History of the Atom A Ancients Socrates l

History of the Atom A. Ancients Socrates l All substances were made up of earth, air, fire and water Democritus l Matter is composed of small particles called “atoms”

B. Dalton l All substances were made up of atoms • Small individual and indestructible parts l MODEL—marble concept • Atom was a small, dense sphere

C. Thomson l Cathode-ray tube experiment • Proved that atoms had negative charges inside them l l Called these charges “electrons” MODEL—”Plum Pudding Model” • Better known as the Chocolate Chip Cookie Model l Cookie is the atom Chips are the electrons http: //www. youtube. com/watch? v=O 9 Goyscbazk

D. Rutherford l Gold-Foil Experiment • When alpha particles passed through gold foil l Proved the atom was mostly empty space • When alpha particles were deflected l Proved atoms had a small, dense core • Called this area a nucleus which contained positive particles called protons • http: //www. youtube. com/watch? v=U 83 swkc. L 32 w

E. Bohr l Planetary Model • Electrons orbited the nucleus like planets orbit the sun

F. Modern Theory l Wave-Mechanical Model • Electrons exist in regions (clouds) around the nucleus l Regions are called orbitals

II Atomic Structure A. Subatomic Particles l Protons • Mass = 1 u • Charge = +1 • Location = in the nucleus l Neutrons • Mass = 1 u • Charge = 0 • Location = in the nucleus l Electrons • • • Mass = 1/1836 th u Charge = -1 Location = outside the nucleus

B. Atomic Number l l Represents the number of protons in an atom Identifies the element • Change the atomic number—change the element l Can also be used to determine the number of electrons • Atom l Protons = electrons • Ion l Changes the number of electrons • (+) ion loses electrons • (-) ions gains electrons

C. Mass Number l Represents the number of protons and neutrons in the nucleus • Represents the mass of an isotope of the atom l Always a whole number • Written two different ways l Co-60 or 60 Co • Mass number is NOT found on the Reference Table

1. Isotope l An isotope is a different form of the same element • Neutrons change • Mass number changes l Example C-12 and C-14 • C-12 protons = 6 neutrons = 6 • C-14 protons = 6 neutrons = 8 • C-12 is stable while C-14 is radioactive

D. Atomic Mass l Weighted average of all isotopes of an element • Example l In nature C-12 exists in 93% of a sample of carbon while C-14 exists in 7% of the sample. What is the atomic mass of carbon? • • • 12 x 0. 93 = 11. 16 14 x 0. 07 = 0. 98 Atomic Mass = 12. 14 u