Problem 78 - Chemistry Matter and Change

Question

What are valence electrons? How many of a magnesium atom’s 12 electrons are valence electrons?

Step-by-Step Solution

Answer

Valence electrons are the outermost electrons of an atom that are involved in chemical reactions and the formation of chemical bonds. Magnesium has an electron configuration of \(1s^2 2s^2 2p^6 3s^2\). The highest energy level is 3, and there are 2 electrons in the 3s subshell. Therefore, a magnesium atom has 2 valence electrons.

Step 1: Definition of Valence Electrons

Valence electrons are the outermost electrons of an atom that are involved in chemical reactions and the formation of chemical bonds. They are present in the highest energy level (principal quantum number) of an atom.

Step 2: Electron Configuration of Magnesium

To determine the number of valence electrons in a magnesium atom, we need to know its electron configuration. Magnesium has 12 electrons and its electron configuration can be represented as: \(1s^2 2s^2 2p^6 3s^2\)

Step 3: Identifying Valence Electrons

The highest energy level (principal quantum number) in the electron configuration of magnesium is 3. Thus, the valence electrons are those in the 3s subshell. There are 2 electrons in the 3s subshell, so a magnesium atom has 2 valence electrons.

Key Concepts

Electron Configuration

Electron configuration is a method used to symbolize the arrangement of electrons within an atom. It assists in understanding how electrons are distributed across different energy levels and orbitals. This is important for predicting chemical behaviors.

Here's how it works: Electrons are organized in shells around the nucleus of an atom, each shell represented by a principal quantum number (n). These shells are further divided into subshells labeled as s, p, d, and f based on their shapes. The periodic table offers a pattern through which electron configurations can be determined.

• The s subshell can hold a maximum of 2 electrons.
• The p subshell can hold up to 6 electrons.
• The d subshell has a capacity of 10 electrons.
• And the f subshell can contain up to 14 electrons.

To find the electron configuration of an element, you fill the subshells in order of increasing energy: starting from the lowest energy level and following through the Aufbau principle, Pauli exclusion principle, and Hund's rule. This allows you to identify the valence electrons crucial for chemical bonding.

Magnesium Atom

Magnesium (Mg) is a chemical element with the atomic number 12, which means it has 12 protons and typically 12 electrons. It belongs to the alkaline earth metal group in the periodic table.

The electron configuration of magnesium helps determine its chemical properties and is written as: \[1s^2 2s^2 2p^6 3s^2\]This configuration indicates how electrons are distributed among the various energy levels. The electrons fill the lower energy levels first before moving to higher levels.

• The 1s subshell is filled with 2 electrons.
• The 2s subshell holds another 2 electrons.
• The 2p subshell contains 6 electrons, completing the second energy level.
• Finally, the 3s subshell has the remaining 2 electrons.

With these last two electrons in the 3s subshell, magnesium has 2 valence electrons that are readily available for bonding, which affects its reactivity and the types of compounds it forms.

Principal Quantum Number

The principal quantum number, often symbolized as \(n\), is essential in quantum mechanics for defining the general size and energy of an electron 'shell' within an atom. Essentially, this number helps in understanding where an electron resides relative to the nucleus.

In an atom:

• The principal quantum number \(n\) can be any positive integer: 1, 2, 3, etc.
• As \(n\) increases, the electron resides farther from the nucleus, at a higher energy level.
• This concept significantly influences the electron configuration and thus how atoms bond with each other.

In the magnesium atom, the electron configuration is \(1s^2 2s^2 2p^6 3s^2\). Here, the highest principal quantum number is 3, which corresponds to the third shell, hosting the 3s subshell with 2 electrons or valence electrons. Understanding \(n\) is crucial because it determines the atom's physical and chemical properties, particularly how it will interact chemically.