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Chapter 3 Atoms and Moles Section 1: Substances Are Made of Atoms Section 2: Structure of Atoms Section 3: Electron Configuration Section 3.1 – Substances Are Made of Atoms OBJECTIVES: State the three laws that support the existence of atoms List the five principles of John Dalton’s atomic theory. Law of Definite Proportions • Also known as the Law of Constant Composition • Two samples of a given compound are made of the same elements in exactly the same proportions by mass, regardless of the size or sources of the samples – NaCl 39.3% Na Sodium 60.7% Cl Chlorine - H2O 11.1% H Hydrogen 88.9% O Oxygen The Law of Conservation of Mass • The mass of the reacting system does not change For example: Table salt If I react 23 g of sodium with 35.5 g of chlorine I get 58.5 g of sodium chloride Water If I react 2 g of hydrogen with 16 g of oxygen I get 18 g of water Law of Multiple Proportions When two elements combine to form more than one compound, the mass ratios of the two compounds are related by small whole numbers Examples • Hydrogen Peroxide (H2O2) 2 atoms of hydrogen react with two atoms of oxygen • Water (H2O) ? Atoms of hydrogen react with ? Atoms of oxygen. Atomic Theory of Matter John Dalton 1. All matter is composed of extremely small particles called atoms. (cannot be created/subdivided or destroyed) 2. All atoms of a given element are identical to one another in chemical and physical properties 3. The atoms of one element are different from the atoms of all other elements in physical and chemical properties. 4. Atoms of different elements combine in simple whole number ratios to form compounds 5. In chemical reactions, atoms are combined, separated, or rearranged but never created, destroyed, or changed. The Electron • Streams of negatively charged particles were found to emanate from cathode tubes. • J. J. Thompson is credited with their discovery (1897). • Thompson measured the charge/mass ratio of the electron to be 1.76 108 coulombs/g. The Atom, circa 1900: • “Plum pudding” model, put forward by Thompson. • Positive sphere of matter with negative electrons imbedded in it. Section 3.2 – Structure of Atoms OBJECTIVES: Describe the evidence for the existence of electrons, protons, and neutrons, and describe the properties of these subatomic particles. Discuss atoms of different elements in terms of their numbers of electrons, protons, and neutrons, and define the terms atomic number and mass number. Define isotope, and determine the number of particles in the nucleus of an isotope Discovery of the Nucleus Ernest Rutherford shot particles at a thin sheet of gold foil and observed the pattern of scatter of the particles. The Nuclear Atom • Since some particles were deflected at large angles, Thompson’s model could not be correct. • Rutherford postulated a very small, dense nucleus with the electrons around the outside of the atom. • Most of the volume of the atom is empty space. Summary of the History of the Atom Model Democritus All matter is made up of small particles called atoms. Dalton See Dalton’s Atomic Theory of Matter J. J. Thompson Discovered the electron Electrons are embedded in a cloud of positive charge Plum Pudding Model Ernest Rutherford The positive charge is centrally located. Discovered the nucleus. Niels Bohr The electrons orbit the nucleus like the sun orbiting the earth James Chadwick Discovered the neutron, located in the nucleus. Electron A subatomic particle that has a negative electric charge. Nucleus An atom’s central region, which is made up of protons and neutrons. Protons A subatomic particle that has a positive charge and that is found in the nucleus of an atom. The number of protons of the nucleus is the atomic number, which determines the identity of an element. Neutron A subatomic particle that has no charge and that is found in the nucleus of an atom. Atomic Number The number of protons in the nucleus of an atom. The number of protons = the number of electrons. The atomic number is the same for all atoms of an element. Mass Number The sum of the number of protons and neutrons of the nucleus of an atom. Mass number – atomic number = number of neutrons Example: Neon: Atomic number: Mass number: 10 20 Number of protons and neutrons (mass number): - Number of protons (atomic number): number of neutrons: 20 10 10 Isotopes Atoms of the same element with different masses. Isotopes have different numbers of neutrons. Mass Number (protons + neutrons) Atomic Number (protons) 11 C 6 Symbol Other Subatomic Particles Protons were discovered by Rutherford in 1919. Neutrons were discovered by James Chadwick in 1932. Subatomic Particles • Protons and electrons are the only particles that have a charge. • Protons and neutrons have essentially the same mass. • The mass of an electron is so small we ignore it. Location Charge Mass unit Proton Nucleus +1 1 Neutron Nucleus 0 1 Electron Orbitals -1 0 Electron Configurations • The way electrons are arranged in atoms. • Aufbau principle- electrons enter the lowest energy first. • This causes difficulties because of the overlap of orbitals of different energies. • Pauli Exclusion Principle- at most 2 electrons per orbital - different spins • Hund’s Rule- When electrons occupy orbitals of equal energy they don’t pair up until they have to. Atomic Orbital Model for Electrons S orbitals • 1 s orbital for every energy level • Spherical shaped • Each s orbital can hold 2 electrons • Called the 1s, 2s, 3s, etc.. orbitals. Pg 86 # 1-4, Pg 89 # 1-2 Section 3.2 Section Review Pg 89 # 1-8 Assignments Worksheet #1 Worksheet #2 Atomic Number Bohr Energy Levels Electron Configurations – Quantum Numbers Worksheet #3 QUIZ Pg 102 # 1-4, Pg 103 # 1-3 Section 3.4 Section Review Pg 104 # 1-13 QUIZ TEST – Chapter 3