Introduction to Covalent Bonding
Welcome! So far, you might have learned about how atoms can swap electrons to become stable (ionic bonding). But what happens when two atoms both want to keep their electrons? Instead of a tug-of-war, they decide to share. This "sharing is caring" approach is what we call covalent bonding.
In this chapter, we will explore how non-metal atoms join together to form molecules and giant structures. Don't worry if it seems like a lot of dots and crosses at first—once you see the pattern, it becomes much easier!
Note: For information on how ions are formed, please refer to the "Ionic bonding" chapter.
What is a Covalent Bond?
A covalent bond is formed when a pair of electrons is shared between two atoms. This usually happens between non-metal atoms.
Why do they do this? Atoms are most stable when they have a full outer shell of electrons (like the Noble Gases in Group 0). By sharing electrons, each atom can "count" the shared pair as part of its own outer shell.
Key Takeaway: One covalent bond = one shared pair of electrons.
Representing Covalent Bonds
Chemists use several models to show how atoms are bonded. Each model has its own strengths and weaknesses.
1. Dot-and-Cross Diagrams
These are the most common diagrams you will see. One atom's electrons are shown as dots \((\cdot)\) and the other atom's electrons are shown as crosses \((\times)\). This helps us see which electron came from which atom.
Example: Hydrogen molecule \( (H_{2}) \)
Each Hydrogen atom has only 1 electron but needs 2 for a full shell. They share their single electrons:
\(H \cdot + \times H \rightarrow H \cdot \times H\)
Example: Water molecule \( (H_{2}O) \)
Oxygen is in Group 6, so it has 6 electrons in its outer shell and needs 2 more. It bonds with two Hydrogen atoms, sharing one pair with each.
2. Structural Formulae
This is a simplified version where we use a straight line to represent a shared pair of electrons. For example, \(H-H\) represents a single covalent bond in a hydrogen molecule. Water would be shown as \(H-O-H\).
3. Ball-and-Stick Models
These use plastic balls to represent atoms and sticks to represent the bonds. They are great for showing the 3D shape of a molecule, but they aren't very realistic because atoms aren't actually hard spheres with sticks between them!
4. Space-filling Models
These show the relative sizes of the atoms and how they are packed together. They look more like a cluster of grapes. They are more realistic for showing volume, but it can be hard to see the bonds clearly.
Did you know? A double bond occurs when atoms share two pairs of electrons (4 electrons total). Oxygen gas \( (O_{2}) \) is held together by a double bond, shown as \(O=O\).
The Size of Atoms and Molecules
It is hard to imagine just how tiny atoms and molecules really are. To help you visualize the scale:
- An atom has a radius of about \(10^{-10}\) meters (that is 0.0000000001 meters!).
- A simple molecule (like water or oxygen) consists of only a few atoms, so it is also extremely small.
Because they are so small, we can't see them with a regular microscope. We have to use special scientific models to understand how they behave.
Simple Molecular vs. Giant Covalent Structures
Covalent bonding can result in two very different types of structures:
1. Simple Molecular Structures
These are made of small, distinct groups of atoms. Examples include \(H_{2}O\) (water), \(CO_{2}\) (carbon dioxide), and \(CH_{4}\) (methane). The atoms inside the molecule are held together by very strong covalent bonds, but the molecules themselves aren't strongly attached to their neighbors.
2. Giant Covalent Structures
In these structures, billions of atoms are all linked together in a massive network of covalent bonds. There are no individual molecules. Examples you need to know include diamond and graphite (both made of carbon) and silicon dioxide (sand).
Common Mistake to Avoid: Students often think that because covalent bonds are strong, all covalent substances have high melting points. This is not true for simple molecules! While the bond is strong, the attraction between molecules is weak. You will learn more about this in the "Types of substance" chapter.
Quick Review: Step-by-Step for Dot-and-Cross Diagrams
If you are asked to draw a covalent molecule, follow these steps:
- Check the Group Number of the atoms to see how many electrons are in their outer shell.
- Work out how many more electrons each atom needs to get a full shell (usually 8, but only 2 for Hydrogen).
- The number of electrons needed is usually the number of bonds the atom will form.
- Draw the atoms overlapping and put the shared pairs in the overlapping section.
- Fill in the rest of the outer electrons for each atom.
Summary Key Points
- Covalent bond: A shared pair of electrons.
- Occurs between: Non-metal atoms.
- Goal: To achieve a stable, full outer electron shell.
- Models: Dot-and-cross, structural (lines), ball-and-stick, and space-filling.
- Scale: Atoms are roughly \(10^{-10}\) meters in size.
Don't worry if this seems tricky at first! The more you practice drawing these diagrams, the more natural it will feel to spot the patterns in the Periodic Table.