1. Laws of Chemical Combination
Chemical reactions follow two key laws discovered by scientists like Lavoisier and Proust.
1.1 Law of Conservation of Mass
This law states that mass is neither created nor destroyed in a chemical reaction. The total mass of reactants equals the mass of products. For example, when sodium carbonate reacts with acetic acid, the mass before and after the reaction remains the same.
1.2 Law of Constant Proportions
In a compound, elements are always present in a fixed ratio by mass, regardless of the source. For example, water always has hydrogen and oxygen in a 1:8 mass ratio, and ammonia has nitrogen and hydrogen in a 14:3 ratio.
Activity: Mix a solution of copper sulphate and sodium carbonate. Weigh before and after mixing to see if the mass changes!
2. What is an Atom?
An atom is the smallest particle of an element that cannot usually exist independently. Atoms are tiny, with sizes measured in nanometres (1 nm = 10⁻⁹ m). For example, a hydrogen atom has a radius of about 10⁻¹⁰ m.
2.1 Symbols of Atoms
Each element has a unique symbol (e.g., H for hydrogen, O for oxygen). Some symbols come from Latin names, like Fe for iron (ferrum). The IUPAC approves these symbols.
2.2 Atomic Mass
Each element has a characteristic atomic mass, measured in atomic mass units (u), where 1 u is 1/12th the mass of a carbon-12 atom. For example, carbon is 12 u, oxygen is 16 u.
2.3 How Do Atoms Exist?
Atoms rarely exist alone; they form molecules or ions. For example, oxygen exists as O₂ molecules, and sodium chloride forms Na⁺ and Cl⁻ ions.
Fun Fact: Millions of atoms stacked together are still thinner than a sheet of paper!
3. What is a Molecule?
A molecule is a group of two or more atoms chemically bonded together, capable of independent existence. Molecules show all the properties of the substance.
3.1 Molecules of Elements
These consist of the same type of atoms. For example, oxygen (O₂) is diatomic (2 atoms), while phosphorus (P₄) is tetra-atomic. Some elements like argon (Ar) are monoatomic.
3.2 Molecules of Compounds
These have atoms of different elements in fixed ratios, e.g., water (H₂O, H:O = 2:1), ammonia (NH₃, N:H = 1:3).
3.3 Ions
Ions are charged particles. Cations (e.g., Na⁺) are positively charged, and anions (e.g., Cl⁻) are negatively charged. Polyatomic ions, like NH₄⁺ or SO₄²⁻, are groups of atoms with a charge.
Activity: Check the formula of water (H₂O). Count the number of hydrogen and oxygen atoms!
4. Writing Chemical Formulae
A chemical formula shows the elements in a compound and their ratios. The valency of an element determines how it combines. For example, magnesium (Mg²⁺) and chloride (Cl⁻) form MgCl₂ by criss-crossing valencies.
Rules:
- Metal symbols are written first (e.g., NaCl).
- Valencies or charges must balance.
- Polyatomic ions are enclosed in brackets if more than one, e.g., (NH₄)₂SO₄.
Try This: Write the formula for calcium oxide using valencies (Ca²⁺, O²⁻)!
5. Molecular Mass
The molecular mass is the sum of the atomic masses of all atoms in a molecule, measured in atomic mass units (u). For example, H₂O = (2 × 1) + 16 = 18 u.
For ionic compounds, we calculate the formula unit mass. For example, NaCl = 23 + 35.5 = 58.5 u.
Activity: Calculate the molecular mass of CO₂ using atomic masses (C = 12 u, O = 16 u)!
Next Chapter
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