Long form of the Periodic Table-Bohr's model:
The periodic table which is most commonly used is the extended or the long from of the periodic table.This is also called bohr's table since it follows bohr's scheme (n+1) rule for arrangement of various electrons around the nucleus.In the long from of the periodic table,the elements are arranged in order of increasing atomic number so that they fall into a number of vertical columns and horizontal rows.The vertical columns are called groups or families while horizontal rows are known as periods.These are discussed below:Periods:The horizontal rows in the periodic table are called periods.There are seven periods in all. These are numbered from I to VII from top bottom.
With the exception of first period,each period starts with an alkali metal and ends up with a noble gas.The number of a period in table represents the outermost energy level of the atom in which the electron enters.
(1) First period contains 2 elements,because for n=1,there is only one orbital,(i.e.,1s) which can accommodate only two electrons.Therefore,first period has only two elements.These are hydrogen (1s1) and helium (1s2).
(2) Second period contains 8 element,because for n=2, there are four orbitals (one 2s and three 2p) which can accommodate 8 electrons.Therefore,second period contains eight elements.It starts with lithium (z=3) in which one electron enters the 2s orbital and ends up with neon (z=10) in which the second shell is complete (2s2,2p6).
(3) Third period contains 8 elements,because for n=3,there are nine orbitals (one 3s, three 3p and five 3d). However,we know from energy level diagram for multielectron atoms (unit as) that 3d orbitals are higher in energy than 4s orbitals.Therefore,3d orbitals,are filled only after filling 4s orbitals.Hence this period involves the filling of only four orbitals (one 3s and three 3p)and contains eight elements from sodium (z=11)to argon (z=18).
(4) Fourth period contains 18 elements, because for n =4,there are 16 orbitals (one 4s, three 4p,five 4d and seven 4f).However,we know from energy level diagram for multielectron atom (unit-as) that 4d and 4f orbitals are higher in energy than 5s orbitals.Therefore,4d and 4f orbitals are filled after the filling of 3d orbitals takes place.Hence this period involves the filling of only nine orbitals (one 4s,five 3d and three 4p).Therefore,fourth period contains eighteen elements from potassium (z=19) to krypton (z=18).
(5) Fifth period,like fourth period,also contains 18 elements. It begins with rubidium (z=37) in which one electron enters the 5s orbital and ends up with xenon (Z=54) in which the filling of 5p orbitals is complete.
(6) Sixth period contains 32 elements (S=55to86) and successive electrons enter into 6s,4f,5d and 6p orbitals in that order.It begins with cesium (z=5) and ends up with radon (z=86).
(7) Seventh period,though expected to have 32 elements,it is incomplete and contains only 24 elements at present.
In order to avoid the table becoming too long,14-lanthanoides occuring in the sixth period and 14-actinoides occurring in the seventh period have been placed at the bottom of the periodic table just as in the mendeleev's table.
Groups:
The vertical rows in the periodic table are called groups.There are eighteen groups in the long form of the periodic table.These are numbered from 1 to 18. Each group consists of a number of elements having the same electronic configuration of the outermost shell (valence shell).
(1) The elements of the first two group i.e.,1 and 2 on the extreme left involve the filling of s-orbitals while those of the last six groups.
(2) The elements placed in between the group 2 and 13 are called transition elements.
(3) Two series of elements each containing 14 elements are placed at the bottom of the periodic table. The first series of elements,with z=58 to z=71 which occur in the periodic table after lanthanum are called lanthanoides while the second series of elements with z=90 to z=103 which occur in the periodic table after actinium are called actinoides.