1. © G. Hill and A. Hunt 2008 OCR Chemistry for AS CD-ROM 1 Atomic structure

2. © G. Hill and A. Hunt 2008 OCR Chemistry for AS CD-ROM Why study chemistry? According to Sir Harry Kroto, chemistry is all about solving problems and having fun. © Geoff Tompkinson/Science Photo Library

3. © G. Hill and A. Hunt 2008 OCR Chemistry for AS CD-ROM Dalton thought that all substances were composed of minute, unbreakable and indivisible particles which he called atoms. Early ideas about atoms © Sheila Terry/Science Photo Library

4. © G. Hill and A. Hunt 2008 OCR Chemistry for AS CD-ROM Evidence for atomic structure Key question 1 How have scientists obtained evidence for the detailed structure of atoms?

5. © G. Hill and A. Hunt 2008 OCR Chemistry for AS CD-ROM Relative atomic mass Atoms are too small to be weighed on a balance, but their masses can be compared. Helium atoms are four times as heavy as hydrogen atoms.

6. © G. Hill and A. Hunt 2008 OCR Chemistry for AS CD-ROM Isotopes Key question 2 Atoms of the same element can have different masses. Analysis of chlorine shows that it contains two types of atom. One with relative mass 35, the other with relative mass 37. How can this be explained?