Modern Periodic Law and Present Form of Periodic Table
According to the modern periodic law, the properties of the elements and their compounds are a periodic function of their atomic numbers. Thus, in the modern periodic table, atomic number (which is equal to the nuclear charge) forms the basis of the classification of elements.
Present Form of Periodic Table
There are several forms of the periodic table. The most popular version is the long form. In that table, there are seven horizontal rows called periods. Each period starts with a new principal quantum number, n, and the electrons are filled up in orbitals according to the Aufbau principle. The first period has two elements, hydrogen (1s1), and helium (1s2), and the first shell (K) is completed. The second period starts with n = 2, and has eight elements. Starting with lithium (2s1), it ends with neon (2s2 2p6) and thus completes the second shell (L). In the third period, shell M starts getting filled, (n=3), and also contains eight elements. It starts with sodium (3s1) and completes at argon (3s2 3p6).
In contrast to the second period, the outer shell is not completely filled in with elements of the third period, and the orbitals of the 3d subshell remain vacant. Elements in which the s subshell is filled are called s-block elements and those in which the p subshell is filled are known as p-block elements.
In the fourth period, the N shell (n = 4) starts filling with potassium (4s1) and is completed with krypton (4s24p6). It has 18 elements, i.e. ten more than in the third period. This is because of the elements in which the filling up of electrons in the 3d orbitals takes place after 4s orbital but before 4p orbitals:
21Sc 1s2 2s2 2p6 3s2 3p6 3d1 4s2
22Ti 1s2 2s2 2p6 3s2 3p6 3d2 4s2
30Zn 1s2 2s2 2p6 3s2 3p6 3d10 4s2
Elements whose d orbitals are filled are called d-block elements. In d-block elements of the fourth period, the M shell is filled until it contains 18 electrons. Similarly, the fifth period (n = 5) has 18 elements. It starts with rubidium (5s1) and ends with xenon (5s2 5p6). The sixth period (n = 6) contains 32 elements in which the electrons enter in 6s, 4f, 5d and 6p orbitals, in that order. It begins with two s elements (Cs and Ba) followed by lanthanum (La) in which the d orbitals of the penultimate shell begin to be filled. But immediately after this, we have 14 elements (58Ce to 71Lu) in which 4f orbitals are in the process of getting filled. Then, the 5d orbitals are filled (from 72Hf to 80Hg) and finally the period ends with six p block elements (81Tl to 86Rn). The seventh period (n = 7) contains two s block elements (87Fr and 88Ra), followed by a d block elements actinium (89Ac), and 14f block elements (90Th to 103Lr) and again ends with d block elements (Z = 104 to 107). This is an incomplete period.
The vertical columns in the periodic table are called groups or families of elements. According to the latest IUPAC (International Union of Pure and Applied Chemistry) recommendation, the groups are numbered from 1 to 18.
Types of Elements
The elements can be classified into four types depending on their electronic configurations (Table 5.4). Thus, we have:
- Noble gases
- Representative elements (s-and p-block elements)
- Transition elements (d-block elements)
- Inner transition elements (f-block elements)
Noble gases constitute group 18 of the periodic table. Except the first element of the group, helium (which has 1s2 configuration) all other elements, namely neon, argon, krypton, xenon and radon have ns2 np6 electronic configuration in the outermost shell. Because of the stable arrangement of electrons in these elements, they exhibit a very low chemical reactivity.
Elements belonging to group 1 (alkali metals; Li ... Fr) with outermost electronic configuration ns1 and those belonging to group 2 (alkaline earth metals; Be... Ra) with outermost electronic configuration ns2 are placed in the s block. Elements of groups 13 to 17 (outermost electronic configuration varying from ns2 np1 to ns2 np5) belong to the p block of the periodic table. The elements of s and p blocks are collectively called representative elements.
Noble gases are at the end of each period of the representative elements; these are also grouped with representative elements. The chemistry of these elements depends on the number of electrons in the valence shell (outermost shell). For groups 1 and 2, the group number indicates the number of valence electrons. Subtracting 10 gives the number of valence electrons for groups 13 to 17 from the group number.
Elements belonging to groups 3 to 12 (in the middle of the periodic table) with an outer electronic configuration (n - 1) d1-10 ns1-2 constitute the d block of the periodic table and are called transition elements. In these elements n is 4, 5 or 6 with corresponding filling of 3d, 4d or 5d orbitals. They are all metals and are characterized by variable oxidation states, formation of coloured ions and complexes.
Inner Transition Elements
At the bottom of the periodic table, there are two rows-one of lanthanides or lanthanide series (z = 58 to 71) and the other of actinides or actinide series (Z = 90 to 107) containing incomplete 4f and 5f orbitals respectively. They also have incomplete (n - 1) d orbitals and are characterized by the outer electronic configuration (n - 2) f1-14 (n - 1) d0-1 ns2. As in each of these series, an inner f electron is added to each element, the two series of elements are called f-block elements or inner transition elements. Among themselves, lanthanides and actinides show similar properties. All the elements of the two series are metals.
Through this periodic classification, we can organize and systematize the study of elements and their compounds. It is seen that the electronic configurations of elements are directly or indirectly related to their physical as well as chemical properties. From this classification, we can understand the cause of periodicity in properties and general trends in the behaviour of elements.