In 1869 th Russian chemist Dmitri Mendleev and German scientist made similar discoveries while working independently..
They found that when they arranged the elements in order of atomic weight,they could place them in horizontal rows,one rows under the other, so that the elements in each vertical column have similar properties.
Such a tabular arrangments of elements in rows and column highlighting in regular repetition of properties of elements is called a periodic table.
But eventually more accurate determinations of atomic weights revealed discrepencies and it was shown that elements are characterized by their atomic number rather than atomic weights.When elements arranged by their atomic number such discrepancies vanished.
A modern version of the periodic table , with the elements arranged by the atomic number can be seen in the pitchure below.
Each entry lists atomic number, atomic symbol and atomic weight.Basically periodic table is divided into PERIODS ( consists of elements in any one horizontal row of the periodic table and GROUPS ( consists of elements in any one column of the periodic table.
There are 7 periods in the periodic table first period in short containing only H and He and Sixth period actually consists of 32 elements but in order for the row to fit on a page part of it appears at the bottom of the table as Lathanides.The 7th period also has some of its elements placed as arow on the bottom of the table as Actinides.
The groups are usually numbered.IUPAC suggested a convention in which columns are numbered 1 to 18.We also can see numbering in Roman numberals and A's and B's.A's are called main group elemtns; the B group are called transition elements.The two rows at the bottom of the table are called inner transition elements.
The point to note is; The elements in any one group have similar properties.For example . the elements in group IA called Alkali metals are soft metal that reacts with water. The group VII A elements known as halogens are also reactive elements.
The elements of the periodic table are divided by a heavy "staircase line" into metals on the left and nonmetals on the right.Most of the elements bordering the staicase line in the periodic table are metalloids or semi metals.