Question

based on information from the periodic table, which statement is true for the element cobalt?
27
co
58.9
a. the atomic number of cobalt is 27, and its average atomic mass is 32.
b. the average atomic number of cobalt is 58.9, and its atomic mass is 27.
c. a cobalt atom contains 27 protons, and most cobalt atoms contain 32 neutrons.
d. a cobalt atom contains 27 protons, and neutral cobalt atoms contain 32 electrons

Explanation:

Step1: Recall periodic table basics

In the periodic table, the top number (27 for Co) is the atomic number. The atomic number equals the number of protons in a neutral atom. The bottom number (58.9 for Co) is the average atomic mass.

Step2: Analyze each option

• Option A: The average atomic mass of Co is 58.9, not 32. So, A is wrong.
• Option B: The atomic number of Co is 27, not the average atomic mass. The average atomic mass is 58.9. So, B is wrong.
• Option C: Atomic number \(Z = 27\) (number of protons). Most common isotope: \(n=\text{mass number}-\text{atomic number}\). Mass number is close to average atomic mass. If we assume mass number \(A\approx59\), then \(n = A - Z=59 - 27=32\).
• Option D: In a neutral atom, number of electrons = number of protons. So, neutral Co has 27 electrons, not 32. So, D is wrong.

Answer:

C. A cobalt atom contains 27 protons, and most cobalt atoms contain 32 neutrons.