Classification ...

In the modern periodic table. the elements are placed in order of increasing atomic number. The long form of periodic table shows all the elements in numerical order. The start of a new period always corresponds to the introduction if the first electron into the $$s-$$ orbital of a new principal quantum number. The number of elements in each period corresponds to the number of electrons required to fill the orbitals according to Aufbau principle.

In the modern periodic table. the elements are placed in order of increasing atomic number. The long form of periodic table shows all the elements in numerical order. The start of a new period always corresponds to the introduction if the first electron into the $$s-$$ orbital of a new principal quantum number. The number of elements in each period corresponds to the number of electrons required to fill the orbitals according to aufbau principle.
$$1s, 2s 2p, 3s 3p, 4s 3d 4p, 5s 4d 5p, 6s 4f 5d 6p, 7s 5f 6s 7p$$
The main groups correspond to elements in which $$s$$ and $$p$$ orbitals are being filled. The transition elements are those in which $$d-$$ orbitals are being filled. Each group contains elements of similar electronic configuration. According to the recommendation of $$IUPAC$$, the group of elements are numbered from $$1$$ to $$18$$. The elements corresponding to filling of $$4f-$$ orbitals are called lanthanons and those corresponding to the filling of $$5f-$$ orbitals are called actinons. These elements are accommodated into two separate horizontal rows below the table.
There are only $$81$$ table elements known. Form these elements one or more isotopes do not undergo spontaneous radioactive decay. No stable isotope exists for any elements above bismuth. Two elements, uranium and thorium for which only radioactive isotopes exist, are found quite abundantly on earth because the half - lives of some of their isotopes are almost as great as the age the earth.