Question

Write the predicted electron configuration for each of the following elements: (a) He (b) Be (c) Co (d) \(\mathrm{Cd}\)

Short answer

(a) He: 1s^2; (b) Be: 1s^2 2s^2; (c) Co: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^7; (d) Cd: 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10}.

Step-by-step solution

Understand Electron Configurations

An electron configuration describes the distribution of electrons in an atom's orbitals. The format uses numbers and letters to indicate the energy levels and types of orbitals (s, p, d, f) where electrons are located. For example, in the notation '{n}{type}', '{n}' represents the principal energy level, and '{type}' represents the type of orbital.

Know the Order of Orbitals

Electrons fill orbitals in a specific sequence known as the Aufbau principle, which usually follows the order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, etc. This order is based on the increasing energy levels of the orbitals.

Electron Configuration for Helium (He)

Helium has an atomic number of 2, which means it has two electrons. Both electrons will occupy the lowest energy orbital, the 1s orbital. Thus, the electron configuration for helium is: \[ \text{He}: 1s^2 \]

Electron Configuration for Beryllium (Be)

Beryllium has an atomic number of 4, indicating it has four electrons. Electrons fill in the 1s and 2s orbitals. The first two fill the 1s orbital and the next two occupy the 2s orbital. Consequently, the electron configuration for beryllium is: \[ \text{Be}: 1s^2 \, 2s^2 \]

Electron Configuration for Cobalt (Co)

Cobalt has an atomic number of 27. Filling the electrons involves the sequence: 1s, 2s, 2p, 3s, 3p, 4s, 3d. Its 27 electrons fill these orbitals: \[ \text{Co}: 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^7 \]

Electron Configuration for Cadmium (Cd)

Cadmium has an atomic number of 48. Filling the electrons proceeds through the order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d. When filled with 48 electrons, it results in:\[ \text{Cd}: 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^{10} \, 4p^6 \, 5s^2 \, 4d^{10} \]

Key concepts

Atomic Orbitals

Atomic orbitals are regions around the nucleus of an atom where there is a high probability of finding electrons. These orbitals have different shapes and sizes and are grouped into different types: s, p, d, and f. Understanding atomic orbitals is crucial for predicting how electrons are distributed in an atom, which is what we mean by electron configuration.
- **s-orbitals**: These are spherical in shape and can contain up to two electrons.
- **p-orbitals**: Consist of three dumbbell-shaped lobes and can hold up to six electrons.
- **d-orbitals**: Have more complex shapes and can hold up to ten electrons.
- **f-orbitals**: Even more complex, containing up to fourteen electrons.
Atomic orbitals provide the framework for electron occupancy. When analyzing elements' electron configurations, one uses the specific order of orbital filling defined by the principles of electron placement, like the Aufbau principle. Exploring these orbitals helps illustrate where and how electrons find their place in an atom's structure, determining the atom's chemical properties.

Aufbau Principle

The Aufbau Principle is a fundamental guideline in chemistry that allows us to determine the electron configuration of elements. The term "Aufbau" comes from a German word meaning "building up." This principle states that electrons occupy the lowest energy orbitals available before moving on to higher energy orbitals.
According to the Aufbau Principle:

• Electrons fill starting from the lowest available orbital, the 1s, then move through 2s, 2p, and so on, following a specific sequence of increasing energy levels.
• This filling order ensures that the atom achieves the most stable electron configuration possible, minimizing its energy.

The steps outlined in the exercise demonstrate electron organization from lower to higher energy levels, reflecting real atomic structures seen in elements like helium, beryllium, cobalt, and cadmium. By applying the Aufbau Principle, we predict these configurations accurately for any element in the periodic table.

Energy Levels

Energy levels are vital for understanding how electrons are organized in an atom. Imagine these levels as steps in a staircase where electrons can "sit." Each step is referred to as a shell or energy level, designated by the principal quantum number (n). These numbers (1, 2, 3, etc.) show how far the energy level is from the nucleus.
The main rules regarding energy levels include:

• Electrons first fill the lowest available energy level, starting with n=1, closest to the nucleus and thus the most stable.
• Higher energy levels (n=2, n=3, etc.) accommodate more electrons as they get farther from the nucleus and involve more orbitals, like p, d, and f.
• The total number of electrons in an atom corresponds to its atomic number, and these electrons fill available energy levels according to specific rules.

In the exercise example, helium, with two electrons, fills only the first energy level (n=1). Beryllium, cobalt, and cadmium fill additional levels as their atomic numbers increase, each requiring a more complex electron distribution. Understanding these levels is crucial for anticipating how electrons arrange themselves in atoms, influencing chemical behavior.