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## Chemistry Grade II ### Start-Up Activity 1. What are the basic building blocks of the following substances: * Water? * Chalk? * Sugar? 2. Why do different materials show different properties? For example, materials such as wood can burn, an iron nail can cut, and salt dissolves in w...

## Chemistry Grade II ### Start-Up Activity 1. What are the basic building blocks of the following substances: * Water? * Chalk? * Sugar? 2. Why do different materials show different properties? For example, materials such as wood can burn, an iron nail can cut, and salt dissolves in water. ### Unit 1: Dalton's Atomic Theory and the Modern Atomic Theory **At the end of this section, you will be able to:** * Compare the views of different philosophers on the nature of matter. * Explain the historical development of the atomic theory. * State the postulates of Dalton's atomic theory. * State the postulates of the modern atomic theory. * State the laws of conservation of mass, definite proportions, and the basis of each of these laws. * Use postulates of Dalton's atomic theory to explain multiple proportions. * Evaluate postulates of Dalton's and the modern atomic model. #### 1.1. Introduction In Grade 9, you learned about the historical development of the atomic theory of matter. In this unit, we will briefly revise it and discuss in detail about early experimental evidence to characterize an atom, the atomic spectra, different models of an atom, atomic mass, isotopes, and the applications of these concepts. #### Activity 1.1 * **Form a group and discuss the following questions, then present your views to the whole class.** 1. How do the beliefs about the structure of matter evolve? 2. Describe the early developments leading to the modern concept of the atom. 3. Can we see atoms with our naked eyes? The philosophers of ancient Greece wondered about the composition of matter. While most philosophers, including Plato and Aristotle, believed that matter is continuously divisible into ever smaller and smaller pieces, or is there an ultimate limit? Democritus disagreed. The Greek philosopher Democritus (460-370 BC) suggested that if you divided matter into smaller and smaller pieces, you would eventually end up with tiny, indestructible particles called *atomos*, or "atoms," meaning "indivisible."