The Nuclear Atom Atomic Scientists’ Song 2:52

Aristotle (460 B.C. – 370 B.C.) emphasized that nature consisted of four elements: air, earth, fire, and water did not believe in discontinuous or separate atoms, but felt that matter was continuous

Democritus (460 B.C. – 370 B.C.) first to suggest the existence of “atoms” (Greek word “atomos” = indivisible) atoms are indivisible and indestructible no experimental support

used scientific method to test Democritus’s ideas Dalton’s atomic theory 1.elements composed of atoms 2.atoms of the same element are alike 3.different atoms can combine in ratios to form compounds 4.chemical reactions can occur when atoms are separated, joined, or rearranged (but atoms are not created nor destroyed) John Dalton ( )

J.J. Thompson ( ) (need to know this guy) discovered the electron thought atom was negative charges stuck in a positive charged lump –referred to as the “plumb pudding model”

Robert A. Millikan ( ) found the quantity of charge carried by an electron (one unit of negative charge) calculated the mass of an electron (1/1840 th the mass of a hydrogen atom)

Ernest Rutherford ( ) (need to know this guy) proposed that the atom is mostly empty space positive charges and almost of the mass are in a small, centralized region called the nucleus “Like howitzer shells bouncing off of tissue paper!”

Rutherford Flash Animation

Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? ?

The Answers Target #1Target #2

Niels Bohr ( ) (need to know this guy) electrons found only in specific circular paths (orbits) around the nucleus based on information about how the energy of an atom changes when it absorbs and emits light called these fixed energies “energy levels”

Erwin Schrodinger (1926) quantum mechanical model –probability of electron locations around the nucleus –not an exact orbit eventually became the electron cloud model

Schrödinger's Cat video 1:41

Werner Heisenberg (1927) (need to know this guy) Heisenberg Uncertainty Principle – impossible to know the exact position and momentum of an electron at the same time “the observer affects the observed” bonn.de/ausstellungen/heisenberg/bilder/heisenb erg_2.jpg

Structure of the Atom

ParticleChargeMass (atomic mass units) Location Proton+ 11nucleus NeutronØ1nucleus Electron x (considered negligible) orbit, level, cloud

Carbon- 12 as a standard carbon- 12 –ALL masses on the periodic table are based on their relationship to carbon-12 the carbon- 12 isotope has been given the atomic weight of exactly and is used as the basis upon which the atomic weight of other isotopes is determined

Even smaller particles quarks –make protons & neutrons –6 types He

Learning Check An atom has 14 protons and 20 neutrons. A.Its atomic number is 1) 142) 163) 34 B. Its mass number is 1) 142) 163) 34 C. The element is 1) Si2) Ca3) Se D. The number of electrons in a neutral atom is 1) 142) 63) 20

Isotopes same element but differ in their number of neutrons the atomic mass on periodic table is the WEIGHTED AVERAGE MASS of “all” the isotopes of that element –this is based on an isotope’s natural abundance the percentage of each isotope of an element that occurs in nature have the same chemical properties (reactivity) but different physical properties (density, melting/boiling point…)

2.3

Chemical symbols for isotopes two different ways to write isotopes –example for sodium sodium- 23 –only shows mass number (23) of the sodium isotope 23 Na –shows the mass number (23) and the atomic # (11) of the sodium isotope 11 X Mass Number Atomic Number Element Symbol A Z

Isotopes? Which of the following represent isotopes of the same element? Which element? 234 X 234 X 235 X 238 X is the element uranium

How many protons, neutrons, and electrons are in-- C 14 6 How many protons, neutrons, and electrons are in-- C protons, 8 (14 - 6) neutrons, 6 electrons 6 protons, 5 (11 - 6) neutrons, 6 electrons Do You Understand Isotopes?

Dangerous, but worth the risk Radioactive Isotopes (don’t need to know) unstable isotopes that break down over time uses: –cobalt 60 radiation treatment for cancer –carbon 14 used to date objects up to 60,000 years old –iodine 125 and iodine 131 ingested and used for medical imaging

The Mass Spectrometer

has many applications, but one of the simplest is to determine the natural abundances of the isotopes of a particular element –the relative atomic mass can be calculated from the data from the mass spectrometer Mass spectrometer video (2:26) 0

magnesium results from the mass spectrometer: –80% 24 Mg –10% 25 Mg –10% 26 Mg Calculate the relative atomic mass of magnesium with the provided data. just a simple weighted mean.80(24) +.10(25) +.10(26) = 24.3 amu