Question

whu some metals exists variable valency?​

Answer 1

ʜᴇʏ ᴜsᴇʀ! ʜᴇʀᴇ's ʏᴏᴜʀ ᴀɴsᴡᴇʀ..

sᴏᴍᴇ ᴍᴇᴛᴀʟs ᴇxɪsᴛ ɪɴ ᴠᴀʀɪᴀʙʟᴇ ᴠᴀʟᴇɴᴄʏ ʙᴇᴄᴀᴜsᴇ ᴛʜᴇʏ ᴄᴀɴ ᴅᴏɴᴀᴛᴇ ᴏʀ ᴀᴄᴄᴇᴘᴛ ᴍᴏʀᴇ ᴛʜᴀɴ ᴏɴᴇ ᴇʟᴇᴄᴛʀᴏɴs ᴀᴛ ᴀ ᴛɪᴍᴇ.

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Hope it's to the point.. and correct.

Answer 2

Because they can donate or losse more than 1 electron in their shell