Problem 11 - Chemistry: The Central Science in SI Units, Global Edition

Question

Consider the element silicon, Si. (a) Write its electron configuration. (b) How many valence electrons does a silicon atom have? (c) Which subshells hold the valence electrons?

Step-by-Step Solution

Answer

(a) The electron configuration of silicon (Si) is \(1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^2\). (b) A silicon atom has 4 valence electrons. (c) The valence electrons are located in subshells 3s and 3p.

Step 1: (a) Determine the electron configuration of silicon

We need to find the electron configuration of silicon (Si). Silicon has an atomic number of 14, which means it has 14 electrons. We will distribute these electrons in various energy levels following the aufbau principle. The electron configuration of silicon Si is: \[ 1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^2 \]

Step 2: (b) Find the number of valence electrons

To determine the number of valence electrons, we need to count the electrons in the outermost energy level. In the case of silicon, the outermost energy level is the third level (n=3). The electron configuration for the third energy level is \(3s^2 3p^2\), which amounts to a total of 2 + 2 = 4 electrons. Therefore, a silicon atom has 4 valence electrons.

Step 3: (c) Identify the subshells holding the valence electrons

To determine which subshells hold the valence electrons, we look at the electron configuration of silicon in the outermost energy level (n=3). The electron configuration for the third energy level is \(3s^2 3p^2\). It means that the valence electrons are located in the 3s and 3p subshells. So, the valence electrons of silicon are located in subshells 3s and 3p.

Key Concepts

Valence Electrons

Valence electrons are the electrons found in the outermost shell of an atom. They play a crucial role in chemical reactions and bonding. For instance, in the case of silicon (Si), its atomic number is 14. Hence, it has 14 electrons. To find the valence electrons, we look at its outermost energy level or shell. The electron configuration of silicon is \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^2\].

Here, the third energy level (n=3) is the outermost shell, containing four electrons: two in the 3s subshell and two in the 3p subshell. Therefore, silicon has four valence electrons.

Understanding valence electrons is essential because they determine how an element behaves in chemical processes. For example:

• Elements with a similar number of valence electrons tend to exhibit similar chemical properties.
• Valence electrons are involved in forming chemical bonds.
• They are crucial for predicting the reactivity of elements.

Subshells

In atomic physics, subshells are subdivisions of electron shells based on the angular momentum quantum number. They determine how electrons are distributed around the nucleus. Each subshell can hold a specific number of electrons:

• s subshells can hold 2 electrons.
• p subshells hold up to 6 electrons.
• d subshells hold up to 10 electrons.
• f subshells can accommodate up to 14 electrons.

Each subshell can be found within different energy levels, with silicon's electron configuration being an excellent example. Silicon's subshell layout is \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^2\].

The outermost electrons of silicon, known as valence electrons, are found in the 3s and 3p subshells.

Understanding subshells is vital for several reasons:

• They explain the arrangement of electrons within atoms.
• They dictate the geometry and shape of an atom's electron cloud.
• Subshells are a key factor in determining an atom's chemical properties.

Aufbau Principle

The Aufbau principle is a widely applied guideline in physics and chemistry for determining the electron configuration of an atom. It translates from German to mean "building up," reflecting the method of adding electrons one at a time to the lowest energy orbitals first.

When filling the electron shells and subshells:

• Electrons occupy the lowest energy level available.
• The order of filling is: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, and so on.
• This helps to achieve the most stable (lower energy) electron arrangement.

For silicon, with 14 electrons, the configuration according to the Aufbau principle becomes \[1s^2 \ 2s^2 \ 2p^6 \ 3s^2 \ 3p^2\].

The principle is crucial as it aids in:

• Predicting the electronic structure of atoms and ions.
• Understanding the atomic spectrum and energy levels.
• Providing insights into atoms' chemical behavior and reactivity.