Hello everyone preparing for the TCAS exam!
Welcome to the lesson on "Covalent Substances," one of the key topics in Physical Science (Applied Science). This topic isn't as difficult as it seems! If you understand the principle of "sharing," you'll grasp covalent bonds in no time. In this chapter, we’ll explore how substances around us—like water, oxygen gas, and even cooking gas—are formed. Let’s get started!
1. What is a Covalent Bond? (A Perfect Share)
The word "Covalent" comes from "Co" (together) and "valence" (outermost electrons). Therefore, a covalent bond is an attractive force resulting from atoms of elements "sharing electrons."
A visual analogy:
Imagine you need 8 baht to buy a snack, but you only have 7. Your friend also has 7 baht. If you stay apart, neither of you can buy the snack. But if you both "put your money in the middle" and say, "This is our money," both of you effectively have 8 baht to spend. That’s how it works!
Key points:
- Usually occurs between non-metal + non-metal (since non-metals love to attract electrons and rarely give them away, so they share them instead).
- Atoms forming bonds try to make their valence electrons count up to 8 (following the Octet Rule) to become stable, just like noble gases.
If it feels tough at first, don't worry:
Just remember: "Covalent = Non-metal + Non-metal + Sharing." That simple rule is enough to help you eliminate many options in the exam!
2. Writing Formulas and Naming Covalent Substances
Naming covalent substances is different from ionic substances because we must "indicate the number of atoms" using Greek prefixes.
Prefixes you need to know:
1 = Mono- | 2 = Di- | 3 = Tri- | 4 = Tetra- | 5 = Penta-
Naming Principles:
1. Name the first element as is.
2. Name the second element, changing the suffix to "-ide."
3. Use the Greek prefixes to indicate the number of atoms before the element name.
Common Mistake:
If the first element has only 1 atom, we do not say "mono." For example, \(CO_{2}\) is called carbon dioxide (not monocarbon dioxide).
Examples:
- \(N_{2}O_{4}\) is called dinitrogen tetroxide.
- \(PCl_{5}\) is called phosphorus pentachloride.
3. General Properties of Covalent Substances
Why is water a liquid while salt is a solid? It’s because of the different attractive forces. Most covalent substances share these characteristics:
1. Low boiling and melting points: Compared to ionic substances, because the intermolecular forces aren't very strong (easily broken with a little heat).
2. States of matter: Can be solids (e.g., table sugar), liquids (e.g., water, alcohol), and gases (e.g., oxygen, carbon dioxide).
3. Electrical conductivity: Most do not conduct electricity in any state (because there are no free-moving ions or electrons).
4. Solubility: Some dissolve in water (like sugar), while others do not (like oil).
Did you know?
Diamond and graphite (pencil lead) are also covalent substances! However, they are a special type called "network covalent solids," where the atoms are so tightly bonded that they have extremely high melting points and are incredibly strong.
4. Molecular Polarity (The Tug-of-War of Electrons)
In a covalent molecule, each atom has a different ability to attract electrons (EN value). Think of it like a game of "tug-of-war."
1. Non-polar molecules: Like two people with equal strength pulling a rope; the electrons stay exactly in the middle. Examples: \(O_{2}\), \(H_{2}\), \(CH_{4}\).
2. Polar molecules: Like one person pulling stronger than the other; the electrons are pulled toward the stronger side, creating a slight electric charge. Examples: \(H_{2}O\) (water), \(NH_{3}\) (ammonia).
Key point:
Substances with the same polarity dissolve in each other ("like dissolves like"). For example, polar water dissolves well in polar alcohol, but water won't dissolve in non-polar oil.
Final Summary: Key Takeaways
- Covalent substances are formed by non-metals + non-metals sharing electrons.
- Naming requires indicating quantities (mono, di, tri...) and adding the "-ide" suffix.
- Main properties: Low boiling/melting points, and generally non-conductive.
- Polarity is determined by the "tug-of-war" for electrons and affects water solubility.
"If you can remember the principle of electron sharing and these basic properties, the covalent substances section of your A-Level Applied Science exam will no longer be difficult. Good luck, everyone!"