Dear aspirant,
As per your query,Firstly you cannot gave drdo exam with Bsc. Chemistry as  the minimum eligibility criteria is that  Candidates having first class bachelor degree (minimum 60 % marks).
First class Bachelor's degree in Engineering or Technology in Following Branches:

• Electronics & Communication Engineering
• Mechanical Engineering
• Computer Science & Engineering
• Chemical Engineering
• Electrical Engineering
• Instrumentation Engineering
• Metallurgical Engineering.

Hope it helps.