Elements, Compounds and Reactions

Overview

This topic forms the foundation of chemistry in the TET-2 syllabus, covering the building blocks of matter and how substances interact. Questions typically test your understanding of atomic structure, the organisation of elements in the periodic table, differences between elements and compounds, and the types of chemical reactions students encounter in classes 6-8.

For GTET, expect 3-5 questions from this area, often combining conceptual understanding with practical applications. You must be clear on definitions, symbols of common elements, valency concepts, and the ability to identify reaction types from given equations. The pedagogy angle may ask how to teach abstract atomic concepts to upper primary students using models and activities.

Mastering this topic also supports related chemistry areas like acids-bases-salts and helps with integrated science questions where chemical principles apply to biological or environmental contexts.

Key Concepts

• **Atoms are the smallest particles** of an element that retain its chemical properties. They consist of protons (positive), neutrons (neutral) in the nucleus, and electrons (negative) orbiting outside.

• **Molecules are groups of atoms** bonded together. A molecule of an element contains identical atoms (O₂, N₂), while a molecule of a compound contains different atoms (H₂O, CO₂).

• **Elements are pure substances** made of only one type of atom and cannot be broken down by chemical means. There are 118 known elements, about 92 occurring naturally.

• **Compounds are pure substances** made of two or more elements chemically combined in a fixed ratio. Properties of compounds differ entirely from their constituent elements.

• **The periodic table arranges elements** by increasing atomic number. Rows are called periods (7 total), columns are called groups (18 total). Elements in the same group have similar properties.

• **Valency is the combining capacity** of an atom, determined by electrons in the outermost shell. It decides how atoms bond to form compounds.

• **Chemical reactions involve** breaking of bonds in reactants and forming new bonds to create products. Mass is conserved—atoms are neither created nor destroyed.

• **Chemical equations use symbols and formulas** to represent reactions. Reactants appear on the left, products on the right, with an arrow showing the direction of change.

Formulas / Key Facts

**Atomic Structure**

• Atomic number = Number of protons = Number of electrons (in neutral atom)