The electron configuration 2-8-7 is given for an atom or ion. Is this configuration an excited state or a ground state?

• A. Ground state of Na
• B. Excited state of Na
• C. Ground state of Cl
• D. Excited state of Cl

Answer: (C)

Verified step by step guidance

1. Identify the element or ion based on the given electron configuration 2-8-7. This means there are 2 electrons in the first shell, 8 in the second shell, and 7 in the third shell, totaling 17 electrons.
2. Recall that the atomic number corresponds to the number of electrons in a neutral atom. An element with 17 electrons is chlorine (Cl), which has the atomic number 17.
3. Determine if the given configuration matches the known ground state electron configuration of chlorine. The ground state electron configuration of chlorine is indeed 2-8-7, which corresponds to the full first and second shells and 7 electrons in the third shell.
4. Compare this configuration to sodium (Na), which has 11 electrons and a ground state configuration of 2-8-1. Since 2-8-7 does not match sodium's ground state, it cannot be sodium's ground or excited state.
5. Conclude that the configuration 2-8-7 represents the ground state of chlorine, not an excited state or any state of sodium.