Give the set of four quantum numbers that represent the indicated electron in each of the following elements. 
Cr³⁺ (21^st electron) 

• A. n = 3; l = 1; ml = 0; ms = -1/2
• B. n = 3; l = 2; ml = 1; ms = -1/2
• C. n = 3; l = 1; ml = 1; ms = +1/2
• D. n = 3; l = 2; ml = 0; ms = +1/2
• E. n = 3; l = 1; ml = 0; ms = +1/2

Answer: (D)

Verified step by step guidance

1. Understand the concept of quantum numbers: Quantum numbers are used to describe the properties of electrons in atoms. There are four quantum numbers: n (principal quantum number), l (azimuthal quantum number), ml (magnetic quantum number), and ms (spin quantum number).
2. Determine the electron configuration of Cr3+: Chromium (Cr) has an atomic number of 24, meaning it has 24 electrons in its neutral state. Cr3+ means it has lost 3 electrons, so it has 21 electrons.
3. Write the electron configuration for Cr3+: The electron configuration for Cr is [Ar] 3d5 4s1. When it loses 3 electrons to become Cr3+, the configuration becomes [Ar] 3d3.
4. Identify the 21st electron: In the electron configuration [Ar] 3d3, the 21st electron is the third electron in the 3d subshell.
5. Assign the quantum numbers to the 21st electron: For the 3d subshell, n = 3, l = 2. The ml value can range from -2 to +2. The ms value can be either +1/2 or -1/2. The correct set of quantum numbers for the 21st electron is n = 3, l = 2, ml = 0, ms = +1/2.