Present basis for the classification of elements modern periodic table

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05 Periodic table

Scientists assumed that the properties of elements are periodic functions of their atomic masses and based on this assumption, Mendeleev placed 63 elements in a vertical column called groups and in horizontal rows called periods.
However, this method of classification could not explain the position of certain elements, rare earth metals, and isotopes. So, it was rejected.
So, Henry Mosley, in the year 1923, gave a new property of elements “Atomic number”. He considered that the atomic number of an element a more fundamental property than the atomic mass.

According to modern periodic law: The properties of elements are a periodic function of their atomic numbers.
When the elements are arranged according to increasing atomic numbers, then the elements having same valence electrons occur at regular intervals.

The arrangement of elements in the modern (long form) periodic table is based on their electronic configurations.

The modern periodic table consists of 18 groups and 7 periods.
Periods: The horizontal rows of elements in a periodic table are called periods. There are seven periods in the long form of periodic table.
- First Period – 2 elements- Very Short Period
- Second Period- 8 elements- Short Period
- Third period-8 elements- Short period
- Fourth period- 18 elements- long period
- Fifth period- 18 elements- long period
- Sixth period- 32 elements- Very long period
- Seventh period- 32 elements- Very long period
The number of elements in a period is fixed by the maximum number of electrons which can be accommodated in the various shells of an atom.
Groups: The vertical columns in a periodic table are called groups. There are 18 groups in the long form of periodic table.
All the elements in a group have similar electronic configuration and show similar properties.

Group number	Group name	Property
Group 1 or IA	Alkali metals	They form strong alkalis with water
Group 2 or IIA	Alkaline earth metals	They also form alkalis but weaker than group 1 elements
Group 13 or IIIA	Boron family	Boron is the first member of this family
Group 14 or IVA	Carbon family	Carbon is the first member of this property
Group 15 or VA	Nitrogen family	This group has non-metals and metalloids
Group 16 or VIA	Oxygen family	They are also known as chalcogens
Group 17 or VIIA	Halogen family	The elements of this group form salts.
Group 18	Zero group	They are noble gases and under normal conditions they are inert.

The diagonal line at the right side of the table separates the elements into two groups: the metals and the nonmetals.
The elements that are on the left of this line tend to be metals, while those to the right tend to be nonmetals.

Here, the metals have been separated from non-metals by some elements called ‘metalloids’ which are placed diagonally in the periodic table.
These metalloids are: Boron (B); Silicon (S); Germanium (Ge); Arsenic (As); Antimony (Sb).