Question

Why do we believe that the valence electrons of calcium and potassium reside in the \(4 s\) orbital rather than in the \(3 d\) orbital?

Short answer

In conclusion, the valence electrons of calcium and potassium reside in the 4s orbital rather than the 3d orbital because the 4s orbital has a lower energy level. According to the aufbau principle, electrons fill orbitals of lower energy first. This is supported by the electron configurations of calcium (\(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2\)) and potassium (\(1s^2 2s^2 2p^6 3s^2 3p^6 4s^1\)), which show that the valence electrons for both elements are indeed located in the 4s orbital.

Step-by-step solution

Understanding electron configurations

Electron configurations represent the distribution of electrons in an atom across various orbitals. The configuration is determined by the aufbau principle, which states that electrons fill orbitals in order of increasing energy levels. The energy levels of orbitals generally follow this order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.

Identify electron configurations of calcium and potassium

First, let's identify the electron configurations of calcium (Ca) and potassium (K). Calcium has an atomic number of 20 and potassium has an atomic number of 19. Their electron configurations can be written as follow: Calcium (Ca): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2\) Potassium (K): \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^1\)

Understanding valence electrons

Valence electrons are the electrons in the outermost energy level of an atom. They are important because they determine the atom's chemical properties, including its reactivity and ability to form bonds. In the case of Calcium and Potassium, the valence electrons are the ones in the 4s orbital.

Explaining why valence electrons reside in the 4s orbital

Electron orbitals are filled in order of increasing energy levels according to the aufbau principle. Though the 3d orbital appears before the 4s orbital in terms of quantum numbers, the 4s orbital is actually lower in energy than the 3d orbital. So, when electrons are being added to the atom, they first fill the lower energy 4s orbital before moving on to the higher energy 3d orbital. In the case of calcium and potassium, their electron configurations show that the valence electrons are indeed in the 4s orbital. This is because electrons will occupy the available orbitals with the lowest energy first, following the aufbau principle, and the 4s orbital fills before the 3d orbital. In conclusion, we believe that the valence electrons of calcium and potassium reside in the 4s orbital rather than the 3d orbital because the 4s orbital has a lower energy level, and electrons fill orbitals of lower energy first according to the aufbau principle.

Key concepts

Electron Configuration

Electron configuration is a way that scientists describe the arrangement of electrons in an atom. This arrangement follows a specific order based on the energy levels of the orbitals. Each energy level can hold a certain number of electrons, and these are distributed among available orbitals such as s, p, d, and f. The filling order is determined by the energy hierarchy, ensuring lower energy orbitals get filled first. For example, calcium and potassium fill their 4s orbitals before moving to other, higher energy orbitals.

Aufbau Principle

The Aufbau Principle is pivotal in understanding how electrons populate orbitals in an atom. According to this principle, electrons occupy orbitals in order of ascending energy levels. This systematic filling follows a well-defined sequence such as 1s, 2s, 2p, 3s, 3p, and so forth. Importantly, the 4s orbital is filled before the 3d orbital because it is lower in energy, despite appearing later in the sequence. This is why the valence electrons of potassium and calcium are found in the 4s orbital as it is energetically favorable to fill it first.

4s Orbital

The 4s orbital holds special significance in the electron configurations of elements like potassium and calcium, as their valence electrons reside here. Despite being labelled as the 4s orbital, it is filled before the 3d orbital, a characteristic that underpins much of elemental chemistry. The orbital accommodates two electrons, making it crucial to the chemical behavior of these elements. The 4s orbital's lower energy relative to 3d is why electrons of calcium and potassium fill it first, influencing their chemical properties and interactions.

Energy Levels

Energy levels, often referred to as "shells," indicate the relative distance and energy of electrons from the nucleus. They determine the manner in which electrons are arranged around the nucleus within orbitals. Lower numbered energy levels, such as n=1, n=2, are closer to the nucleus and are filled first when building an electron configuration. The 4s orbital occupies the fourth energy level, yet holds a lower energy state compared to the 3rd level 3d orbitals. This energy arrangement explains the electron fill order crucial for understanding atomic structure.

Chemical Properties

Chemical properties of an element are intrinsically connected to its electron configuration, especially the arrangement and presence of valence electrons. Valence electrons in particular play a predominant role in determining how an atom will chemically interact with others. For calcium and potassium, the electrons in the 4s orbital dictate their reactivity and bonding tendencies. This is because these electrons are on the outermost shell, readily available to interact. Elements sharing similar valence configurations often exhibit similar chemical behavior due to this strategic electron arrangement.