An element has the mass number 31 and it has 5 electrons on the M level. Which is the element, what is it's position in periodic table and how...

In order to identify the element, first we have to find it's atomic number, Z (the number of protons). Also we know that the number of protons is equal to the number of electrons.

We don't know the number of protons but we can find out the number of electrons, knowing that the number of electrons of the M level is 5. This means that the levels K and L have been occupied with 10 electrons, 2 on the K level and 8 on the L level. The total number of electrons of the element is 2+8+5=15 electrons. That means that we have also 15 protons and, in this way, we've found out the atomic number Z=15. Searching on periodic table, we'll find on the 15th position the element phosphorus, P.

Phosphorus, P is located on the 5th group  (column) and the 3rd period (row) of the periodic table.

To find out the number of neutrons, we know that atomic mass, A=31, is equal with the sum of the number of protons, Z, and the number of neutrons, n:

A=Z+n

So, the number of neutrons is:

n=A-Z=31-15=16

n=16 neutrons