Question

Write the full electron configuration \(\left(1 s^{2} 2 s^{2}, \text { etc. }\right)\) for each of the following elements. a. bromine, \(Z=35\) b. xenon, \(Z=54\) c. barium, \(Z=56\) d. selenium, \(Z=34\)

Short answer

a. Bromine (Z=35): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5\) b. Xenon (Z=54): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6\) c. Barium (Z=56): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^2\) d. Selenium (Z=34): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^4\)

Step-by-step solution

a. Electron Configuration for Bromine (Z=35)

Following the aufbau principle, we start filling electrons in the lowest energy orbitals first, until all 35 electrons are placed: 1. 1s (2 electrons) → 1s² 2. 2s (2 electrons) → 2s² 3. 2p (6 electrons) → 2p⁶ 4. 3s (2 electrons) → 3s² 5. 3p (6 electrons) → 3p⁶ 6. 4s (2 electrons) → 4s² 7. 3d (10 electrons) → 3d¹⁰ 8. 4p (5 electrons) → 4p⁵ Putting it together, we get the electron configuration for bromine: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^5\)

b. Electron Configuration for Xenon (Z=54)

Following the aufbau principle, we place all 54 electrons in the atomic orbitals: 1. 1s (2 electrons) → 1s² 2. 2s (2 electrons) → 2s² 3. 2p (6 electrons) → 2p⁶ 4. 3s (2 electrons) → 3s² 5. 3p (6 electrons) → 3p⁶ 6. 4s (2 electrons) → 4s² 7. 3d (10 electrons) → 3d¹⁰ 8. 4p (6 electrons) → 4p⁶ 9. 5s (2 electrons) → 5s² 10. 4d (10 electrons) → 4d¹⁰ 11. 5p (6 electrons) → 5p⁶ Putting it together, we get the electron configuration for xenon: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6\)

c. Electron Configuration for Barium (Z=56)

Following the aufbau principle, we place all 56 electrons in the atomic orbitals: 1. 1s (2 electrons) → 1s² 2. 2s (2 electrons) → 2s² 3. 2p (6 electrons) → 2p⁶ 4. 3s (2 electrons) → 3s² 5. 3p (6 electrons) → 3p⁶ 6. 4s (2 electrons) → 4s² 7. 3d (10 electrons) → 3d¹⁰ 8. 4p (6 electrons) → 4p⁶ 9. 5s (2 electrons) → 5s² 10. 4d (10 electrons) → 4d¹⁰ 11. 5p (6 electrons) → 5p⁶ 12. 6s (2 electrons) → 6s² Putting it together, we get the electron configuration for barium: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^2\)

d. Electron Configuration for Selenium (Z=34)

Following the aufbau principle, we place all 34 electrons in the atomic orbitals: 1. 1s (2 electrons) → 1s² 2. 2s (2 electrons) → 2s² 3. 2p (6 electrons) → 2p⁶ 4. 3s (2 electrons) → 3s² 5. 3p (6 electrons) → 3p⁶ 6. 4s (2 electrons) → 4s² 7. 3d (10 electrons) → 3d¹⁰ 8. 4p (4 electrons) → 4p⁴ Putting it together, we get the electron configuration for selenium: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^4\)

Key concepts

The Aufbau Principle

The concept of the Aufbau principle is crucial in understanding electron configuration. Essentially, "Aufbau" is a German term meaning "building up". The principle describes the way electrons fill the atomic orbitals of an atom. According to it, electrons start to fill the lowest energy orbital first before moving onto higher ones.

To visualize this, consider an apartment building where each floor represents an energy level. Electrons "fill" the ground floor first because it's easy and takes less energy, just like people filling the first floor before moving to the second. As more electrons are added, they gradually move to higher floors.

• Electrons first fill the 1s orbital, which is the lowest energy level.
• They proceed to the 2s, 2p, 3s, and so on sequentially up to higher orbitals.

This pattern continues until all electrons are properly distributed among orbitals based on the amount of energy each level requires. This principle helps chemists predict and explain chemical reactions and bonding.

Understanding Atomic Orbitals

Atoms contain regions known as atomic orbitals, areas where there’s a high probability of finding an electron. Each orbital can hold a maximum of two electrons. Understanding these orbitals is important for writing electron configurations.

The basic types of atomic orbitals are s, p, d, and f:

• s-orbitals are spherical and hold up to 2 electrons.
• p-orbitals have a dumbbell shape and can accommodate up to 6 electrons, filling three sublevels: px, py, and pz.
• d-orbitals are more complex in shape and can have up to 10 electrons across five sublevels.
• f-orbitals are the most intricate and can hold up to 14 electrons across seven sublevels.

Recognizing these distinct orbitals is key when determining where electrons are located within an atom. It affects how elements interact with one another and helps in predicting their chemical behavior.

Electron Filling Order

When determining electron configuration, understanding the electron filling order is crucial. This order dictates how electrons are distributed across orbitals following the Pauli exclusion principle and Hund’s rule, aiding in stability and energy efficiency.

The generalized electron filling order adheres to the increasing energy levels:

• Begin at 1s, the lowest energy level that fills first.
• Progress through 2s, 2p, then 3s, 3p, moving forward as energy levels rise.
• Notice for 3d orbitals, even though they belong to the third principal energy level, they fill after 4s because they have higher energy.
• Continue this process, ensuring higher energy levels like 4p, 5s, 4d, and so forth are filled after their lower-energy counterparts.

A helpful mnemonic to remember this is the diagonal rule, visualizing a sequence starting from 1s and tracing diagonally downward through higher energy levels. This ordered pattern facilitates a systematic way to determine an element's electron configuration.