…A… are formed when atoms of different elements combine in a …B… ratio. Here, A and B refer to — Mole Concept and Some Basic Concepts of Chemistry Chemistry Question

💡 Solution & Explanation

# Solution **Understanding the Blanks:** - **Blank A**: When atoms of different elements combine, they form **compounds**. - **Blank B**: These atoms combine in a **fixed** (or **definite**) ratio. **Step-by-step reasoning:** 1. **Definition of compounds**: A compound is a pure substance formed by the chemical combination of two or more different elements in a definite proportion by mass. 2. **Fixed ratio requirement**: The key characteristic distinguishing compounds from mixtures is that compounds always have atoms combining in a **fixed (or definite) ratio**. For example, $H_2O$ always contains hydrogen and oxygen in a 2:1 atomic ratio. 3. **Why this matters**: This fixed ratio arises from the valency and bonding requirements of the atoms involved. Whether you have one molecule of water or one mole, the H:O ratio remains constant at 2:1. **Answer: A = Compounds, B = Fixed (or Definite)** This is a fundamental principle of chemistry—compounds have invariant composition, which is the primary difference from physical mixtures where component ratios can vary.