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)