Question

Write the full electron configuration \(\left(1 s^{2} 2 s^{2}, \text { etc. }\right)\) for each of the following elements. a. helium, \(Z=2\) b. neon, \(Z=10\) c. argon, \(Z=18\) d. krypton, \(Z=36\)

Short answer

a. Helium: \(1s^2\) b. Neon: \(1s^2 2s^2 2p^6\) c. Argon: \(1s^2 2s^2 2p^6 3s^2 3p^6\) d. Krypton: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\)

Step-by-step solution

a. Helium, \(Z=2\)

Since helium has an atomic number (Z) of 2, it has 2 electrons. Following the Aufbau principle, we will fill the 1s subshell first, which can hold a maximum of 2 electrons: Electron Configuration: \(1s^2\)

b. Neon, \(Z=10\)

Neon has an atomic number (Z) of 10, meaning it has 10 electrons. Following the Aufbau principle, we will fill the subshells in order of increasing energy levels: 1. 1s can hold 2 electrons: \(1s^2\) 2. 2s can hold 2 electrons: \(2s^2\) 3. 2p can hold 6 electrons: \(2p^6\) Electron Configuration: \(1s^2 2s^2 2p^6\)

c. Argon, \(Z=18\)

Argon has an atomic number (Z) of 18, meaning it has 18 electrons. Following the Aufbau principle, we will fill the subshells in order of increasing energy levels: 1. 1s can hold 2 electrons: \(1s^2\) 2. 2s can hold 2 electrons: \(2s^2\) 3. 2p can hold 6 electrons: \(2p^6\) 4. 3s can hold 2 electrons: \(3s^2\) 5. 3p can hold 6 electrons: \(3p^6\) Electron Configuration: \(1s^2 2s^2 2p^6 3s^2 3p^6\)

d. Krypton, \(Z=36\)

Krypton has an atomic number (Z) of 36, meaning it has 36 electrons. Following the Aufbau principle, we will fill the subshells in order of increasing energy levels: 1. 1s can hold 2 electrons: \(1s^2\) 2. 2s can hold 2 electrons: \(2s^2\) 3. 2p can hold 6 electrons: \(2p^6\) 4. 3s can hold 2 electrons: \(3s^2\) 5. 3p can hold 6 electrons: \(3p^6\) 6. 4s can hold 2 electrons: \(4s^2\) 7. 3d can hold 10 electrons: \(3d^{10}\) 8. 4p can hold 6 electrons: \(4p^6\) Electron Configuration: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\)

Key concepts

Atomic Number

The atomic number, often represented as Z, is a fundamental concept in understanding electron configuration. It tells us the number of protons present in an atom's nucleus, which is the same as the number of electrons in a neutral atom. For example, helium has an atomic number of 2, indicating it has 2 protons and, in a neutral state, 2 electrons as well.
This number not only identifies an element uniquely, but also dictates its position in the periodic table. Furthermore, knowing the atomic number allows us to determine the electron configuration of that element.
Every electron configuration starts with this value, guiding us in arranging electrons across different energy levels or "shells" as per the Aufbau Principle. Understanding the atomic number paves the way for demystifying the behavior of elements through their electrons.

Aufbau Principle

The Aufbau Principle is essential for configuring electrons in atoms. The term 'Aufbau' is German for "building up," as it describes the gradual filling of electrons into atomic orbitals.
According to this principle, electrons occupy the lowest energy orbitals available before shifting to higher ones. For instance, with helium, having atomic number 2, its two electrons first fill the 1s orbital since it's the lowest in terms of energy.
This principle simplifies the filling process, allowing us to predict how electrons are distributed in subshells. It's important to remember that orbitals are filled in a specific sequence—starting from the 1s orbital, then 2s, followed by 2p, and so on. The principle not only helps in constructing the electron configuration but also in understanding the stability and reactivity of elements.

Subshell Filling Order

The subshell filling order, also known as the electron filling sequence, is a crucial aspect of determining how electrons are allocated among an atom's orbitals. Each subshell can hold a max number of electrons, and they fill up in a specific order based on energy levels.
Typically, the order starts with the 1s subshell, progressing through 2s, 2p, 3s, 3p, 4s, and so on, following the n+l rule. This rule helps prioritize subshells based on their combined principal quantum number (n) and azimuthal quantum number (l), where a lower sum indicates the subshell is filled first.
By knowing this filling order, we can accurately predict the electron configuration for any element, making it a key concept in chemistry. An understanding of this sequence helps illustrate why elements behave the way they do across the periodic table.