8, 18, (18 + x), 9, 2 where x varies from 0 to 13, that is from Lanthanum to Lutecium. Number of the electrons in valency shell, in the case of all elements remains the same although atomic number increases. As they have the same number of valency electrons, the chemical nature is also similar. This elaborates their positions in the same group and in the same place of the periodic table.
(iv) Anomalous pairs of elements : Now the basis of classification is atomic number, therefore, this anomaly disappears as the elements occupy their normal position in the new periodic table.
(v) Position of isotopes : Since the isotopes of same element possess same atomic number they should occupy one and the same position in the periodic table.
(vi) Position of VIII group elements : In long periods 18 elements are to be distributed among 8 groups; 1 to 7 groups get 2 elements each and zero group accommodates inert elements, the remaining three elements are placed at one place in a new group, called as VIII group. This lack of space justifies the induction of VIII group in the periodic table.
(vii) Transuranic elements : These elements form a series known as actinide series, it starts from actinium and ends at lawrencium (89-103). This sequence has been placed outside the periodic table. The electronic configuration of the these elements is written as 2, 8, 18, 32, (18 + x), 9, 2, where x varies from zero (for actinium) to 14 (for lawrencium). Number of the valency electrons is the same for all these elements although atomic number increases. Thus, their chemical behavior is same. This explains their position outside the periodic table at one place.
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