1. Chapter 3 Atoms and Elements 3.3The Atom

2. Dalton’s Atomic Theory Atoms • are tiny particles of matter • of an element are similar to each other but are different from atoms of other elements • of two or more different elements combine to form compounds • are rearranged to form new combinations in a chemical reaction

3. Subatomic Particles Atoms contain subatomic particles. • Protons have a positive (+) charge. • Electrons have a negative (–) charge. • Like charges repel, and unlike charges attract. • Neutrons are neutral.

4. Rutherford’s Gold-Foil Experiment In Rutherford’s gold-foil experiment, positively charged particles • were aimed at atoms of gold • mostly went straight through the atoms • were deflected only occasionally Conclusion: There must be a small, dense, positively charged nucleus in the atom that deflects positive particles that come close.

5. Rutherford’s Gold-Foil Experiment (continued)

6. Structure of the Atom An atomconsists • of a nucleus that contains protons and neutrons • of electrons in a large, empty space around the nucleus

7. Atomic Mass Scale On the atomic mass scale for subatomic particles: • 1 atomic mass unit (amu) is defined as 1/12 of the mass of the carbon-12 atom. • A proton has a mass of about 1 (1.007) amu. • A neutron has a mass of about 1 (1.008) amu. • An electron has a very small mass, 0.000549 amu.

8. Particles in the Atom

9. Learning Check Identify each statement as describing a(n) 1) proton, 2) neutron, or 3) electron. A. __ found outside the nucleus B. __ has a positive charge C. __ is neutral D. __ found in the nucleus

10. Solution Identify each statement as describing a(n) 1) proton, 2) neutron, or 3) electron. A. 3 found outside the nucleus B. 1 has a positive charge C. 2 is neutral D. 1, 2 found in the nucleus