Question

how many normal modes of vibration are possible for a (1) linear molecule and (2) non- linear molecules​

Answer 1

Answer:

In general, a non-linear molecule with N atoms has 3N – 6 normal modes of vibration, but a linear molecule has 3N – 5 modes, because rotation about the molecular axis cannot be observed. A diatomic molecule has one normal mode of vibration, since it can only stretch or compress the single bond

Answer 2

ɪɴ ɢᴇɴᴇʀᴀʟ, ᴀ ɴᴏɴ-ʟɪɴᴇᴀʀ ᴍᴏʟᴇᴄᴜʟᴇ ᴡɪᴛʜ ɴ ᴀᴛᴏᴍs ʜᴀs 3ɴ – 5 ɴᴏʀᴍᴀʟ ᴍᴏᴅᴇs ᴏғ ᴠɪʙʀᴀᴛɪᴏɴ, ʙᴜᴛ ᴀ ʟɪɴᴇᴀʀ ᴍᴏʟᴇᴄᴜʟᴇ ʜᴀs 3ɴ – 5 ᴍᴏᴅᴇs, ʙᴇᴄᴀᴜsᴇ ʀᴏᴛᴀᴛɪᴏɴ ᴀʙᴏᴜᴛ ᴛʜᴇ ᴍᴏʟᴇᴄᴜʟᴀʀ ᴀxɪs ᴄᴀɴɴᴏᴛ ʙᴇ ᴏʙsᴇʀᴠᴇᴅ. ᴀ ᴅɪᴀᴛᴏᴍɪᴄ ᴍᴏʟᴇᴄᴜʟᴇ ʜᴀs ᴏɴᴇ ɴᴏʀᴍᴀʟ ᴍᴏᴅᴇ ᴏғ ᴠɪʙʀᴀᴛɪᴏɴ, sɪɴᴄᴇ ɪᴛ ᴄᴀɴ ᴏɴʟʏ sᴛʀᴇᴛᴄʜ ᴏʀ ᴄᴏᴍᴘʀᴇss ᴛʜᴇ sɪɴɢʟᴇ ʙᴏɴᴅ