1. Chapter 4, Section 1 Introduction to Atoms

2. Democritus A philosopher in 430 B.C. who theorized that matter was made up of small pieces that could not be cut into smaller parts Atomos means uncuttable Atom = the smallest particle of an element See BrainPop “Atoms”

3. Atomic Theory The ancient Greeks could not prove atoms existed because they could not experiment Table Talk: What would prevent the ancient Greeks from experimenting? The theory took better shape in the 1600s A series of models was developed from experimental evidence

4. Dalton’s Model: atoms are like smooth hard balls that cannot be broken into smaller pieces. (1803) Thomson’s “Plum Pudding” Model: atoms have negatively charged electrons embedded in a positive sphere. (1897) Rutherford’s Model: atoms have positively charged particles (protons) in their nucleus which account for the atom’s mass. (1911) Bohr’s Model: electrons orbit the nucleus in a definite pattern. (1913) Cloud Model: electrons move in every direction around the nucleus (1920s) DEVELOPING ATOMIC THEORY

5. Modern Atomic Model Atoms are mostly empty space composed of: – Electrons (e - ) = negatively charged particles; no mass – Protons (+) = positively charged particles in an element’s nucleus – Neutrons = no electric charge; about the same mass as a proton

6. Table Talk: Describe (not just name) the parts of an atom. Use this picture to help you.

7. Modern Atomic Model (cont’d.) At the center of the atom is a tiny, massive nucleus containing protons and neutrons. Surrounding the nucleus is a cloudlike region of moving electrons. See BrainPop “Atomic Model”

8. Modern Atomic Model (cont’d.) In atoms, the number of protons = the number of electrons The charges balance, making the atom neutral The number of neutrons in an atom does not have to equal the number of protons Table Talk: Do you think neutrons affect the charge of an atom?

9. Most of an atom’s volume is the space in which the electrons move. To picture the scale of an atom, imagine the nucleus were the size of an eraser on a pencil. If you put this nucleus on the pitcher’s mound of a baseball stadium, the electrons could be as far away as the top row of seats!

10. Atomic Number The number of protons in the nucleus of an element This number shows up as the top number for each element on the periodic table of elements The number of protons are fixed for an element, and all atoms of that element have the same number of protons Table Talk: How many protons does Helium (He) have? Chlorine (Cl)? Argon gas (Ar)?

11. Isotopes Many atoms have the same number of protons and neutrons, but not always Isotopes are atoms of the same element that have different numbers of neutrons Isotopes are identified by their mass numbers: # protons + # neutrons = mass # Table Talk: What is the atomic mass of a tritium isotope? How does it compare to that of deuterium?

12. Carbon Isotopes See BrainPop “Isotopes” Table Talk: Compare and contrast these carbon isotopes.

13. What do the numbers mean? Family: vertical column of elements having similar properties Fe = chemical symbol for Iron Atomic mass = sum of protons and neutrons and is an average of all the naturally occurring isotopes for that element Atomic number = # of protons end Try it! Using the period table, name the atomic number chemical symbol and atomic mass for several elements