Answer:

To understand why the correct answer is option 'A', let's analyze the given information step by step.

Step 1: Determine the electron configuration of the monatomic species
The monatomic species has 18 electrons. To determine its electron configuration, we need to follow the Aufbau principle, which states that electrons fill atomic orbitals in order of increasing energy. The electron configuration of a neutral argon atom (Ar) is 1s² 2s² 2p⁶ 3s² 3p⁶.

Since the monatomic species has a net charge of 2−, it means it has gained 2 extra electrons. Therefore, its electron configuration would be 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d².

Step 2: Compare the number of electrons with a neutral argon atom
Now, let's compare the number of electrons in the monatomic species with a neutral argon atom. Both have the same electron configuration up to the 3p orbital, which is 1s² 2s² 2p⁶ 3s² 3p⁶. Therefore, they have the same number of electrons.

Step 3: Analyze the other options
Let's evaluate the other options to further confirm that option 'A' is correct:
- Option 'B': It states that the monatomic species has more protons than electrons. However, since it is a monatomic species with a net charge of 2−, it means it has gained 2 extra electrons, which makes the number of protons and electrons equal.
- Option 'C': It states that the monatomic species has 2 unpaired electrons. However, the electron configuration of the species shows that all the electrons are paired within the 3d orbital, so there are no unpaired electrons.
- Option 'D': It states that the monatomic species has 20 protons. However, this information cannot be determined solely based on the number of electrons and the net charge. The number of protons in an atom determines its atomic number, which is not given in the question.

Therefore, option 'A' is the correct answer as the monatomic species has the same number of electrons as a neutral argon atom.