Which of the following correctly describes the number of protons, neutrons, and electrons in a neutral atom of barium-137 (Ba-137)?

• A. 81 protons, 56 neutrons, 81 electrons
• B. 56 protons, 137 neutrons, 56 electrons
• C. 56 protons, 81 neutrons, 56 electrons
• D. 137 protons, 56 neutrons, 137 electrons

Answer: (C)

Verified step by step guidance

1. Identify the atomic number of barium (Ba) from the periodic table. The atomic number represents the number of protons in the atom.
2. Recognize that the number of protons in a neutral atom is equal to the number of electrons, so the number of electrons will be the same as the atomic number.
3. Understand that the mass number (given as 137 in Ba-137) is the sum of protons and neutrons in the nucleus.
4. Calculate the number of neutrons by subtracting the atomic number (number of protons) from the mass number: \(\text{neutrons} = \text{mass number} - \text{atomic number}\).
5. Compare the calculated values of protons, neutrons, and electrons to the options provided to determine which correctly describes the neutral Ba-137 atom.