Elements, Compounds and Reactions

Overview

This topic forms the backbone of chemistry content in KTET Category II and III papers. Understanding atoms, molecules, the periodic table and chemical reactions is essential not only for answering direct content questions but also for demonstrating your ability to teach these foundational concepts to upper primary and secondary students.

KTET typically tests your grasp of basic atomic structure, the logic behind periodic table organisation, the distinction between elements and compounds, and the types of chemical reactions students encounter in classes 6–10. Questions often blend factual recall with application—expect scenarios asking how you would explain a concept or identify the type of reaction in a given equation.

Mastering this topic requires you to connect abstract ideas (like electron configuration) to observable phenomena (like why sodium reacts violently with water). This conceptual linking is precisely what examiners and future students will expect from you.

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Key Concepts

• **Atom**: The smallest particle of an element that retains the element's chemical properties; consists of protons, neutrons (in the nucleus) and electrons (orbiting the nucleus).

• **Molecule**: Two or more atoms chemically bonded together; can be of the same element (O₂, N₂) or different elements (H₂O, CO₂).

• **Element**: A pure substance made of only one type of atom; cannot be broken down into simpler substances by chemical means (e.g., iron, oxygen, gold).

• **Compound**: A substance formed when two or more different elements chemically combine in a fixed ratio (e.g., water H₂O always has 2 hydrogen atoms for every 1 oxygen atom).

• **Atomic Number (Z)**: Number of protons in the nucleus; defines which element an atom is.

• **Mass Number (A)**: Total number of protons + neutrons in the nucleus.

• **Valency**: The combining capacity of an atom, determined by the number of electrons it can lose, gain or share to achieve a stable configuration.

• **Chemical Reaction**: A process where reactants transform into products through bond breaking and bond formation, always conserving mass (Law of Conservation of Mass).

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Formulas / Key Facts

| Concept | Formula / Fact | |---------|----------------| | Mass Number | A = Number of protons + Number of neutrons | | Atomic Number | Z = Number of protons = Number of electrons (in neutral atom) | | Number of Neutrons | Neutrons = A − Z | | Avogadro's Number | 6.022 × 10²³ particles per mole | | Law of Conservation of Mass | Total mass of reactants = Total mass of products | | Periodic Table Groups | Group 1: Alkali metals, Group 17: Halogens, Group 18: Noble gases | | Periodic Trends | Atomic size increases down a group; decreases across a period (left to right) | | Electronegativity Trend | Increases across a period; decreases down a group |

**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)**: Transfer of electrons; oxidation = loss of electrons, reduction = gain of electrons

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Worked Examples

### Example 1: Calculating Subatomic Particles **Problem**: An atom has atomic number 11 and mass number 23. Find the number of protons, electrons and neutrons.

**Solution**:

• Protons = Atomic number = 11
• Electrons = Protons (for neutral atom) = 11
• Neutrons = Mass number − Atomic number = 23 − 11 = 12

**Answer**: 11 protons, 11 electrons, 12 neutrons (This is sodium, Na)

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### Example 2: Identifying Reaction Type **Problem**: Classify the reaction: CaCO₃ → CaO + CO₂

**Solution**:

• One compound (calcium carbonate) breaks down into two simpler substances
• This is a **decomposition reaction**
• Specifically, thermal decomposition (happens on heating)

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### Example 3: Balancing a Chemical Equation **Problem**: Balance: Fe + O₂ → Fe₂O₃

**Solution**:

• Count atoms: Reactants have 1 Fe, 2 O; Products have 2 Fe, 3 O
• Balance Fe: Put 4 before Fe on left, 2 before Fe₂O₃ on right → 4Fe + O₂ → 2Fe₂O₃
• Now balance O: Right side has 6 O, so put 3 before O₂ on left
• **Balanced equation**: 4Fe + 3O₂ → 2Fe₂O₃
• Verify: 4 Fe on each side, 6 O on each side ✓

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Common Mistakes

| Wrong Thinking | Correct Fix | |----------------|-------------| | Confusing atomic number with mass number | Remember: Atomic number = protons only; Mass number = protons + neutrons | | Thinking molecules must contain different elements | Molecules can be of same element (O₂, N₂) or different elements (H₂O) | | Believing compounds can be separated by physical means | Compounds require chemical methods for separation; mixtures use physical methods | | Mixing up oxidation and reduction | Use mnemonic "OIL RIG": Oxidation Is Loss, Reduction Is Gain (of electrons) | | Forgetting to balance equations | Always verify: count each type of atom on both sides must be equal | | Assuming group number equals valency for all elements | Works for groups 1–3; for groups 15–17, valency = 18 minus group number |

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Quick Reference

• **Element vs Compound**: Element = one type of atom; Compound = two or more different atoms chemically bonded in fixed ratio

• **Periodic Table Logic**: Rows (periods) = number of electron shells; Columns (groups) = similar valence electrons and chemical properties

• **Balancing equations**: Never change subscripts; only adjust coefficients

• **Metals vs Non-metals**: Metals lose electrons (form positive ions); Non-metals gain electrons (form negative ions)

• **Reactivity Series** (descending): K > Na > Ca > Mg > Al > Zn > Fe > Cu > Ag > Au

• **Valency shortcut**: Group 1 = 1, Group 2 = 2, Group 13 = 3, Group 14 = 4, Group 15 = 3, Group 16 = 2, Group 17 = 1, Group 18 = 0