Modern Periodic Law

Modern Periodic Law

In 1913, a British Physicist Henry Moseley showed that the atomic number is a more fundamental property of an element than its atomic weight. This observation led to the development of modern periodic law. The modern periodic law states that ' the physical and chemical properties of the elements are periodic function of their atomic numbers.'

This means that when the elements are arranged in order of increasing atomic numbers, the elements with similar properties recur after regular intervals. The periodic repetition is called periodicity. The physical and chemical properties of the elements are related to the arrangement of electrons in the outermost shell. Thus, if the arrangement of electrons in the outermost shell (valence shell) of the atoms is the same, their properties will also be similar. For example, the valence shell configurations of alkali metals show the presence of one electron in the s-orbital of their valence shells.

Similar behaviour of alkali metals is attributed to the similar valence shell configuration of their atoms. Similarly, if we examine the electronic configurations of other elements, we will find that there is repetition of the similar valence shell configuration after certain regular intervals with the gradual increase of atomic number. Thus we find that the periodic repetition of properties is due to the recurrence of similar valence shell configuration after certain intervals. It is observed that similarity in properties is repeated after the intervals of 2, 8, 18, or 32 in their atomic numbers.

Long form of the Periodic Table: The periodic table is constructed on the basis of repeating electronic configurations of the atoms when they are arranged in the order of increasing atomic numbers. The long form of the Periodic table is given in a modified form in page number 70. Readers are advised to follow the periodic table closely while studying the structural features of the long form of the Periodic Table.

Structural Features of the Long form of the periodic Table: The long form of the periodic table consists of horizontal rows called periods and vertical columns called groups.

Periods: In terms of electronic structure of the atom, a period constitutes a series of elements whose atoms have the same number of electron shell i.e., principal quantum number (n). There are seven periods and each period starts with a different principal quantum number.

The first period corresponds to the filling of electrons in the first energy shell (n = 1). Now this energy level has only one orbital (1s) and, therefore, it can accommodate two electrons. This means that there can be only two elements (hydrogen, 1s¹ and helium, 1s² ) in the first period.

The second period starts with the electron beginning to enter the second energy shell (n = 2). Since there are only four orbitals (one 2s-and three 2p- orbitals) to be filled, it can accommodate eight electrons. Thus, second period has eight elements in it. It starts with lithium (Z = 3) in which one electron enters the 2s-orbital. The period ends with neon (Z = 10) in which the second shell is complete (2s²2p⁶).

The third period begins with the electrons entering the third energy shell (n = 3). It should be noted 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 to n = 3 are filled before fourth energy level begins to be filled. Hence, third period contains only eight elements from sodium (Z = 11) to argon (Z = 18).

The fourth period corresponding to n = 4 involves the filling of one 4s and three 4p-orbitals (4d and 4f orbitals have higher energy than 5s-orbital and are filled later). In between 4s and 4p-orbitals, five 3d-orbitals are also filled which have energies in between these orbitals. Thus, altogether nine orbitals (one 4s, five 3d and three 4p ) are to be filled and therefore, there are eighteen elements in fourth period from potassium (Z = 19) to krypton (Z = 36). The elements from scandium (Z = 21) to zinc (Z = 30) are called 3d- transition series.

The fifth period beginning with 5s-orbital (n=5) is similar to fourth period. There are nine orbitals (one 5s, five 4d and three 5p) to be filled and, therefore, there are eighteen elements in fifth period from rubidium (Z = 37) to xenon (Z = 54).

The sixth period starts with the filling of 6s-orbitals (n= 6). There are sixteen orbitals (one 6s, seven 4f, five 5d, and three 6p) in which filling of electrons takes place before the next energy level starts. As such there are thirty two elements in sixth period starting from cesium (Z = 55) and ending with radon (Z = 86). The filling up of 4f orbitals begins with cerium (Z = 58) and ends at lutetium (Z = 71). It constitutes the first f-inner transition series which is called lanthanide series.

The seventh period begins with 7s-orbital (n = 7). It would also have contained 32 elements corresponding to the filling of sixteen orbitals (one 7s, seven 5f, five 6d and three 7p), but it is still incomplete. At present there are 23 elements in it. The filling up of 5f- orbitals begins with thorium (Z = 90) and ends up at lawrencium (Z = 103). It constitutes second f-inner transition series which is called actinide series. It mostly includes man made radioactive elements. In order to avoid undue extension of the periodic table the 4f and 5f- inner transition elements are placed separately.

The number of elements and the corresponding orbitals being filled are given below.

     Principal     Orbitals      Electrons to          Number of

Period    Valence       being filled  be accommo-       

     shell (=n)    up     dated electrons

First       N = 1 1s      2        2

Second   N = 2 2s, 2p         2+6   8

Third      n = 3 3s, 3p         2+6   8

Fourth    n = 4 4s, 3d, 4p   2 +10+6      18

Fifth       n = 5 5s, 4d, 5p   2+10+6       18

Sixth      n = 6 6s, 4f, 5d, 6p        2+14+10+6 32

Seventh  n = 7 7s, 5f, 6d, 7p        2+14+10+6 32

The first three periods containing 2, 8 and 8 elements respectively are called short periods, the next three periods containing 18, 18 and 32 elements respectively are called long periods.

Groups

A vertical column in the periodic table is known as group. A group consists of a series of elements having similar configuration of the outer energy shell. There are eighteen vertical columns in long from of the periodic table. According to the recommendations of the International Union of Pure and Applied Chemistry (IUPAC), these groups are numbered from 1 to 18. Previously, these were numbered from I to VII as A and B, VIII and zero groups elements. The elements belonging to the same group are said to constitute a family. For example, elements of group 17 (VII A) constitute halogen family.