Bohr Diagrams and Lewis Dot Structures Study Guide

Learning Objectives for Modeling the Atom

• Utilize knowledge of the Periodic Table of Elements to construct a Bohr Diagram including specific electron configurations.

• Determine the specific number of valence electrons an element possesses.

• Apply the knowledge of valence electrons to accurately draw a Lewis Dot Structure.

Review of Electron Cloud Capacities

• The electron cloud around an atom consists of multiple levels, each with a maximum capacity for electrons:

  • Level $1$: Can hold a maximum of $2$ electrons.

  • Level $2$: Can hold a maximum of $8$ electrons.

  • Level $3$: This level is "happy" with $8$ electrons, though it has the physical capacity to hold up to $18$ electrons.

The Bohr Diagram

• Definition: A Bohr Diagram is a visual model of an atom that features the nucleus located at the center, with electrons illustrated around it on distinct energy levels or electron clouds.

Step-by-Step Bohr Diagram Construction

1. Draw the Nucleus: Represent the center of the atom (often indicating the number of protons or the element name).

2. Determine Total Electrons: Identify the total number of electrons in the atom based on its atomic number.

3. Arrange Electrons in Clouds:

   • Place up to $2$ electrons in the first energy level.

   • Place up to $8$ electrons in the second energy level.

   • Place up to $8$ electrons in the third energy level (to ensure stability).

Practice Examples for Bohr Diagrams

• Hydrogen ($H$):

  • Total Electrons: $1$

  • Drawing: One electron placed on the first energy level.

• Helium ($He$):

  • Total Electrons: $2$

  • Drawing: Two electrons placed on the first energy level.

• Lithium ($Li$):

  • Total Electrons: $3$

  • Total Electron Clouds: $2$

  • Drawing: Two electrons in the first cloud and one electron in the second cloud.

• Fluorine ($F$):

  • Nucleus: $(+9)$

  • Configuration: $2$ electrons in the first level, $7$ electrons in the second level.

• Aluminum ($Al$):

  • Nucleus: $(+13)$

  • Configuration: $2$ electrons in the first level, $8$ electrons in the second level, and $3$ electrons in the third level.

Valence Electrons

• Definition: Valence electrons are the specific electrons located on the outermost electron cloud of an atom.

• Significance: They determine how an element interacts and bonds with others.

• Examples of Valence Counts:

  • Fluorine: Has $7$ valence electrons.

  • Lithium: Has $1$ valence electron.

Determining Valence Electrons from the Periodic Table

• Valence electron counts are determined by the element's Group.

• Groups are the vertical columns (oriented up and down) found on the Periodic Table.

• Rules for Counting:

  • Group $1$: Elements have $1$ valence electron.

  • Group $2$: Elements have $2$ valence electrons.

  • Transition Metals (Groups $3$ through $12$): Skip this middle section when learning the standard rules, as these elements do not always follow the basic valence electron patterns.

  • Groups $13$ through $18$: To find the number of valence electrons, "drop the $1$." For example, Aluminum ($Al$) is in Group $13$ and therefore has $3$ valence electrons.

• The Helium Exception: Although Helium ($He$) is located in Group $18$, it is an exception to the "drop the $1$- rule; it only has a total of $2$ valence electrons because its only electron cloud is full with $2$.

Lewis Dot Structures

• A Lewis Dot Structure is a simplified representation of an atom using its chemical symbol surrounded by dots representing its valence electrons.

Lewis Structure Summary

1. Draw the Chemical Symbol: Write the letters representing the element (e.g., $H$, $Li$, $N$).

2. Determine Valence Electrons: Use the atom's group number (skipping groups $3$ through $12$ and dropping the leading "$1$" for groups $13$ through $18$).

3. Draw the Dots: Place the valence electrons around the chemical symbol.

Practice Examples for Lewis Structures

• Hydrogen ($H$): Belongs to the $1\text{st}$ group; it has a structure with the symbol $H$ and one dot.

• Beryllium ($Be$): In the $2\text{nd}$ group; it has $2$ valence electrons.

• Boron ($B$): In the $3\text{rd}$ group (skipping transition metals); it has $3$ valence electrons.

• Aluminum ($Al$): Located in Group $13$ ($13-10=3$); it has $3$ dots surrounding the symbol.

• Nitrogen ($N$): Group $15$; it has $5$ valence electrons. Representation: $\text{:}\\dot{N}\text{.}$

Question and Discussion: Evaluating Accuracy

• Question: Is the following Lewis Structure for Carbon ($C$) correct: $C\text{:}$?

• Clarification: No, it is not correct. Carbon is in Group $14$ and must have $4$ valence electrons, but the example provided only showed $2$.

• Question Regarding Sodium ($Na$): Which Bohr diagram is correct and why?

  • Sodium has the symbol $Na$. A correct Bohr diagram must show its $11$ total electrons: $2$ in the first cloud, $8$ in the second cloud, and $1$ in the third cloud. An incorrect diagram might show the wrong distribution or count.

Practice and Assessment

• Upcoming Quiz: There will be a quiz on Friday covering these topics.

• Instruction: Use the remaining time to practice drawing structures, ask questions, and verify work accuracy.