Calculate the mass number for an element which has the atomic number 12, knowing that the number of protons is the same with the number of...

We'll use the following notation:

A-mass number

Z-atomic number

The mass number could be calculated as a sum of protons and neutrons, and knowing that the atomic number is given by the number of the protons, that means that Z=12, so we'll have 12 protons.

In the enunciation is given the information that the number of protons and the number of neutrons are equal, so we'll have 12 neutrons, too.

A=protons+neutrons=12+12=24

The electronical configuration will be found out based on the fact that the number of protons is also equal to the number of electrons, so the element will have 12 electrons.

We'll fill up the  levels, according to the maximum number of electrons accepted on each level.

On the first level K, the element will have 2 electrons.

On the next level L, the element will have 8 electrons.

On the following level, M, the element will have the rest of 2 electrons.

The total number of electrons:2+8+2=12.

Searching for the element in the periodic table, knowing it's atomic number Z=12, it will be located on the 12th position, and this element is Magnesium.