An atom contains 13 protons. What is the approximate atomic mass of this atom in atomic mass units (u)?

• A. 40 u
• B. 13 u
• C. 27 u
• D. 20 u

Answer: (C)

Verified step by step guidance

1. Identify the element by its number of protons. The number of protons defines the atomic number, which determines the element. Here, the atom has 13 protons, so it is aluminum (Al).
2. Understand that the atomic mass (in atomic mass units, u) is approximately the sum of the number of protons and neutrons in the nucleus of the atom.
3. Since the number of protons is 13, find the most common isotope of aluminum, which has a mass number (protons + neutrons) close to 27. This means the atom has about 14 neutrons (27 - 13 = 14).
4. Recognize that the atomic mass listed on the periodic table is a weighted average of all isotopes, but for a single atom, the approximate atomic mass is the mass number (protons + neutrons).
5. Therefore, the approximate atomic mass of this atom is about 27 u, which corresponds to the sum of 13 protons and 14 neutrons.