What is a Molecule?
A molecule is a group of two or more atoms that are chemically bonded. This means that they are held tightly together by attractive forces. A molecule is the smallest part of an element or compound. It can exist independently and shows all the properties of that substance. Atoms can be of the same or different elements.
Molecules of Elements
Molecules of an element are made up of the same type of atoms. Some elements, like argon (Ar) and helium (He), have only one atom. But most non-metals are different.
For example:
- Oxygen (O₂) has two oxygen atoms. It's a diatomic molecule.
- If three oxygen atoms combine, it makes ozone (O₃).
The number of atoms in a molecule is called atomicity.
Here’s a table of atomicity for some non-metals:
| Type of Element | Name | Atomicity |
|---|---|---|
| Non-Metal | Argon | Monoatomic |
| Helium | Monoatomic | |
| Oxygen | Diatomic | |
| Hydrogen | Diatomic | |
| Nitrogen | Diatomic | |
| Chlorine | Diatomic | |
| Phosphorus | Tetra-atomic | |
| Sulphur | Poly-atomic |
Molecules of Compounds
Different atoms join to form molecules of compounds. This happens in definite proportions. Here are some examples:
| Compound | Combining Elements | Ratio by Mass |
|---|---|---|
| Water (H₂O) | Hydrogen, Oxygen | 1:8 |
| Ammonia (NH₃) | Nitrogen, Hydrogen | 14:3 |
| Carbon dioxide (CO₂) | Carbon, Oxygen | 3:8 |
Activity
Look at the table for mass ratios of different elements. Now, find the ratio by number of atoms in these compounds.
For Water (H₂O):
| Element | Ratio by Mass | Atomic Mass | Simplest Ratio / Atomic Mass |
|---|---|---|---|
| H | 1 | 1 | |
| O | 8 | 16 |
Thus, the ratio by number of atoms for water is H:O = 2:1.
What is an Ion?
Compounds made of metals and non-metals contain charged particles called ions. An ion can be a single atom or a group of atoms with a charge.
Ions can be:
- Negatively charged ions: called anions.
- Positively charged ions: called cations.
For example, in sodium chloride (NaCl):
- Sodium ion (Na⁺) is a cation.
- Chloride ion (Cl⁻) is an anion.
A group of atoms with a charge is called a polyatomic ion.
Here are some ionic compounds and their elements:
| Ionic Compound | Constituent Elements | Ratio by Mass |
|---|---|---|
| Calcium oxide | Calcium, Oxygen | 5:2 |
| Magnesium sulphide | Magnesium, Sulphur | 3:4 |
| Sodium chloride | Sodium, Chlorine | 23:35.5 |
Scenario-Based Questions and Answers
Scenario 1: You have a sample of pure oxygen. How many atoms are in a molecule of this oxygen?
Answer: In pure oxygen, the molecule is O₂. This means there are 2 atoms in a molecule.
Scenario 2: You have water (H₂O) in your bottle. What is the ratio of hydrogen atoms to oxygen atoms?
Answer: The ratio of hydrogen to oxygen in water is 2:1.
Scenario 3: If you combine 1 atom of carbon with 2 atoms of oxygen, what compound do you form?
Answer: You will form carbon dioxide (CO₂).
Scenario 4: What type of ion is formed when chlorine gains an electron?
Answer: When chlorine gains an electron, it forms a negatively charged ion called an anion (Cl⁻).
Scenario 5: You mix magnesium with sulfur. What type of compound do you create?
Answer: You create magnesium sulphide, which is an ionic compound.