Problem 14 - Chemistry The Central Science In Si Units

Question

What is the Lewis symbol for each of the following atoms or ions? \((\mathbf{a}) \mathrm{Be},(\mathbf{b}) \mathrm{Rb},(\mathbf{c}) \mathrm{I}^{-},(\mathbf{d}) \mathrm{Se}^{2-} .\)

Step-by-Step Solution

Answer

(a) Be with 2 dots, (b) Rb with 1 dot, (c) I^- with a full octet, (d) Se^{2-} with a full octet.

Step 1: Understand the Concept of Lewis Symbols

The Lewis symbol represents the valence electrons of an atom or ion. These electrons are depicted as dots around the element's symbol. Each dot represents one valence electron, and the arrangement reflects the typical pattern of electron distribution around the nucleus.

Step 2: Determine the Number of Valence Electrons

For each atom or ion, determine the number of valence electrons based on its position in the periodic table. The group number often indicates the count of valence electrons: - (a) Beryllium (Be) is in group 2, so it has 2 valence electrons. - (b) Rubidium (Rb) is in group 1, so it has 1 valence electron. - (c) Iodide ion (I^-) has gained one electron to achieve a full outer shell similar to Xenon, with 8 valence electrons. - (d) Selenide ion (Se^{2-}) has gained two electrons resulting in 8 valence electrons, achieving the same configuration as Krypton.

Step 3: Draw the Lewis Symbol for Each

Draw the element symbol and place dots around it, representing the valence electrons. Dots can be placed on all four sides (right, left, top, bottom), with a maximum of two dots per side: - (a) Be: Be with one dot above and one below. - (b) Rb: Rb with a single dot on any side. - (c) I^-: I with two dots on each side, representing a full octet. - (d) Se^{2-}: Se with two dots on each side, indicating the full octet.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining how an element will interact or bond with other elements. These electrons are present in the highest energy level of an atom's electron configuration. Understanding valence electrons helps you predict an element's chemical properties and its potential to form certain types of bonds.

Each element's valence electrons coincide with its group number in the periodic table, making it easier to identify them:

• Elements in Group 1, such as Hydrogen or Lithium, have 1 valence electron.
• Group 2 elements, like Beryllium, contain 2 valence electrons.
• The elements in Group 17 (Halogens) strive to gain an extra electron to complete their outer shell, achieving an octet with 8 valence electrons.

These electrons play a pivotal role in forming Lewis Symbols, as they are depicted as dots surrounding the element's symbol.

Periodic Table Groups

The periodic table is organized into vertical columns known as groups. These groups are fundamental to understanding the reactivity and properties of elements. Each group contains elements that share significant similarities in chemical behavior due to having the same number of valence electrons.

• Group 1: Known as the alkali metals, these elements have a single valence electron and are highly reactive.
• Group 2: Alkaline earth metals, such as Beryllium, which have 2 valence electrons, are slightly less reactive.
• Group 17: Halogens, like Iodine, need one more electron to reach a full outer shell configuration.
• Group 18: Noble gases are characterized by having a complete valence electron shell, generally 8 electrons, which makes them very stable and non-reactive.

The element’s group number gives a quick insight into its valence count and its Lewis structure characteristics.

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom. This configuration is pivotal for predicting and explaining how elements will bond chemically. Electrons tend to occupy the lowest energy levels available, and these configurations follow a predictable pattern across the periodic table.

For instance, consider Beryllium and Rubidium:

• Beryllium (Be) has an electron configuration of 1s² 2s², indicating the two valence electrons in the 2s orbital.
• Rubidium (Rb) has a more extended configuration due to its placement in period 5, with one valence electron in the 5s orbital.

The electron configuration directly influences how many valence electrons are available for bonding, and thus, the resulting Lewis symbol. Understanding this aspect provides insights into elements forming certain ions or compounds.

Ions

Ions refer to atoms or molecules that have gained or lost electrons, resulting in a net electrical charge. In the formation of ions, atoms may strive to achieve a stable electron configuration similar to the nearest noble gas.

In the case of negative ions, or anions:

• Iodide ( I^{-} ) gains one electron, giving it a full set of valence electrons (8) like Xenon.
• Selenide ( Se^{2-} ) captures two additional electrons, achieving a configuration similar to Krypton, also reaching octet stability.

Positive ions, or cations, occur differently: Rubidium, for example, loses its single valence electron to form a cation, further decreasing its electron count to resemble the electron configuration of the noble gas Krypton. The concept of ions highlights the purpose behind electron gain or loss, often driven by the pursuit of a stable, full valence shell resembling the electron config of noble gases.