1115 elements are known today and many more are being invented.
Mendeleev’s periodic table helped to discover the new elements.
Modern periodic law states, ‘Properties of elements are periodic function of their atomic number.’
Modern periodic table is based on atomic number.
Modern periodic table is based on electronic configuration of elements.
There are 7 periods (horizontal rows) and 18 groups (vertical columns) in long form of periodic table.
There are 2, 8, 8, 18, 18, 32, 29 elements in 1st, 2nd, 3rd, 4th, 5th, 6th, and 7th period of periodic table respectively.
Atomic number was discovered by Moseley.
Atomic radius goes on increasing down the group due to increase in number of shells.
Atomic radius goes on decreasing along the period due to increase in nuclear charge which results in increase in force of attraction between nucleus and valence electrons.
Ionisation energy goes on decreasing i.e., tendency to lose electron increases down the group due to increase in atomic size.
Ionisation energy goes on increasing i.e. tendency to lose electron decreases along a period from left to right due to decrease in atomic size.
Metallic character increases down the group due to decrease in ionisation energy and therefore, increase in tendency to lose electrons.
Metallic character decreases along a period due to increase in ionisation energy and therefore, decrease in tendency to lose electrons.
Metalloids are elements which resemble both with metals and non-metals, e.g., B, Si, Ge, As, Sb etc.
Second ionisation energy is energy required to remove one electron from unipositively charged ion. It is greater than 1st ionisation energy due to greater effective nuclear charge.
Melting point of elements (metals) decreases down the group, e.g., Li(180.5°C), Na, (97.5°C), K(63.4°C).
Melting point of non-metals increase down the group , e.g., F2(-219.6°C), Cl2 (-101°C), Br2(-7.2°C), I2(113.6°C).
Non-metallic character increases and metallic character (electropositive character) decreases along a period from left to right.
Lanthanides and Actinides are placed at the bottom because they do not resemble with any other group elements but resemble with each other.
Elements after U(92) are synthetic elements or man-made elements. They are also called ‘Transuranic elements”.