Final answer:
An atom of Mg-25 has 13 neutrons. The mass number of an atom with 5 protons and 7 neutrons is 12. These calculations are based on subtracting the atomic number from the mass number for neutrons, and adding the protons and neutrons together for the mass number.
Step-by-step explanation:
An atom of Mg-25 has 25 as its mass number and magnesium (Mg) has an atomic number of 12, which represents the number of protons. Therefore, the number of neutrons in an atom of Mg-25 is calculated by subtracting the atomic number from the mass number:
Number of neutrons = mass number - number of protons
= 25 - 12
= 13
So, an atom of Mg-25 has 13 neutrons.
The mass number of an atom with 5 protons and 7 neutrons is the sum of protons and neutrons:
Mass number = number of protons + number of neutrons
= 5 + 7
= 12
Therefore, the mass number of an atom with 5 protons and 7 neutrons is 12.