A Molecular View of Matter

Energy cannot be created or destroyed in a chemical reaction or in a physical change. 

With the dawn of the nuclear age in the 1940s, scientists, and then the world, became aware that matter can be converted into energy. In nuclear reactions (Chapter 22), matter is transformed into energy. The relationship between matter and energy is given by Albert Einstein s now famous equation 

The combined amount of matter and energy available in the universe is fixed. 

Throughout this book, we will study atoms and molecules in much more detail. For now, let s look at some of the basic ways that chemists represent and think about these important particles. 

The Greek philosopher Democritus (470-400 bc) suggested that all matter is composed of tiny, discrete, indivisible particles that he called atoms. Llis ideas, based entirely on philosophical speculation rather than experimental evidence, were rejected for 2000 years. By the late 1700s, scientists began to realize that the concept of atoms provided an explanation for many experimental observations about the nature of matter. 

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