Molecular Shapes, Lewis Dot Structures, and Valence Electrons

Shapes of Molecules

Introduction

Not all molecules have the same shape, and these differences in molecular geometry can significantly affect their chemical properties and functions. Understanding molecular shapes is essential for predicting reactivity, polarity, and interactions in chemical and biological systems.

• Molecular shape refers to the three-dimensional arrangement of atoms in a molecule.

• Shapes are determined by the number and arrangement of electron pairs around the central atom.

• Before discussing the impact of shape, it is necessary to learn how to determine the shape of molecules.

Lewis Dot Structures Theory

Representing Valence Electrons

A Lewis Dot Structure is a diagram that shows the valence electrons of atoms as dots around the chemical symbol. This representation helps visualize bonding and lone pairs in molecules.

• Valence electrons are the electrons in the outermost shell of an atom.

• Bonding pairs are shared electrons between atoms, shown as two dots or a single line (–).

• Lone pairs are nonbonding pairs of electrons placed on the outside of the atom symbol.

Periodic Table Groups and Valence Electrons

The number of valence electrons for main group elements can be determined from their group number on the Periodic Table.

Group Number	Number of Valence Electrons
1A (1)	1
2A (2)	2
3A (13)	3
4A (14)	4
5A (15)	5
6A (16)	6
7A (17)	7
8A (18)	8

These values are used to construct Lewis Dot Structures for molecules.

Lewis Dot Structure Examples

• Each atom's valence electrons are placed as dots around the symbol.

• For diatomic elements (e.g., F2), each atom shares electrons to complete its octet.

• Bonding pairs are represented by lines or pairs of dots between atoms.

Example: Fluorine (F2) Each fluorine atom has 7 valence electrons. They share one pair to form a single bond, completing their octets.

Bonding Patterns for Nonmetals

The number of bonds an atom can make is related to the number of electrons needed to complete its octet.

Group	Valence Electrons	Typical Number of Bonds
1A (1)	1	1
4A (14)	4	4
5A (15)	5	3
6A (16)	6	2
7A (17)	7	1

Additional info: Hydrogen only requires two electrons to be complete.

Calculating Valence Electrons in Molecules

Step-by-Step Calculation

To determine the total number of valence electrons in a molecule:

1. Identify the group number for each atom in the molecule.

2. Multiply the number of atoms by the number of valence electrons for each type.

3. Add up all the valence electrons.

Example 1: CO2 Carbon (4A): 4 electrons Oxygen (6A): 2 × 6 = 12 electrons Total: 4 + 12 = 16 electrons

Example 2: H2S Hydrogen (1A): 2 × 1 = 2 electrons Sulfur (6A): 6 electrons Total: 2 + 6 = 8 electrons

Practice Problems

• Calculate the total number of valence electrons for: N2O4, SF6, BF3, CCl4, C2H6

• Answers: a) 34, b) 48, c) 24, d) 32, e) 14

Lewis Dot Structures for Multiple Bonds

Double and Triple Bonds

Single bonds contain two electrons, double bonds contain four electrons, and triple bonds contain six electrons.

• Double bonds are represented by two lines or four dots between atoms.

• Triple bonds are represented by three lines or six dots between atoms.

Example: O2 (Oxygen molecule) Each oxygen atom has 6 valence electrons. They share two pairs (four electrons) to form a double bond.

Example: N2 (Nitrogen molecule) Each nitrogen atom has 5 valence electrons. They share three pairs (six electrons) to form a triple bond.

Drawing Lewis Dot Structures for Compounds

Steps for Drawing Lewis Dot Structures

1. Determine the arrangement of atoms (central atom is usually the least electronegative).

2. Calculate the total number of valence electrons.

3. Attach each bonded atom to the central atom with a pair of electrons.

4. Place the remaining electrons to complete octets (two for hydrogen).

Example 1: CH4 (Methane) Carbon is the central atom. Attach four hydrogens with single bonds. Place remaining electrons to complete octets.

Example 2: CO2 (Carbon dioxide) Carbon is the central atom. Attach two oxygens. Use double bonds to complete octets.

Additional info: The octet rule states that atoms tend to form bonds until they are surrounded by eight valence electrons (except hydrogen, which requires only two).