Database design help and PHP and SQL

Hi I have a chemistry database containing two tables: "Elements" and "Compounds". For my purposes, three (and always three) Elements make a Compound. My Elements table comprises the following fields: ID -- Name My Compounds table comprises the following fields: ID1 -- ID2 -- ID3 (ID1, ID2 and ID3 make up a triple compound key and each refers to an ID in the Elements table). I am looking for a coder to help with the following: 1. Is this schema correct? Or should I have three duplicate Elements tables and have each relate to the Compounds table? (This seems clumsy to me but if it's the correct way...) If it's (in)correct, WHY is it (in)correct? 2. I need various PHP scripts written to generate SQL statements to manipulate the above tables. Thanks Jake