Question

$4$ protons, $5$ neutrons, and $4$ electrons are there in an atom of beryllium. Its mass number is:

• A. $10$
• B. $5$
  
• C. $8$
• D. $9$
  

Answer 1

The correct option is D $9$

Mass number is the sum of number of protons and number of neutrons.
Mass number of beryllium $=4+5=9$.