Question

Write the complete electron configurations for atoms of the following elements. (a) \(\mathrm{Na}\) (b) \(\mathrm{Mn}\) (c) \(\mathrm{Se}\)

Short answer

The electron configurations are as follows: (a) Sodium (Na): 1s² 2s² 2p⁶ 3s¹ (b) Manganese (Mn): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁵ (c) Selenium (Se): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴.

Step-by-step solution

Identify the Atomic Number

First, identify the atomic number of each element. This can be found on the periodic table. The atomic number tells you the number of electrons in a neutral atom. Na has atomic number 11, Mn has atomic number 25, and Se has atomic number 34.

Apply the Aufbau Principle

Second, apply the Aufbau Principle: Electrons occupy the lowest energy level first. Meaning, fill the innermost electron shell (1s) first, followed by 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.

Write Electron Configuration of Sodium (Na)

Na has 11 electrons. Following the sequence of electron filling, Na has electron configuration of 1s² 2s² 2p⁶ 3s¹.

Write Electron Configuration of Manganese (Mn)

Mn has 25 electrons. Following the sequence of electron filling, Mn has electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁵.

Write Electron Configuration of Selenium (Se)

Se has 34 electrons. Following the sequence of electron filling, Se has the electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴.

Key concepts

Atomic Number

The atomic number of an element is a fundamental property that defines the identity of an atom. It tells us how many protons are present in the nucleus of an atom. For neutral atoms, it also equals the number of electrons surrounding the nucleus.

• Why it Matters: The atomic number helps in determining an element's position in the periodic table and its chemical properties.
• Example: Sodium ( \( \text{Na} \) ) has an atomic number of 11, which means it has 11 protons and 11 electrons.

When writing electron configurations, knowing the atomic number is crucial because it tells you exactly how many electrons you need to account for. For example, Manganese ( \( \text{Mn} \) ) has an atomic number of 25, indicating 25 electrons in a neutral atom. Similarly, Selenium ( \( \text{Se} \) ) has an atomic number of 34.Different elements have unique atomic numbers, and this number allows us to write accurate electron configurations by accounting for all the electrons present. This foundational concept is integral when applying the Aufbau Principle.

Aufbau Principle

The Aufbau Principle is a key concept when determining the electron configuration of an atom. It outlines the sequence in which electrons fill the atomic orbitals around the nucleus. The term "Aufbau" is German for "building up," which accurately describes the method's approach as electrons fill from lower to higher energy levels.

• Basic Idea: Electrons are added to the lowest available energy orbital first.
• Filling Order: Often remembered by the sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, etc.
• Why it Matters: Correct application of the Aufbau Principle ensures each electron is placed in the most stable, lowest energy position available, providing an accurate picture of how electrons are organized around the atom's nucleus.

When writing electron configurations, such as for Sodium ( \( \text{Na} \) ), we first fill the 1s orbital before moving on to 2s and 2p, resulting in the configuration \( 1s^2 2s^2 2p^6 3s^1 \) . Similarly, for Manganese ( \( \text{Mn} \) ) and Selenium ( \( \text{Se} \) ), applying the Aufbau Principle helps systematically fill energy levels up to the needed electron count.

Energy Levels

In chemistry, energy levels (also known as electron shells or principal quantum levels) describe the distinct layers around an atom where electrons are likely to be found. These levels are represented by principal quantum numbers (n = 1, 2, 3, ...) and each sublevel is further divided into orbitals like s, p, d, and f.

• Main Concept: Electrons fill energy levels in a specific order according to their distance from the nucleus and amount of energy.
• Orbital Distribution: s orbitals can hold 2 electrons, p can hold 6, d can hold 10, and f can accommodate 14 electrons.
• Importance: Understanding energy levels explains why elements have a specific electron configuration and chemical properties.

When considering energy levels, we start filling electrons from the innermost shell (n = 1) outwards. For Selenium ( \( \text{Se} \) ), this means filling 1s, 2s, 2p, 3s, and 3p before the 4s level. Once lower shells are filled appropriately, transition to 3d and 4p levels occurs. Each step increases the stability of the electron configuration by minimizing potential energy.