Question

State the steps used to draw Lewis structures.

Short answer

To draw Lewis structures, follow these steps: (1) Determine the total number of valence electrons in the molecule. (2) Arrange the atoms with the least electronegative atom (excluding hydrogen) in the center and the others surrounding it. (3) Assign electrons to atoms by creating single bonds between the central atom and surrounding atoms. (4) Distribute remaining valence electrons as lone pairs to achieve the octet or duet rule. (5) Check all the atoms to ensure they follow the octet or duet rule and form double or triple bonds if needed. (6) Verify the stability of the Lewis structure by comparing the total number of electrons and checking for resonance structures. Practice with various molecules to become proficient in drawing Lewis structures.

Step-by-step solution

1. Introduction to Lewis Structures

A Lewis structure is a diagram that represents the electron distribution around atoms in a molecule. It uses lines to represent chemical bonds and dots to represent electrons, particularly the valence electrons. Lewis structures are helpful for understanding and predicting molecular geometries, bond angles, and chemical reactivity.

2. Determine the Total Number of Valence Electrons

You will first need to determine the total number of valence electrons in the molecule. To do this, calculate the sum of the valence electrons of all the atoms present in the molecule. In case of ions, add or subtract electrons based on whether it is an anion or a cation, respectively. Make sure to account for any charges that the molecule may carry.

3. Arrange the Atoms

Place the least electronegative atom (excluding hydrogen) in the center of the molecule. Then, arrange the other atoms around the central atom. Hydrogen atoms are generally terminal atoms, meaning they will be on the outside of the molecule. In polyatomic ions, place the charged atoms closer to the central atom.

4. Assign Electrons to Atoms

Begin assigning the valence electrons to the atoms by creating single bonds between the central atom and the atoms surrounding it. To form a single bond, draw a line representing two shared electrons between the atoms. After forming single bonds, distribute the remaining valence electrons as lone pairs to achieve the octet rule (except for hydrogen, which follows the duet rule). Lone pairs are represented as dots, with each dot representing one electron.

5. Check the Octet Rule

The octet rule states that, with a few exceptions, atoms should have eight electrons in their valence shell to achieve stability. Check all the atoms in the molecule to ensure they follow the octet rule (or duet rule in the case of hydrogen). If an atom does not obey the octet rule, you may need to form double or triple bonds to satisfy the rule.

6. Verify the Lewis Structure's Stability

Once the structure is complete, double-check that all atoms follow the octet or duet rule and that the total number of valence electrons is consistent with your initial calculation. Also, check for resonance structures, which occur when multiple valid Lewis structures represent the same molecule. A resonance structure is signified by a double-headed arrow between the different valid structures. By following these steps and practicing with various molecules, you will become proficient in drawing Lewis structures for different chemical compounds.

Key concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom and play a crucial role in determining how atoms interact with each other. They are the electrons involved in forming chemical bonds.
Each element has a different number of valence electrons. For example, hydrogen has one, while oxygen has six. The number of valence electrons can be found using the periodic table since each column corresponds to a specific number.

• Group 1 elements have 1 valence electron.
• Group 17 elements have 7 valence electrons.

Calculating the total number of valence electrons for a molecule or compound is an essential step in drawing Lewis structures. This helps in determining how the electrons will be distributed among the atoms in the molecule.

Octet Rule

The octet rule is a fundamental concept in chemistry that suggests atoms are most stable when their outer shell is complete with eight electrons. This configuration is similar to that of noble gases, which are highly stable and unreactive.
The rule is applied when drawing Lewis structures to ensure that atoms achieve this "octet" by sharing or transferring electrons through bonds. However, there are exceptions.

• Hydrogen is satisfied with only two electrons (duet rule).
• Boron often operates with less than eight due to its availability for six electrons.
• Nonmetals in period 3 and beyond can exceed an octet, utilizing d-orbitals for expansion (such as phosphorus and sulfur).

Understanding the octet rule helps in predicting molecular structures and the stability of the resulting configuration.

Chemical Bonds

Chemical bonds are the forces holding atoms together in a molecule. They result from the sharing or transferring of valence electrons between atoms. There are three primary types of chemical bonds:

• Covalent Bonds: Formed when two atoms share one or more pairs of valence electrons. Covalent bonds can be single, double, or triple, depending on the number of shared electrons. Double and triple bonds occur when atoms need to fulfill the octet rule.
• Ionic Bonds: Occur when electrons are transferred from one atom to another, resulting in oppositely charged ions. This bond is typical in compounds formed between metals and nonmetals.
• Metallic Bonds: Found in metals, where electrons flow freely between atoms, contributing to properties like conductivity and malleability.

By understanding chemical bonds, one can predict how different atoms will interact to form stable molecules.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule, which is influenced by the type and number of chemical bonds. Understanding molecular geometry is important as it affects the physical and chemical properties of a substance.
The VSEPR (Valence Shell Electron Pair Repulsion) theory is commonly used to predict molecular geometry. This theory states that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsive forces. Some common geometries include:

• Linear: Occurs with molecules like CO₂, where there are two regions of electron density.
• Tetrahedral: Seen in molecules like CH₄ with four bonds around the central atom.
• Trigonal Planar: Found in molecules like BH₃ with three surrounding bonds.
• Bent or V-shaped: Common in water (H₂O), where the presence of lone pairs changes the geometry from linear to bent.

Identifying molecular geometry helps chemists understand a molecule's reactivity, polarity, phase of matter, and interaction with other molecules.