Neils Bohr constructed the long form of the periodic table based on the electronic configurations of the elements. In the table, the vertical columns are termed as "groups" and the horizontal rows as 'periods'. There are 18 groups and 7 periods in the periodic table. All the elements are arranged in the increasing order of their atomic numbers and the atomic number increases by one unit from one element to the immediate next element as we move, from left to right in the periodic table.
Differentiating Electron
The electron by which the electron configuration of the given element differs from that of its preceeding element is called the differentiating electron and thatelectron is the last entering electron of its atom._
Periods in the long form of the periodic table: In each period, the differentiating electron enters the "s " orbital in the first element and "p " orbital in the last element. When s and p orBitals are completely filled with electrons as ns2np6, the element is the noble gas with stable configuration of octet.
Table 2.2: Periods and energy levels in periodic table
Period | Energy levels | Number of elements in the period | |||
1st period | Is | 2 | |||
2nd period | 2s | 2p | 8 | ||
3rd period | 3s | 3P | 8 | ||
4th period | 4s | 3d | 4p | 18 | |
5th period | 5s | 4 d | 5p | 18 | |
6th period | 6s | 4f | 5 d | 6p | 32 |
7th period | 4 s | 5f | 6 d | - | - |
Table 2.3 : Electronic Configurations of the First and Last group i.e. zero group elements in the Periods
First | Electronic | Last - | Electronic | |
Period | element | configuration | element | configuration |
1 | H | Is ' | He | Is 2 |
2 | Li | [He ] 2s 1 | Ne | [He 1 2s 2 2p 6 |
3 | Na | [Ne]3s1 | Ar | TM?13s23p6 |
4 | K | [Ar ] 4s 1 | Kr | [Ar ] 3d10 4s 2 4p6 |
5 | Rb | [Kr] 5s ' | Xe | [Kr \4di0 5s 25p6 |
6 | Cs | [Xe ] 6s 1 | Rn | [Xe ] 4/14 5d10 6s2 6p6 |
7 | Fr | IRn] Is 1 | - | - |
Each electron in the atom is distinguished by four quantum numbers. Among the four quantum numbers, the principal quantum number (n) defines the main energy levels of the shell. In the periodic table, as the shells are filled with electrons in the order n =1 ,n =2,n =3,..., the periods are formed serially.
In the first period, as the first energy level (K shell) is filled with two electrons, there are only two elements. They are hydrogen(Z- l;ls,!) and helium (Z= 2; Is2). In the second period, the differentiating electron enters the 2s orbital (2nd shell) in lithium and the second energy level is completely filled 2s2.2p6 with electrons in neon. Thus the second principal energy level is now completely filled with eight electrons and hence the second period has eight elements. In the third period, the differentiating electron enters 3s orbital in sodium, (Z = 11, ... 3s,1 ). After "3s " is completed, 3p orbitals get filled with differentiatirfg electrons upto argon (Z = 18,... 3s2 3p6). Then the differenting electron does not enter 3d orbitals but enters ' 4s ' orbital. Hence the 3rd period also has only eight elements.
In the fourth period, the energy level 4s ; is first filled i.e. from potassium (Z= 19,... 4s1). After 4s is filled the differentiating electron enters 3d orbitals from scandium (Z = 21, ... 3d14s2) to zinc (Z = 30 3d10 4s2)and these ten elements are called first transition series (3d transition series )or d block elements. Then the differentiating electron enters the 4p Orbitals upto krypton (Z = 36, ... 3d104s24p6). Krypton attains stability due to completely filled orbitals(4s2 4p6). Thus, the fourth period has 18 elements. .
In the fifth period, the energy levels 5s, 4 J and 5p are filled in a similar order with differentiating electrons as in the fourth' period. The period starts with rubidium (Z= 37,... 5s1) and ends with xenon(Z = 54, ... 5s24d105p6 ). The fifth period has 18 elements which include second transition, series(4d transition series) i.e. d. block elements from yttrium (Z = 39) to cadmium (Z = 48).
In the sixth period, the energy levels 6s,4f, 5d,and 6p get filled with differentiating electrons in the same sequence. After 6s orbital is filled, the differentiating electrons enter the 4f orbitals to form the first inner transition series with an exception of La (Z=57) that gets the differentiating e-1 into 5d. 14 elements from cerium (Z= 58) to lutetium (Z = 71) get their diffrentiating electrons into 4f orbitals. These elements are therefore called 'f' block elements. Then 5d orbitals get filled, to give the third transition series (5d transition series) or d -block elements and next 6p orbitals get filled upto radon (Z = 86). The sixth period is the longest period with a total of 32 elements.
The seventh period is incomplete and has about 20 elements. The period includes the second inner transition series (actinides) from thorium (Z = 90) to Lawrencium (Z= 103). The long form of the periodic'table is shown in the fig. 2.2.
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