The reason why there are 18 groups in the Periodic table is mentioned below.
- The elements can have minimum 1 to maximum 8 electrons in the outermost orbit. And based on this arrangement, there are total 18 groups in Periodic table.
All the elements found in nature have the outermost electrons between 1 to 8.
But the question is;
From where does this number 8 come from?
The answer is, 2 electrons from s-orbitals and 6 electrons from p-orbitals.
s-orbitals can hold maximum 2 electrons and p-orbitals can hold maximum 6 electrons.
Hence, there can be maximum 8 electrons in the outermost orbit of a main group elements.
If the s-orbitals and p-orbitals are completely filled with electrons, then the elements will be stable.
And if these orbitals are not completely filled, then the element won’t be stable.
Anyway, the main point here is that there can be 1 to 8 electrons in the outermost orbit.
Those elements which have 1 electron in outermost orbit, lie in the group 1.
Those elements which have 2 electrons in outermost orbit, lie in the group 2.
Similarly, there are many elements between group 3 and group 12, which have 1 and 2 valence electrons in the outermost orbit.
Elements with 3 electrons in outermost orbit are in group 13.
Elements with 4 electrons in outermost orbit are in group 14.
Elements with 5 electrons in outermost orbit are in group 15.
Elements with 6 electrons in outermost orbit are in group 16.
Elements with 7 electrons in outermost orbit are in group 17.
Elements with 8 electrons in outermost orbit are in group 18.
In this way, all the elements on the Periodic table are having minimum 1 to maximum 8 electrons in outermost orbit.
See the Periodic table with electron shells of all the elements, for better idea.
Based on these valence electrons, the classification of elements is done which gives us the final number of columns as 18.
Hence, there are 18 groups on the Periodic table.
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