2The Roots of Atomic Theory DemocritusAristotleJohn DaltonIdeas versus ScienceWhy was it hard for Democritus to defend his ideas?
3Dalton’s Atomic Theory All elements are composed of tiny indivisible particles called atoms.Atoms of the same element are identical. The atoms of any one element are different from those of any other element.Atoms of different elements can chemically combine with one another in simple whole-number ratios to form compounds.Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element are never changed into another element as a result of a chemical reaction.
4Defining the AtomAn atom is the smallest particle of an element that retains the properties of that element.100,000,000 atoms = 1 centimeter
5The ElectronUsing a vacuum pump, scientists passed electricity through glass tubes with an anode at one end and a cathode at the other. Cathode rays led to the discovery of negatively charged particles called electrons.
6ElectronsJJ Thomson did experiments using cathode rays, discovered that opposite charges attract and like charges repelRobert Millikan determined electrons negative charge and very small massfound in cloud (sublevels) outside the nucleus; valence electrons are the outermost electronsPlum Pudding Model
7Atomic NucleusErnest Rutherford proposed that the atom is mostly empty space (gold foil experiment)all the positive charge and almost all the mass is concentrated in a small regionRutherford called this region the nucleusnucleus is the center core of an atom composed of protons and neutrons
8Rutherford’s Nuclear Model The atom is composed of a dense, positively charged nucleus surrounded by negative electrons.What force causes the deflection of alpha particles?
9Protons and Neutrons atoms are electrically neutral protons are positively chargedalmost 2000 times the mass of an electronneutrons have no charge, but equal mass to a protonprotons and neutrons are collectively called nucleons
10Summary of Atom Anatomy The nucleus of the atom is made of protons and neutrons.Electrons orbit the nucleus in a cloud.Protons+ chargeNeutronsno chargeElectrons- charge
11Atomic Numberelements are different due to different numbers of protonsnumber of protons is the atomic numbernumber of protons equals number of electrons in a neutral atom
12Mass Numbertotal number of protons and neutrons in an atom is the mass numbernumber of neutrons = mass # - atomic #oxygen has an atomic # 8, mass # 16atomic number can be written as the subscript and mass number as the superscriptcan also designate element by mass numbergold-197
13Isotopesatoms with same number of protons but different number of neutrons are called isotopesdifferent mass numbers; chemically alike due to same # of protons and electronshydrogen-1 (mass number 1, no neutrons)hydrogen-2 (mass number 2, 1 neutron)deuterium
14Atomic Mass Unitdefined as one twelfth the mass of a carbon-12 atom (amu)mass of a single proton (or neutron) is about 1 amuWhy isn’t atomic mass always a whole number?Atomic mass is the weighted average mass of the atoms in a naturally occurring sample of the element.
15Isotope AbundanceAnalyzing an element’s mass can indicate the most abundant isotope for that element.Using the atomic mass, you can usually assume that the most common isotope is the atomic mass rounded to the nearest whole number.
16Example Problems Find the weighted average of 3 assignments… Term paper 20% A = 4.0Midterm 30% B = 3.0Final 50% C = 2.0Student receives a “B” on term paper, “C” on midterm, and “B” on final. What is the overall grade for this student?
17Example Problems Calculate the average atomic mass of nitrogen… What is the average atomic mass?
18Example ProblemsThree isotopes of Element X have atomic masses and relative abundances of amu (78.99%), amu (10.00%), and amu (11.01%). Calculate the atomic mass.
19RadioactivityNuclear reactions involve an atom of one element changing into an atom of another element.radioactive atoms undergo changes that alter their identitiesunstable nuclei lose energy by emitting radiation through radioactive decay