CBSE Class 10 Answered

Placing hydrogen in the modern periodic table wasn’t easy.

Firstly hydrogen resembles Group 1 elements that are also called alkali metals.

The electronic configuration of Hydrogen is 1s¹

 And the general outer electronic configuration of alkali metals is ns¹.

Thus, hydrogen should belong to group 1.

But hydrogen also resembles elements of group 17 called as halogens.

Halogens have a general outerelectronic configuration of ns²np⁵ .They can easily accept one electron to complete its octet.

Hydrogen also needs one electron to complete its octet.

So, hydrogen resembles elements of group 17 and should belong to group 17.

 In addition to the above resemblance of hydrogen with alkali metals as well as halogens, hydrogen has some unique properties.

One of the properties being size of the ion it forms. Size of H plus ion  is approximately 1.5 x 10^-3  picometer. This size is so small that it does not exist as an individual ion. Rather it always associate with other atoms or molecules.

Thus it does not belong to either group 1 or group 17 and is rather placed separately in the  modern periodic table.