How many protons, neutrons and electrons does a magnesium atom with a mass number of 25 contain?

To find the number of protons, neutrons, and electrons in a magnesium atom with a mass number of 25, we first need to know the standard atomic number for magnesium. Magnesium has an atomic number of 12, which means it normally has 12 protons.

Given that the mass number (sum of protons and neutrons) is 25, we can find the number of neutrons by subtracting the atomic number (protons) from the mass number:

Number of neutrons = Mass number - Atomic number
= 25 - 12
= 13 neutrons

Since atoms are electrically neutral, the number of protons (positive charge) should equal the number of electrons (negative charge). Therefore, in a neutral magnesium atom, the number of electrons would also be 12.

To summarize:

Protons: 12
Neutrons: 13
Electrons: 12