MODERN PERIODIC TABLE
In 1913, the English Physicist Henry Moseley, through his X-ray diffraction experiments, proved that the properties of elements depend on the atomic number and not on the atomic mass. Consequently, the modern periodic table was prepared by arranging elements in the increasing order of their atomic number.
This modern periodic table is the extension of the original Mendeleev’s periodic table and known as the long form of periodic table.
Atomic number of an element (Z) not only indicates the number of protons (positive charge) but also the number of electrons (negative charge). The physical and chemical properties of elements depend not only on the number of protons but also on the number of electrons and their arrangements (electronic configuration) in atoms.
Hence, the modern periodic law can be stated as follows: “The Chemical and Physical properties of elements are periodic functions of their atomic numbers”. Based on the modern periodic law, the modern periodic table is derived.
· All the elements are arranged in the increasing order of their atomic number
· The horizontal rows are called periods.
· There are seven periods in the periodic table.
· The elements are placed in periods based on the number of shells in their atoms
· Vertical columns in the periodic table starting from top to bottom are called groups. ere are 18 groups in the periodic table
· Based on the physical and chemical properties of elements, they are grouped into various families.
We know that the electrons in an atom are accommodated in shells around the nucleus. Each shell consists of one or more subshells in which the electrons are distributed in certain manner. These subshells are designated as s, p, d, and f. The maximum number of electrons that can be accommodated in s, p, d, and f are 2, 6, 10 and 14 respectively.
Based on the arrangement of electrons in subshells, the elements of periodic table are classi ed into four blocks as shown in Table 4.6
While arranging the electrons of elements of group 1 and 2, the last electron is added to s subshell. ese elements are called s-block elements. The elements of group 1 (except hydrogen) are metals. ey react with water to form solutions that change the colour of a vegetable dye from red to blue. ese solutions are said to be highly alkaline or basic. Hence they are called alkali metals.
The elements of group 2 are also metals. ey combine with oxygen to form oxides, formerly called "earths," and these oxides produce alkaline solutions when they are dissolved in water. Hence, these elements are called alkaline earth metals.
The last electron in these elements is lled in p subshells and hence these elements are called p block elements. ese elements are in group 13 to 18 in the periodic table. ey include boron, carbon, nitrogen, oxygen, uorine families in addition to noble gases (Except helium). The p-block is home to the biggest variety of elements and is the only block that contains all three types of elements: metals, nonmetals, and metalloids.
e elements of group 3 to 12 have their valence electrons in their outermost d subshells. ese elements are called d block elements. ey are found in the centre of the periodic table. eir properties are intermediate to that of s block and p block elements and so they are called transition elements.
Part of the group 3 elements have their valence electrons in inner f subshell. ey are known as f block elements or inner transition elements. ey are placed at the bottom of the periodic table. ere are two series in f block elements. The elements that follow Lanthanum are called “Lanthanides” and that follow Actinium are called “Actinides”.
· The table is based on a more fundamental property i.e., atomic number.
· It correlates the position of the element with its electronic configuration more clearly.
· The completion of each period is more logical. In a period, as the atomic number increases, the energy shells are gradually filled up until an inert gas configuration is reached.
· It is easy to remember and reproduce.
· Each group is an independent group and the idea of subgroups has been discarded.
· One position for all isotopes of an element is justified, since the isotopes have the same atomic number.
· The position of the eighth group (in Mendeleev‘s table) is also justified in this table. All transition elements have been brought in the middle as the properties of transition elements are intermediate between left portion and right portion elements of the periodic table.
· The table completely separates metals from nonmetals. The nonmetals are present in upper right corners of the periodic table.
· The positions of certain elements which were earlier misfit (interchanged) in the Mendeleev’s periodic table are now justified because it is based on atomic number of the elements.
· Justification has been offered for placing lanthanides and actinides at the bottom of the periodic table.
Hydrogen is the lightest, smallest and first element of the periodic table. Its electronic configuration (1s ) is the simplest of all the elements. It occupies a unique position in the periodic table. It behaves like alkali metals as well as halogens in its properties.
In the periodic table, it is placed at the top of the alkali metals.
· Hydrogen can lose its only electron to form a hydrogen ion (H+) like alkali metals.
· It can also gain one electron to form the hydride ion (H-) like halogens.
· Alkali metals are solids while hydrogen is a gas.
Hence the position of hydrogen in the modern periodic table is still under debate as the properties of hydrogen are unique.
The elements Helium, Neon, Argon, Krypton, Xenon and Radon of group 18 in the periodic table are called as Noble gases or Rare gases. ey are monoatomic gases and do not react with other substances easily, due to completely lled subshells. Hence they are called as inert gases. ey are found in very small quantities and hence they are called as rare gases.
These gases are chemically inert or non-reactive in nature because they have stable electronic structures which are very difficult to change.
Though they are found rare, they have many uses.
· Helium is used for filling weather balloon, as it has very low density.
· Neon gas is used in discharge lamps for the orange column.
· Argon gas is filled in electrical bulbs to prevent evaporation of the filament.
· Radon is a radioactive gas.