Question

why do you think the noble gases are placed in a separate group

Answer 1

The groups are elements with the same number of electrons in the outer shell at least for the more common elements. The noble gases like helium, neon. argon, krypton and xenon are placed in a separate group because these are inert ( un-reactive ) and present in very low concentration in the atmosphere.

Answer 2

ɴᴏʙᴇʟ ɢᴀsᴇs ᴀʀᴇ ᴄʜᴇᴍɪᴄᴀʟʟʏ ɪɴᴇʀᴛ ᴀɴᴅ ᴀʀᴇ ᴘʀᴇsᴇɴᴛ ɪɴ ᴇxᴛʀᴇᴍᴇʟʏ ʟᴏᴡ ᴄᴏɴᴄᴇɴᴛʀᴀᴛɪᴏɴ ɪɴ ᴀᴛᴍᴏsᴘʜᴇʀᴇ. ᴛʜᴜs ᴏᴡɪɴɢ ᴛᴏ ᴛʜᴇɪʀ sɪᴍɪʟᴀʀ ɪɴᴇʀᴛ ʙᴇʜᴀᴠɪᴏᴜʀ ᴀɴᴅ sɪᴍɪʟᴀʀ ᴇʟᴇᴄᴛʀᴏɴɪᴄ ᴄᴏɴғɪɢᴜʀᴀᴛɪᴏɴ, ᴛʜᴇʏ ᴀʀᴇ ᴘʟᴀᴄᴇᴅ ɪɴ ᴀ sᴇᴘᴀʀᴀᴛᴇ ɢʀᴏᴜᴘ.