Question

Which of the following atoms contains the least number of neutrons?

• A. ${}_{92}^{235}U$
• B. ${}_{92}^{238}U$
• C. ${}_{93}^{239}Np$
• D. ${}_{93}^{240}Np$

Answer 1

The correct option is A ${}_{92}^{235}U$

The number of neutrons is the difference between the mass number and atomic number.

${}_{92}^{235}U$ contains the least number of neutrons
The number of neutrons in ${}_{92}^{235}U$ $=235-92=143$
The number of neutrons in ${}_{92}^{238}U$ $=238-92=146$
The number of neutrons in ${}_{93}^{239}Np$ $=239-93=146$
The number of neutrons in ${}_{93}^{240}Np$ $=240-93=147$