Question

The mass number of an atom is 31. The M shell of this atom contains 5 electrons. How many neutrons does this atom have?

• A. 31
• B. 16
• C. 15
• D. 5

Answer 1

Answer:

B. 16

To find the number of neutrons, follow these steps:

1. The mass number AA of the atom is given as 31.

2. The M shell contains 5 electrons. This means the total number of electrons (and hence protons, since atom is neutral) is calculated considering:

   • K shell: 2 electrons

   • L shell: 8 electrons

   • M shell: 5 electrons
     Total electrons = 2 + 8 + 5 = 15

3. Number of protons ZZ = Number of electrons = 15

4. Number of neutrons N=A−Z=31−15=16N=A−Z=31−15=16

So, the atom has 16 neutrons.