The electron configuration of the calcium ion is Ca2+, 1s^2 2s^2 2p^6 3s^2 3p^6. Provide the electron configuration for each of the following (atom or ion) in the space provided: (0 points) Se? Complete the orbital diagram provided for a chromium atom. Show any unpaired electrons as single barbed arrow pointing up, and paired electrons as a pair of arrows (one up and one down) (5 points). How many electrons does a Cr atom have in its 3d subshell? B) How many of those chromium electrons are unpaired?
The Correct Answer and Explanation is:
Electron Configuration:
Se (Selenium) atom:
Selenium has an atomic number of 34, so its electron configuration is:
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴
Orbital Diagram for Cr (Chromium) atom:
Chromium (Cr) has an atomic number of 24. Its ground state electron configuration is:
[Ar] 4s¹ 3d⁵
(Note: Chromium is an exception to the standard filling order due to extra stability from half-filled d orbitals.)
Orbital Diagram (3d and 4s only):
makefileCopyEdit3d: ↑ ↑ ↑ ↑ ↑
4s: ↑
Subquestions:
A) How many electrons does a Cr atom have in its 3d subshell?
➡️ 5 electrons
B) How many of those chromium electrons are unpaired?
➡️ 5 unpaired electrons
Textbook-Style Explanation
Chromium (Cr), with an atomic number of 24, exhibits a unique electron configuration compared to the expected Aufbau principle pattern. According to the Aufbau principle, electrons fill orbitals in order of increasing energy, and we might expect chromium to be [Ar] 4s² 3d⁴. However, the actual observed configuration is [Ar] 4s¹ 3d⁵. This occurs because a half-filled d subshell (five electrons in the five 3d orbitals) provides additional stability due to symmetrical distribution and minimized electron repulsion.
In the case of chromium, the 4s orbital contains one electron, and the 3d subshell contains five electrons. These five 3d electrons occupy all five d orbitals singly, following Hund’s rule, which states that electrons will occupy degenerate orbitals singly and with parallel spins before pairing occurs. This arrangement maximizes total spin and minimizes repulsion, contributing to a lower energy and more stable configuration.
As a result, all five electrons in the 3d subshell of chromium are unpaired. This leads to strong paramagnetic behavior because unpaired electrons generate magnetic fields that align with external magnetic fields.
For selenium (Se), with atomic number 34, its electron configuration follows the expected order without exceptions: 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴. Here, selenium ends in the p-block, and its four 4p electrons indicate that selenium has two unpaired electrons in the 4p orbitals.
Understanding these electron configurations is crucial for predicting chemical bonding, magnetic properties, and reactivity patterns of the elements.
