In 1911, Moseley discovered atomic number. According to him, physical and chemical properties of elements do not depend upon atomic weight but on atomic number, that is the number of protons and atomic number is the basic properties of elements and not atomic mass. Therefore, Moseley considered atomic number as the basis of classification and not the atomic weight. Moseley proposed the modern periodic law according to which the physical and chemical properties of elements are the periodic functions of their atomic numbers. It means that when elements are placed in increasing order of their atomic number then after regular interval elements of same properties are repeated.
Mendeleev's periodic table was based on atomic masses while in Modern periodic table elements were arranged in the order of increasing atomic numbers in seven horizontal rows (period) and 18 vertical columns (groups). Elements of a group possess similar valence shell electronic configuration hence they exhibit similar chemical properties.
Study of the electronic configuration of different elements revealed that there is a close relationship between the electronic configuration of elements and their properties. On placing elements in increasing order of their atomic number, elements of the same configuration appear after regular intervals. As a result of which elements of similar properties also appear after regular intervals. Hence periodicity in the properties of elements at a regular interval of atomic number is due to the repetition of similar valence shell electronic configuration.
Considering electronic configuration of elements Werner, Bury, Rang, etc. proposed modern periodic table or long form or extended form of periodic table. This table is also called Bohr's periodic table since Bohr, first of all, described the arrangement of electrons round the nucleus of an atom.
Present form of the periodic table
There are 7 periods and 18 groups in the modern periodic table. The detail of the periods and groups are as follows:
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Like Mendeleev's periodic table, the modern periodic table also has seven blocks which are called periods. They are numbered 1, 2, 3, 4, 5, 6 and 7 from top to bottom. The number of a period is in accordance with the highest principal quantum number of the elements in the period. First is very short period containing two elements, second and third are short periods containing eight elements each, fourth and fifth are long periods having 18 elements each, sixth is very long period having 32 elements out of which 14 elements are placed in a horizontal row below the table seventh is incomplete period having 28 elements. Theoretically, there should be 32 elements, rest of the elements are yet to be discovered. Apart from it, the sixth period has 14 elements (atomic number 58 to 71), which are called lanthanides and the 14 elements of the seventh period (atomic number 90 to 103) are called actinides. They are kept in separate rows at the bottom of the periodic table.
The modern periodic table has 18 vertical blocks which are called groups: A number of ways are there to give numbers to these groups:
Characteristics of Long form of periodic table or Modern periodic table
The important characteristics of modern periodic table are as below:
Advantages of modern periodic table
The long form of the periodic table has eliminated most of the drawbacks of Mendeleev's periodic table. Some of them are below:
Most of the drawbacks of Mendeleev's periodic table have been eliminated in the long form of periodic table but some drawbacks of Mendeleev's periodic table have not been removed still in the modern periodic table, such as:
Hakimuddin Kuwakhedawala is based in India with over 15 years experience as a teacher. Teaching and writing are his passion. Most of the articles of Hakimuddin are related to education, exam, environmental problems, etc.
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The basic difference between the old periodic table and the modern metallic table is well explained in the write-up. Atomic mass of an element is also dependent on an atomic number. In many cases the higher the atomic number higher the atomic mass. As such the sequence followed in both the tables is almost same except a few changes. The chemical reactivity of a particular element is depending on the atomic number than mass. So it is more logical to follow the modern periodic table which is definitely a refined format of the original periodic table.