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Classification of Elements and Periodicity in Properties

Modern Periodic Table

Why are certain elements grouped together in the modern periodic table of elements? For example, we can find Helium and Argon grouped together as inert gases. Also, what is the basis of grouping elements in the periodic table? Let us find out. First, we will understand the basis for organizing elements. And then, we will learn about the characteristics of the modern periodic table.

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Modern Periodic Law

The modern periodic law states that the physical and chemical properties of the elements are the periodic functions of their atomic numbers.

Scientists arranged elements in increasing order of their atomic numbers from left to right across each row. And discovered that the elements having similar properties repeat after regular intervals.

Why atomic number and not atomic mass?

Atomic mass is the total mass of the protons and neutrons present in a nucleus of an atom. Whereas, the atomic number is the number of protons in a nucleus. Also, the number of protons in the nucleus is equal to the electrons present outside the nucleus.

We know that the nucleus is deep-seated inside an atom. But the electrons outside it, especially the ones in the outermost shell, are free to move around. Hence they take part in chemical reactions. For this reason, the properties of an element depend on the atomic number rather than the atomic mass.

Read about the Historical Development of the Periodic Table.

Long Form of Periodic Table

The modern or long form of the periodic table is based on the modern periodic law. The table is the arrangement of elements in increasing order of their atomic numbers. The modern periodic table is the present form of the periodic table. And it consists of 18 vertical columns and 7 horizontal rows.

Modern Periodic Table of elements

Modern Periodic Table

Groups in the Modern Periodic Table

  • Groups are the vertical columns in the modern or long form of the periodic table.
  • There are 18 groups in the periodic table.
  • These groups are numbered from 1 to 18.
  • Each group consists of elements having the same outer shell electronic configuration.

Periods in the Modern Periodic Table

  • Periods are the horizontal rows in the modern or long form of the periodic table.
  • There are 7 periods in the periodic table.
  • These are numbered as 1, 2, 3, 4, 5, 6 and 7 from top to bottom.
  • The 1st period consists of only two elements – Hydrogen and Helium.
  • While the 2nd and 3rd period consists of 8 elements each.
  • The 4th and 5th period consists of 18 elements each
  • On the other hand, the 6th period consists of 32 elements.
  • The 7th period of the periodic table now has four new elements. They are 113-Nihonium, 115-Moscovium, 117-Tennessine, and 118 –Oganesson. This addition has completed the 7th period with 32 elements.
  • Also, the long form of the periodic table consists of a separate panel at the bottom. It consists of 14 elements of the 6th period called the lanthanoids. And 14 elements in the 7th period called the actinoids.
  • Each period represents the number of shells or energy levels present in an atom of an element.

Cause of Periodicity in the Modern Periodic Table

  • The cause of periodicity in properties is the repetition of similar outer electronic configuration after certain regular intervals.
  • For example, all the elements of group 1 i.e. alkali metals have a similar outer electronic configuration, ns1. Here n refers to the Principal Quantum Number of the outermost shell.
  • In the same way, the elements of group 17 have a similar outer electronic configuration, ns2 np5. Hence have similar properties to one another. They are halogens.
  • Similarly, the elements of group 18 have an outer electronic configuration of ns2 np6. They have full filled orbitals. They comprise of non-reactive elements called Inert gases. Helium and Argon have electron configuration similar to the elements of this group. So they are together and known as Inert gases!
  • Likewise, elements in a group have similar properties to those of all the other members of the same group. This is because they all have a similar electron configuration of the outer shell.

Read about the Metallic and Non-Metallic Characters here.

Furthermore, based on the entry of the valence electrons in the respective sub-shell, the periodic table is divided into s, p, d and f block.

Modern Periodic Table of elements

Learn more about D and F Block elements here.

A Solved Question for You

Q: What is the basic difference in approach between the Mendeleev’s and the Modern Periodic Law?

Ans: Mendeleev’s Periodic Law states that the properties of elements are periodic functions of their atomic weightsWhereas, the Modern Periodic Law states that the properties of elements are periodic functions of their atomic numbers.

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