Elements, Compounds and Reactions

Overview

This topic forms the chemistry core of Paper II Mathematics and Science, covering the fundamental building blocks of matter and how they interact. For TS TET, you need a solid grasp of atomic structure, the organisation of the periodic table, and the types of chemical reactions that students in classes 6-8 encounter. Questions typically test your ability to distinguish between elements and compounds, interpret chemical symbols and formulas, and identify reaction types.

Understanding this topic is essential not just for content questions but also for pedagogy questions that ask how to teach abstract concepts like atoms and molecules to upper primary students. The examiner expects you to know both the subject matter and age-appropriate ways to explain it. Focus on definitions, symbols, valency basics, and the five main reaction types—these appear repeatedly in TET papers.

Key Concepts

• **Atom**: The smallest particle of an element that retains its chemical identity. Cannot be divided by ordinary chemical means. Example: One atom of iron (Fe).

• **Molecule**: Two or more atoms chemically bonded together. Can be of the same element (O₂, N₂) or different elements (H₂O, CO₂). The smallest unit of a compound that shows all its properties.

• **Element**: A pure substance made of only one type of atom. Cannot be broken into simpler substances by chemical reactions. 118 elements known; 94 occur naturally.

• **Compound**: A substance formed when two or more elements chemically combine in a fixed ratio. Properties differ from constituent elements. Example: Water (H₂O) is liquid, though hydrogen and oxygen are gases.

• **Mixture vs Compound**: Mixtures have variable composition and components retain properties; compounds have fixed composition and new properties emerge.

• **Valency**: The combining capacity of an atom, determined by electrons in the outermost shell. Helps predict how atoms bond. Example: Oxygen has valency 2, so it forms H₂O (two hydrogen atoms per oxygen).

• **Chemical Formula**: Symbolic representation showing types and numbers of atoms in a molecule. H₂SO₄ means 2 hydrogen, 1 sulphur, 4 oxygen atoms.

• **Chemical Equation**: Shorthand for a chemical reaction showing reactants → products with balanced atoms on both sides.

Formulas / Key Facts

**Atomic Structure Basics**

• Protons (positive) and neutrons (neutral) in nucleus; electrons (negative) orbit in shells
• Atomic number = number of protons = number of electrons (in neutral atom)
• Mass number = protons + neutrons
• First shell holds max 2 electrons; second and third hold max 8 each

**Periodic Table Organisation**

• Rows = Periods (7 periods); Columns = Groups (18 groups)
• Group 1: Alkali metals (Li, Na, K) — highly reactive, valency 1
• Group 2: Alkaline earth metals (Mg, Ca) — valency 2
• Group 17: Halogens (F, Cl, Br, I) — reactive non-metals, valency 1
• Group 18: Noble gases (He, Ne, Ar) — inert, valency 0
• Metals on left and centre; non-metals on right; metalloids along the zigzag line

**Common Valencies**

• Hydrogen: 1, Oxygen: 2, Nitrogen: 3, Carbon: 4
• Sodium: 1, Magnesium: 2, Aluminium: 3, Iron: 2 or 3

**Five Types of Chemical Reactions** 1. Combination: A + B → AB (e.g., 2H₂ + O₂ → 2H₂O) 2. Decomposition: AB → A + B (e.g., 2H₂O → 2H₂ + O₂) 3. Displacement: A + BC → AC + B (e.g., Zn + CuSO₄ → ZnSO₄ + Cu) 4. Double Displacement: AB + CD → AD + CB (e.g., NaCl + AgNO₃ → AgCl + NaNO₃) 5. Oxidation-Reduction (Redox): Involves gain/loss of oxygen or electrons

Worked Examples

**Example 1: Writing a Chemical Formula**

*Problem*: Write the formula for magnesium chloride.

*Solution*:

• Magnesium (Mg) has valency 2
• Chlorine (Cl) has valency 1
• Cross-multiply valencies: Mg₁Cl₂
• Formula: MgCl₂

**Example 2: Balancing a Chemical Equation**

*Problem*: Balance the equation: Fe + O₂ → Fe₂O₃

*Solution*:

• Count atoms: Left side has 1 Fe, 2 O; Right side has 2 Fe, 3 O
• Step 1: Balance Fe by putting 4 before Fe and 2 before Fe₂O₃

4Fe + O₂ → 2Fe₂O₃

• Step 2: Now right side has 6 O, so put 3 before O₂

4Fe + 3O₂ → 2Fe₂O₃

• Verify: Left = 4 Fe, 6 O; Right = 4 Fe, 6 O ✓

**Example 3: Identifying Reaction Type**

*Problem*: Classify: CaCO₃ → CaO + CO₂

*Solution*:

• One reactant breaks into two products
• This is a **decomposition reaction**
• Specifically, thermal decomposition (limestone heated to form quickite and carbon dioxide)

Common Mistakes

• **Confusing atoms and molecules**: Students think H₂ is an atom. Correction: H₂ is a molecule made of two hydrogen atoms; a single H is an atom.

• **Believing compounds are mixtures**: Students assume salt water and sodium chloride are the same type. Correction: Salt water is a mixture (variable, separable); NaCl is a compound (fixed ratio, chemically bonded).

• **Forgetting to balance equations**: Writing H₂ + O₂ → H₂O without balancing. Correction: Always count atoms on both sides; the balanced equation is 2H₂ + O₂ → 2H₂O.

• **Misreading the periodic table**: Confusing group number with period number. Correction: Groups are vertical columns (determine valency); periods are horizontal rows (determine number of shells).

• **Assuming all metals are solid**: Students forget mercury (Hg) is liquid at room temperature. Correction: Mercury is the only liquid metal; bromine is the only liquid non-metal.

Quick Reference

• **Element** = one type of atom; **Compound** = two or more elements chemically combined in fixed ratio

• Atomic number = protons; Mass number = protons + neutrons

• Valency = combining capacity; cross-multiply to write formulas

• Periodic table: Groups (vertical) show similar properties; Periods (horizontal) show increasing atomic number

• Five reaction types: Combination, Decomposition, Displacement, Double Displacement, Redox

• Always balance equations: atoms of each element must be equal on both sides