An atom with an atomic number of 12 and a mass number of 25 can best be described by which of these? a) an atom that contains 12 protons and 13 neutrons. b) an atom that contains 13 protons and 12 neutrons. c) an atom that contains 12 protons and 25 neutrons. d) an atom that contains 25 protons and 12 neutrons.

The correct answer is Option A.

Explanation:

An atom consists of 3 subatomic particles: Electrons, neutrons and protons.

Atomic number is equal to the number of electrons or number of protons.

Atomic number = Number of electrons = Number of protons    (1)

Mass number is the sum of number of protons and number of neutrons.

Mass number = Number of protons + Number of neutrons    (2)

We are given:

Atomic number = 12

Mass number = 25

From equation 1:

Number of protons = Number of electrons = 12

From equation 2:

Number of neutrons = 25 - 12 = 13

Thus, an atom contains 12 protons and 13 neutrons.

Hence, the correct answer is Option A.