Modern Periodic Table

  • This classification was proposed by Henry Moseley in 1913
  • In this classification, Mendeléev’s law was modified and atomic number (instead of atomic mass) was taken as the basis of classification. 

Note: Atomic number is the number of protons in an atom.

          Atomic mass is the sum of number of protons and neutrons in an atom

  • Elements were arranged in increasing order of atomic number .
    (Atomic number is equal to the number of protons in the nucleus of an atom.)
  • The law was modified to;

‘The properties of elements are the periodic function of their atomic number’

  • This led to the classification known as Modern Periodic Table.
  • Modern Periodic Table takes care of the limitations we saw in Mendeléev’s periodic table
  • The table has 18 Groups (vertical columns) and 7 Periods (horizontal rows).

What is the increasing order of Atomic Number?

Atomic Number is the number of protons present in the nucleus of an atom.

In an atom, number of protons is normally equal to number of electrons

Hence, we can say that atoms are arranged on the basis of their electron configuration .

(Number of electrons in different shells)

 

Position of elements in Modern Periodic Table

  1. Modern Periodic Table has 18 Groups and 7 Periods.
  2. Elements that belong to the same period contain the same number of shells but different number of electrons in the valence shell .

Example,

Element

Atomic Number

*Electronic Configuration

Lithium

3

2,1

Beryllium

4

2,2

* Electronic configuration is the arrangement of electrons in energy levels around the nucleus of an atom

So Lithium is placed before Beryllium in the Periodic Table

Both Lithium and Beryllium have the same number of shells (K and L shells) and they belong to the same period on the periodic table.

We can also notice that the number of electrons in the valence shell increases by one as we go across the same period .

 

Total number of elements is each period

Based on the Bohr-Bury scheme of electronic configuration, the maximum number of electrons that can be accommodated in each shell is 2n 2 .

 

Note - For the M shell, though it can accommodate 18 electrons, the outermost shell can have only 8 electrons, so the third period also has only 8 elements .

 

Period

No. of Elements

1

2

2

8

3

8

4

18

5

18

6

32

7

Rest of the elements (The period is incomplete)

 

3. Elements that belong to the same group contain different number of shells but the number of valence electrons is the same .

Example, 

Element

Atomic Number

Electronic Configuration

Lithium

3

2,1

Sodium

11

2,8,1

Potassium

19

2,8,8,1

 

Since all have 1 valence electron (electron in outermost shell) they are placed in same group

It can be seen that they have similar properties.

Also Lithium has 2 shells (K and L) , Sodium has 3 shells (K, L, M) and Potassium has 4 shells (K, L, M, N) hence we can say that;

Number of shells increases as we go down the group.

 

Notice the trends in electronic configuration across the period and down the group in 2nd and 3rd period of the periodic table

 

Different groups in the periodic table

Group

Explanation

Alkali Metals

Group 1 metals having valency 1. They form alkalies (bases) on reaction with water

Alkaline Earth Metals

Group 2 metals having valency 2. They also form alkalies (bases) on reaction with water. "Earth" is an old term given to nonmetallic substances that are insoluble in water and resistant to heating — properties shared by these oxides.

Transition Metals

These metals belong to the 3rd to 12th groups and show variable valencies

Metalloids

These are 7 elements that show properties of both metals and nonmetals

Nonmetals

They are present towards the right hand side of the periodic table and form acidic oxides

Halogens

They are elements in group 17, having 7 valence electrons and require only one more electron to form the noble gas configuration

Noble gases

They are generally unreactive as they have completely filled outermost shell. 

Lanthanides

Lanthanides are the 15 metallic elements with atomic numbers 57–71 (from Lanthanum through Lutetium) placed separately at the bottom of the periodic table

Actinides

Actinides are the 15 metallic elements with atomic numbers 89 - 103 (from Actinium through Lawrencium) placed separately at the bottom of the periodic table

What were Limitations of Mendeléev Periodic Table which were corrected in Modern Periodic Table

  • Isotopes could not be placed in Mendeléev’s Periodic Table but they could be placed in the Modern Periodic Table as they have the same atomic number .

Example: Chlorine has 2 isotopes  Cl-35 and CL-37. Both have different atomic masses 35u ad 37u but same atomic number 17

  • Cobalt correctly placed before Nickel in Periodic Table

Atomic Mass of Nickel is 58.7 and Cobalt is 58.9. Still Cobalt was placed before Nickel in Mendeléev Periodic Table to place elements of similar properties in the same group.

However, the Atomic Number of Cobalt is 27 and Nickel is 28 so Cobalt is correctly placed before Nickel in the periodic table.

  • Noble Gases were placed correctly is a separate group in the Modern Periodic Table  

They were not placed in Mendeléev’s Periodic Table as noble gases were not discovered during that time.

They were discovered later and were placed in group 18 as the last and separate group .

Position of elements in Modern Periodic Table

Anomalous Position of Hydrogen in Periodic Table

‘Anomalous' means unusual

Why is the position of Hydrogen in the Periodic Table called anomalous?

Hydrogen does not have a fixed position in the periodic table as it resembles both Alkali Metals as well as Halogens .

However,

Atomic Number of Hydrogen is 1

So it is placed in first group and first period

It has 1 valence electron

Hence it is placed in first group

 

Difference between Mendeléev’s Periodic Table and Modern Periodic Table and 

 

Mendeléev’s Periodic Table

Modern Periodic Table

It was proposed by Dmitri Ivanovich Mendeléev in 1873 .

It was proposed by Henry Moseley in 1913

Elements were arranged in order of increasing atomic mass

Elements were arranged in order of increasing atomic number

There were 7 periods (horizontal row) and 8 groups (vertical columns) in the table

There were 7 periods (horizontal row) and 18 groups (vertical columns) in the table

This classification was less accurate and had limitations

This classification was more accurate and corrected those limitations

Isotopes could not be placed as the basis of classification was atomic mass.

Isotopes could be placed as the basis of classification is atomic number.

Some elements weren’t placed periodically. Example: Cobalt was placed after Nickel

Elements were placed periodically. Example: Cobalt was correctly placed after Nickel

Position of Hydrogen could not be justified due to similarities in properties with alkali metals and halogens.

Hydrogen correctly placed separately.

Metals, Nonmetals and Metalloids in Periodic Table

Position of Metals

Metals are placed on left hand side of Periodic Table

Position of Nonmetals

Nonmetals are placed on right hand side of Periodic Table

Position of Metalloids

Metalloids are separated by a Zig zag line or Border Line

The elements on this border lines are called semi-metal or metalloids

These borderline elements are boron, silicon, germanium, arsenic, antimony, tellurium and polonium

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CA Maninder Singh is a Chartered Accountant for the past 14 years and a teacher from the past 18 years. He teaches Science, Economics, Accounting and English at Teachoo