Modern Periodic Table

The periodic table based on the modern periodic law is called the Modern Periodic Table. Presently, the accepted modern periodic table is the Long Form of Periodic Table. It may be regarded as an extended form of Mendeleev’s table in which the subgroups A and B have been separated.

Features of Long Form of Periodic Table

The long form of periodic table helps to understand the reason why certain elements resemble one another and why they differ from other elements in their properties. The arrangement of elements in this table is also in keeping with their electronic structures (configuration). It is divided into columns and rows. The columns represent the groups or family and the rows represent the periods.

Groups

There are 18 vertical columns in the periodic table. Each vertical column is called a group. The groups have been numbered from 1 to 18 (in Arabic numerals).

All elements present in a group have similar electronic configurations and have same number of valence electrons. In case of group 1 (alkali metals) and group 17 elements (halogens), as one moves down a group, more and more shells are added.

All elements of group 1 have only one valence electron. Li has electrons in two shells, Na in three, K in four and Rb has electrons in five shells. Similarly all the elements of group 17 have seven valence electrons however the number of shells is increasing from two in fluorine to five in iodine.

Periods

There are seven horizontal rows in the periodic table. Each row is called a period. The elements in a period have consecutive atomic numbers. The periods have been numbered from 1 to 7 (in Arabic numerals).

In each period a new shell starts filling up. The period number is also the number of the shell which starts filling up as we move from left to right across that particular period.

For example, in elements of 3rd period (N = 3), the third shell (M shell) starts filling up as we move from left to right. The first element of this period, sodium (Na 2,8,1) has only one electron in its valence shell (third shell) while the last element of this period, argon (Ar 2,8,8) has eight electrons in its valence shell.

  • The first period is the shortest period of all. It contains only two elements - H and He.
  • The second and third periods are called short periods containing 8 elements each.
  • The fourth and fifth periods are long periods containing 18 elements each.
  • The sixth and seventh periods are very long periods containing 32 elements each.

Types of Elements

1. Main Group Elements

The elements present in groups 1 and 2 on left side and groups 13 to 17 on the right side of the periodic table are called representative or main group elements. Their outermost shells are incomplete, which means their outermost shell has less than eight electrons.

2. Noble Gases

Group 18 on the extreme right side of the periodic table contains noble gases. Their outermost shells contain 8 electrons except He which contains only 2 electrons. Their main characteristics are:

  1. They have 8 electrons in their outermost shell (except He which has 2 electrons).
  2. Their combining capacity or valency is zero.
  3. They do not react and so are almost inert.
  4. All the members are gases.

3. Transition Elements

The middle block of periodic table (groups 3 to 12) contains transition elements. Their two outermost shells are incomplete. Since these elements represent a transition (change) from the most electropositive element to the most electronegative element, they are named as transition elements.

Their important characteristics are:

  1. All these elements are metals and have high melting and boiling points.
  2. They are good conductors of heat and electricity.
  3. Some of these elements get attracted towards magnet.
  4. Most of these elements are used as catalyst.
  5. They exhibit variable valencies.

4. Inner Transition Elements

These elements, also called rare-earth elements, are shown separately below the main periodic table. These are two series of 14 elements each. The first series called lanthanoids consists of elements 58 to 71 (Ce to Lu). They all are placed along with the element 57, lanthanum (La) in the same position (group 3, period 6) because of very close resemblance between them. It is only for the sake of convenience that they are shown separately below the main periodic table.

The second series of 14 rare-earth elements is called actinoids. It consists of elements 90 to 103 (Th to Lr) and they are all placed along with the element 89, actinium (Ac) in the same position (group 3, period 7) but for convenience they are shown below the main periodic table.

In all rare-earths (lanthanoids and actinoids), three outermost shells are incomplete. They are therefore called inner transition elements. The element lanthanum is not a lanthanoid and the element actinium is not an actinoid.

5. Metals

Metals are present in the left hand portion of the periodic table. The strong metallic elements; alkali metals (Li, Na, K, Rb, Cs, Fr) and alkaline earth metals (Be, Mg, Ca, Sr, Ba, Ra) occupy groups 1 and 2 respectively.

6. Non-metals

Non-metals occupy the right hand portion of the periodic table. Strong non-metallic elements i.e., halogens (F, Cl, Br, I, At) and chalkogens (O, S, Se, Te, Po) occupy groups 17 and 16 respectively.

7. Metalloids

Metalloids are the elements that show mixed properties of both metals and non-metals. They are present along the diagonal line starting from group 13 (Boron) and going down to group 16 (Polonium).

Merits of the Modern Periodic Table

The following points overcame the defects of Mendeleev’s periodic table, that is why, it was accepted by scientists across the world.

  1. Position of isotopes: All isotopes of an element have the same atomic number and therefore, occupy the same position in the modern periodic table.

  2. Anomalous pairs: The anomaly regarding all these pairs disappears when atomic number is taken as the basis for classification. For example, cobalt (at. no. 27) would naturally come before nickel (at. no. 28) even though its atomic mass is little more than that of nickel.

  3. Electronic configuration: This classification is according to the electronic configuration of elements, i.e., the elements having a certain pattern of electronic configuration are placed in the same group of the periodic table. It relates the properties of elements to their electronic configurations.

  4. Separation of metals and non-metals: The position of metals, non-metals and metalloids are clearly established in the modern periodic table.

  5. Position of transition metals: It makes the position of the transition elements quite clear.

  6. Properties of elements: It reflects the differences, the trends and the variations in the properties of the elements in the periodic table.

  7. This table is simple, systematic and easy way of remembering the properties of different metals.