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

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

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)

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

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

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

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

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

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

The Structure of the Atom Chapter 3 Section 2   The Structure of the Atom

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

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  

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

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

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 

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

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

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.

Chapter 3 Section 3 Counting Atoms

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

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

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

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

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

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

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?

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

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?

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