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)
• Mass number = Protons + Neutrons
• Electrons in shells: 2 in first shell, 8 in second, 8 in third (for first 20 elements)

**Common Element Symbols to Remember**

• Hydrogen (H), Oxygen (O), Nitrogen (N), Carbon (C), Sulphur (S)
• Sodium (Na), Potassium (K), Iron (Fe), Copper (Cu), Silver (Ag), Gold (Au)
• Chlorine (Cl), Calcium (Ca), Magnesium (Mg), Aluminium (Al), Zinc (Zn)

**Valency of Common Elements**

• Valency 1: H, Na, K, Cl, Ag
• Valency 2: O, Mg, Ca, Zn, Cu (cupric)
• Valency 3: Al, Fe (ferric), N
• Valency 4: C, Si

**Types of Chemical Reactions**

• Combination: A + B → AB (e.g., 2H₂ + O₂ → 2H₂O)
• Decomposition: AB → A + B (e.g., 2H₂O → 2H₂ + O₂)
• Displacement: A + BC → AC + B (e.g., Zn + CuSO₄ → ZnSO₄ + Cu)
• Double displacement: AB + CD → AD + CB (e.g., NaCl + AgNO₃ → AgCl + NaNO₃)

**Periodic Table 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

Worked Examples

**Example 1: Finding Valency from Electron Configuration**

Question: Sodium has atomic number 11. Find its valency.

Solution:

• Atomic number 11 means 11 electrons
• Distribution: 2 in first shell, 8 in second shell, 1 in third shell (2, 8, 1)
• Outermost shell has 1 electron
• Valency = 1 (it's easier to lose 1 electron than gain 7)

**Example 2: Writing a Chemical Formula**

Question: Write the formula for aluminium oxide.

Solution:

• Aluminium (Al) has valency 3
• Oxygen (O) has valency 2
• Cross-multiply valencies: Al₂O₃
• Formula: Al₂O₃

**Example 3: Identifying Reaction Type**

Question: Identify the type of reaction: CaCO₃ → CaO + CO₂

Solution:

• One reactant (CaCO₃) breaks into two products (CaO and CO₂)
• Single compound splitting into simpler substances
• This is a decomposition reaction (thermal decomposition of limestone)

Common Mistakes

• **Confusing atoms and molecules**: Students think H₂O is an atom. Correction: H₂O is a molecule made of 3 atoms (2 hydrogen, 1 oxygen). Atoms are individual units like one H or one O.

• **Mixing up atomic number and mass number**: Students add protons and electrons for atomic number. Correction: Atomic number equals protons only. Mass number equals protons plus neutrons.

• **Assuming compound properties match element properties**: Students expect sodium chloride (NaCl) to be reactive like sodium metal. Correction: Compounds have completely different properties from their elements. NaCl is stable table salt, not a reactive metal.

• **Forgetting to balance equations**: Students write H₂ + O₂ → H₂O and consider it complete. Correction: Atoms must be equal on both sides. Balanced equation: 2H₂ + O₂ → 2H₂O.

• **Confusing displacement and double displacement**: Students identify any exchange reaction as displacement. Correction: Single displacement has one element replacing another. Double displacement involves exchange between two compounds.

Quick Reference

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

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

• Valency = electrons in outermost shell (if ≤4) or 8 minus outermost electrons (if >4)

• Periodic table: Periods are horizontal rows; Groups are vertical columns with similar properties

• Law of conservation of mass: Total mass of reactants = Total mass of products

• For teaching atoms to students: Use ball-and-stick models, clay models, and visual animations to make abstract concepts concrete