Question

The number of neutrons in the element ${ }_{4} Be ^{9}$ is :

• A. 3
• B. 5
• C. 7
• D. 9

Answer 1

Answer: (B)

5

Answer 2

We know that Atomic number = number of protons = number of electrons Mass number = number of proton $+$ number of neutrons
$\therefore$ number of neutrons = mass number $-$ atomic number
$=9-4=5$