Problem 14 - Chemistry: The Central Science in SI Units, Global Edition

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

The Lewis symbols for the given atoms and ions are: (a) Be (Beryllium): Be •• (b) Rb (Rubidium): Rb • (c) I⁻ (Iodide Ion): \[ \mathrm{[I \cdot \cdot \cdot \cdot \cdot \cdot \cdot ]^{-}} \] (d) Se²⁻ (Selenide Ion): \[ \mathrm{[Se \cdot \cdot \cdot \cdot \cdot \cdot \cdot \cdot ]^{2-}} \]

Step 1: Determine the Valence Electrons for Each Atom or Ion

For each atom or ion, we will identify its atomic number (which determines its place in the periodic table), which will guide us in determining the number of valence electrons. In case of ions, we will also account for any gained or lost electrons.

Step 2: Create the Lewis Symbols for Each Atom or Ion

Using the chemical symbol for the element and the number of valence electrons, we will place dots around the symbol to represent the valence electrons. Keep in mind to distribute the electrons according to the octet rule, and add brackets and charge, if it is an ion. Now let's find the Lewis symbols for each given atom or ion:

Step 3: (a) Be (Beryllium)

Atomic Number = 4 Valence Electrons = 2 Lewis Symbol for Be: Be ••

Step 4: (b) Rb (Rubidium)

Atomic Number = 37 Valence Electrons = 1 Lewis Symbol for Rb: Rb •

Step 5: (c) I- (Iodide Ion)

Atomic Number = 53 Valence Electrons = 7, adding 1 electron for the negative charge Lewis Symbol for I-: \[ \mathrm{[I \cdot \cdot \cdot \cdot \cdot \cdot \cdot ]^{-}} \]

Step 6: (d) Se2- (Selenide Ion)

Atomic Number = 34 Valence Electrons = 6, adding 2 electrons for the double negative charge (-2) Lewis Symbol for Se2-: \[ \mathrm{[Se \cdot \cdot \cdot \cdot \cdot \cdot \cdot \cdot ]^{2-}} \]

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons in an atom and play a crucial role in chemical bonding. These electrons are found in the highest energy level of an atom and are instrumental in determining how an element reacts with others. Understanding valence electrons is essential when drawing Lewis symbols.

Here's why they are important:

• They are responsible for the chemical properties of elements.
• Interactions between valence electrons dictate the bonding behavior of atoms.
• These electrons are the ones involved in forming covalent or ionic bonds.

To determine the number of valence electrons:

• Look at the group number in the periodic table (for main-group elements).
• Account for changes when dealing with ions, as electrons can be added or removed based on the ion's charge.

For example, beryllium has two valence electrons, making its Lewis symbol as simple as "Be with two dots" representing these electrons.

Octet Rule

The octet rule is a fundamental concept in chemistry, serving as a guideline for understanding atom stability. It states that atoms tend to achieve a total of eight electrons in their valence shell, mimicking the electron configuration of noble gases.

Here are key points about the octet rule:

• Elements will share, lose, or gain electrons to complete their valence shell with eight electrons.
• This rule helps predict how atoms will bond in molecules, guiding the creation of covalent or ionic bonds.
• Lewis symbols effectively illustrate this rule by showing how atoms adjust their electron count to achieve stability.

However, there are some exceptions. For instance, hydrogen strives for just two electrons, aligning itself with the stability of helium. Recognizing that Lewis symbols show atoms striving to fulfil the octet rule helps explain why ions like Iodide (I⁻) or Selenide (Se²⁻) adjust their electron count.

Periodic Table

The periodic table is a chart organizing elements based on their atomic number, electron configurations, and recurring chemical properties. It provides crucial insights for understanding elements and their behavior.

Learning how the periodic table works can help you:

• Identify the number of valence electrons an element has, as elements in the same group often share these.
• Predict bonding behavior and the formation of molecules due to similarities in electron configurations within groups.
• Understand trends such as electronegativity, atomic radius, and ionization energy, which influence chemistry concepts like bonding and reactivity.

For instance, elements like beryllium and rubidium can be found within Group 2 and Group 1, respectively, offering clues to their one or two valence electrons. The periodic table acts as an invaluable tool for predicting how elements, such as the ions I⁻ and Se²⁻, will react in chemical settings.

Ions in Chemistry

In chemistry, ions are atoms or molecules that have a net electrical charge due to the loss or gain of one or more electrons. Understanding ions is crucial as they feature prominently in various chemical reactions.

Here's what you need to know about ions:

• Cations are positively charged ions formed when an atom loses electrons.
• Anions are negatively charged ions created when an atom gains electrons.
• Ions are central to ionic compounds, where oppositely charged ions attract each other to form stable structures.

For instance, the iodide ion (I⁻) gains an electron, resulting in a negative charge, while the selenide ion (Se²⁻) gains two electrons for a double negative charge. Learning how to write Lewis symbols for ions involves adjusting the number of dots to reflect their electron settings fully, often enclosing the symbol in brackets with a charge shown outside.