Question

1. which electron configuration represents an atom in an excited state? 2-8-1 2-7-1 2-7 2-6

Explanation:

Step1: Recall electron - configuration rules

In the ground - state, electrons fill the lowest - energy orbitals first. An excited state occurs when an electron is promoted to a higher - energy orbital.

Step2: Analyze each option

• For 2 - 8 - 1, it follows the normal filling order (2 in the first shell, 8 in the second shell, 1 in the third shell) and is likely a ground - state configuration for an element like sodium.
• For 2 - 7 - 1, the second shell is not completely filled (it can hold 8 electrons) while there is an electron in the third shell. This indicates that an electron has been promoted from the second shell to the third shell, which is an excited state.
• For 2 - 7, it is a possible configuration for a halogen in the ground - state of its outer shell (e.g., fluorine).
• For 2 - 6, it is a possible configuration for an element like oxygen in the ground - state of its outer shell.

Answer:

2 - 7 - 1