Atoms: The Building Blocks of Matter

1. Atoms: The Building Blocks of Matter Chapter 3 Jessica Baird, Zhenhao Li, Brianna Mays, Joey Powell

2. Chapter 3 Section 1 The Atom: From Philosophical Idea to Scientific Theory

3. Democritus vs. Aristotle • Democritus called nature’s basic particle, the atom (Greek for “indivisible”) •  Aristotle did not believe in atoms, but thought that all matter was continuous (could keep being divided)

4. Foundations of Atomic Theory • Almost all chemists by the late 1700s agreed that an element was a substance that could not be broken down further chemically  • Chemists also agreed that elements could combine to form compounds that have different physical and chemical properties than those of the elements used to form them • Ex. NaCl has different physical and chemical properties than chlorine (Cl) and Sodium (Na)   • There was controversy over whether elements always combined in the same ratios when forming a particular compound

5. Law of Conservation of Mass • Mass is neither created or destroyed during normal chemical reactions or physical changes

6. Law of Definite Proportions • A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of sample or source

7. Law of Multiple Proportions • If two or more different compounds are composed of the same two elements, the ratio of the masses of the second element combined with a certain mass of the first element is always a ratio of small whole numbers

8. Dalton's Atomic Theory • All matter is composed of atoms • Atoms of a given element are identical in size, mass, and other properties • Atoms cannot be subdivided, created, or destroyed •  Atoms of different elements combine in simple whole – number ratios to form chemical compounds     • In chemical reactions, atoms are combined, separated, or rearranged

9. Modern Atomic Theory • Some parts of Dalton's theory was incorrect • Atoms can be divided into smaller particles • Atoms of the same element can have different masses (isotopes) • All matter is composed of atoms • Atoms of any one element differ in properties from atoms of another element

10. Chapter 3 Section 2   The Structure of the Atom

11. Atoms • The smallest particle of an element that retains the chemical properties of that element·      •  An atom consists of two regions the nucleus and the electron cloud·      • The nucleus has at least one positively charged proton and one neutrally charged neutron·      • The electron cloud surrounds the nucleus and has negatively charged electrons      •  Protons, neutrons and electrons are called subatomic particles

12. Discovery of the Electron • In the late 1800s many experiments were conducted that used an electric current that passed through various gases at low pressure      • These experiments, in glass tubes called cathode-ray tubes, would lead to the discoveries of the subatomic particles

13. Cathode Rays and Electrons •  When a current was passed through a cathode-ray tube the opposite side of the cathode glowed, this was caused by a stream of particles called cathode rays • Other experiments showed that when an object was placed between the cathode and the other end of the tube a shadow was cast and that when a paddle wheel was placed inside the tube it moved • These experiments and others proved that a cathode ray had sufficient mass and was negatively charged       • JJ Thomson experimented further with the cathode rays and renamed them electrons

14. Charge and Mass of the Electron •  Robert A. Millikan found that the mass of the electrons is about one-thousandth the mass of an atom • These experiments showed that two other inferences could be made about atoms:         1. Because electrons are negative and atoms are neutral, there must be something with a positive charge         2. Atoms must contain other particles that account for most of their mass

15. Discovery of the Atomic Nucleus • Ernest Rutherford, Hans Geiger and Ernest Marsden bombarded a thin piece of gold foil with positively charged particles • They expected the particles to go through but some of them deflected back •  Later they found that this was because atoms have a very small, very dense area with a                                        positive charge called the nucleus 

16. Composition of the Atomic Nucleus • The nucleus contains protons and neutrons and there is an equal number of protons and electrons in an atom •  Most of the mass is made up of neutrons and protons •  The number of protons in an atom determine its identity

17. Forces in the Nucleus •  When two protons are really close to each other they attract • This is the same for two neutrons and a proton and a neutron •  These forces are called nuclear forces

18. The Sizes of Atoms • The radius of an atom is the distance from the center of the nucleus to the outer part of the electron cloud • Atomic radii are about 40 to 270 picometers while the radii of the nucleus is about 0.001 picometer.

19. Chapter 3 Section 3 Counting Atoms

20. Number of Atoms • The atomic number of an element is the number of protons in the nucleus of each atom that element • The atomic number is found on the periodic table on the top, above the name of the element • Mass number is the total number of protons and neutrons in the nucleus of an isotope, it is found under the name of the element

21. Isotopes • Isotopes are atoms of the same element that have different masses • Mass Number-Atomic Number= Neutrons • Nuclide- General term for any isotope of any element

22. Atomic Mass • The Atomic Mass Unit (AMU) was based of off the Carbon-12 Isotope, which was 12 amu • 1 amu is equal to 1/12 the mass of an oxygen-12 atom

23. Average Atomic Mass • Average atomic mass is the weighted average of the atomic masses of the naturally occurring isotopes of an element • Average atomic mass is calculated by multiplying the mass of the isotope by its relative abundance in decimal form

24. Moles and Atoms • A mole is the amount of a substance equal to the number of particles in 12 grams Carbon-12 • Avogadro's Number: • 6.02 x 1023 • Avogadro's number is named after Amedeo Avogadro, a chemist • The number is used as a conversion between a number of things and moles • The mass of one mole of a pure substance is called the molar mass

25.  Conversions!!!!! Grams Use: Molar Mass  Moles Use: Avogadro's # # of Atoms

26. Problems 1) How many molecules of carbon dioxide are found in 2.50 moles of carbon dioxide? 2) How many moles of O2 are represented by 7.45 x 1024 molecules of O2?  3) What would be the mass of 3.75 x 1021 atoms of iron?

27. Answers 1) 2.50 mol x 6.02 x 1023 molecules/mol             Answer = 1.51 x 1024 molecules 2)  7.45 x 1024 molecules/6.02 x 1023 molecules/mol             Answer = 12.4 moles 3) 3.75 x 1021 molecules/6.02 x 1023 molecules/mol = 0.00623 moles 0.00623 mol x 55.8 g/mol             Answer = 0.348 g

28. More Problems 1) How many molecules of water would be found in 54.0g of water? 2) A certain laboratory procedure requires the use of .100 moles of magnesium. How many grams of magnesium would you mass out on the balance?

29. More Answers 1) Molar mass of H2O = 18 g  54.0 g/ 18.0 g/mol=3 mol (3.00 mol)(6.02x1023 molecules/mol)             Answer = 1.81 x 1024 molecules 2) 0.100 mol x 24.3 g/mol             Answer = 2.43 g of magnesium