ELECTRONIC CONFIGURATION AND THE POSITION OF ELEMENTS IN THE PERIODIC TABLE-The Long Form of Periodic Table
The long form of the periodic table is an improved form of the periodic table which is based upon modern periodic law. The long form of periodic table is given on page 67. Let us now, study the structural features of the periodic table.
DLSCRIPTION OF PERIODS
A horizontal row of a periodic table is called a period. A period consists of a series of elements having same valence shell. There are seven periods in all, which are numbered as 1, 2, 3, 4, 5, 6 and 7.
There is a close connection between the electronic configurations of the elements and the long form of the periodic table. As pointed out earlier in Unit 5 that the principal quantum number n defines the main energy level of the electron also called main energy shell. Each period of the periodic table begins with the filling of new energy shell. In fact, the number of the period also represents the highest principal quantum number of the elements present in it. The number of elements in each period is equal to the number of electrons which can be accommodated in the orbitals belonging to that electron shell.
The first period corresponds to the filling of electrons in first energy shell (i.e., n = 1). Now this energy level has only one orbital (i.e., n =1) and, therefore, !t can accommodate two electrons. This means that there can be only two elements in the first period.
The second period starts with the electrons beginning to enter the second energy shell (n = 2). There are only four orbitals (one 2s and three 2p orbitals) to be filled which can accommodate eight electrons. Thus, second period has eight elements in it
Long Form of Periodic Table
Fig. 6.1. Long form of periodic table
The third period begins with the electrons entering the third energy shell (n = 3). It may be recalled that out of nine orbitals of this energy level (one s, three p and five d), the five 3d orbitals have higher energy than 4s orbitals. As such only four orbitals (one 3s and three 3p) corresponding ton= 3 are filled before the fourth energy level begins to be formed. Hence. there are only eight elements in the third period.
The fourth period corresponds to n = 4. It starts with the filling of 4s-orbitals. However, after the 4s but before the 4p orbitals, there are five 3d orbitals also to be filled. Thus, in all, nine orbitals (one 4s, five 3d and three 4p) have to be filled and as such there are eighteen elements in fourth period. It may be noted that the filling of 3d-orbitals starts from SC (Z = 21). The elements from SC (Z = 21) to Zn (Z = 30) are called 3d-transition series.
Similarly. we can show that there are 18 elements in the fifth period and 32 elements in the sixth period. Seventh period is still incomplete.
DESCRIPTION OF GROUP
A vertical column of the periodic table is called a group. A group consists of a series of elements having similar configuration of the outer energy shell. For example, all the group 1 elements have ns1 valence shell electronic configuration. There are eighteen vertical columns in the long form of the periodic table. According to the recommendation of the International Union of Pure and Applied Chemistry (IUPAC), these groups are numbered from 1 to 18.
It may be noted that the elements belonging to same group are said to constitute a family. For example,
• The elements of group 1 are known as alkali metals.
• The elements of group 2 are known as alkaline earth metals.
• The elements of group 17 are known as halogens.
• The elements of group 18 are known as noble gases.