1. Atoms & Their Structure Chapter 2 Section 1 Part 1

2. Objectives  How have historic experiments led to the development of the modern model of the atom?  How is the modern model of the atom different from previous models?  What information is available in an element block of the periodic table?

3. Important Vocabulary  Atom  Atomic theory  Law of conservation of mass/matter  Law of definite proportions  Electron  Proton  Electron cloud  Isotope  Neutron  Nucleus  Atomic number  Mass number  Atomic mass unit  Atomic mass

4. The Atom  An atom is the smallest particle of an element that retains its identity in a chemical change  The concept of the atom intrigued a number of early scientists and philosophers  However, they could not observe individual atoms

5. History of Atoms  The idea of the atom first came from a Greek philosopher Democritus nearly 2500 years ago  It started with a set of simple questions, “If you take a piece of chalk and break it in half, are both halves still chalk? If you continue to divide each piece, is there a limit to the division before it becomes something else?”  Democritus thought there was a limit which he called atomos, meaning “unable to be cut”  This idea that matter is made of the fundamental particles called atoms is known as the atomic theory of matter

6. Democritus' Theory of Atoms 1.All matter consists of invisible particles called atoms. 2.Atoms are indestructible. 3.Atoms are solid but invisible. 4.Atoms are homogenous. 5.Atoms differ in size, shape, mass, position, and arrangement.  He believed that the movements of atoms caused the changes in matter he observed  However, he had no way of proving the existence of atoms

7. Contributors to Atomic Theory  Democritus  Antoine Lavoisier  Joseph Proust  John Dalton  J.J Thomson  Hantaro Nagaoka  Ernest Rutherford  Niels Bohr  Erwin Schrodinger  Werner Heisenberg  Robert Millikan  Eugene Goldstein  James Chadwick  Irene Joliot-Curie

8. Antoine Lavoisier  In 1782, the French taxman Lavoisier made measurements of chemical reactions in sealed containers  He observed that the mass of reactants before the reaction was equal to the mass of the products after the reaction  He concluded that when an chemical reaction occurs, matter is neither created nor destroyed but only changed  This idea became known as the Law of Conservation of Mass/Matter

9. Joseph Proust  In 1799, a French chemist Joseph Proust observed that the compostion of water is always 11.2% hydrogen and 88.8% oxygen by mass  He studied many other compounds and observed that compounds were always in certain proportions by mass  This principle is referred to as the Law of Definite Proportions

10. John Dalton  Was a teacher, chemist, meteorologist, & physicist  He is best known for his development of the modern atomic theory  He also did research into color blindness (sometimes called Daltonism)

11. Dalton’s Atomic Theory  Was established in the early 1800’s before the Periodic Table  Dalton believed that a few kinds of atoms made up all matter  He also believed that elements were composed of only one kind of atom and compounds were made from two or more kinds of atoms  He reasoned that only whole numbers of atoms could combine to form compounds

12. Five Principles of Dalton’s Theory 1.All matter is composed of atoms which cannot be subdivided, created, or destroyed 2.Atoms of a given element are identical in their physical and chemical properties 3.Atoms of different elements differ in their physical and chemical properties 4.Atoms of different elements combine in simple, whole-number ratios to form compounds 5.In chemical reactions, atoms are combined, separated, or rearranged

13. The Problem with Dalton’s Theory  It explained most of the chemical data that existed during his time  However, it doesn’t all hold true today  Atoms can be divided into particles  Atoms can be created and destroyed  It doesn’t account for molecules & allotropes  Thus, it changed