History, Theory, Structure

 How can we describe something we can’t even see?  Compare to Black Box Experiment  Can’t see internal structure  Discovery by experimenting with how matter behaves  Constantly changing and/or refining definition

 Our current understanding of atomic structure is based on: Nucleus Contains Protons (+) and Neutrons (N) +N++N+ Electron Shells (Not exact locations) e- Drawn as an outer “wall” only for reference; Energy

“Ancient”“Modern”  Aristotle:matter may be continuously divided without changing its intrinsic properties  Democritus:matter is composed of tiny, indivisible particles; atoms may not be divided  Dalton: Atoms may not be further divided.  Thomson: pudding theory; electrons  Rutherford: gold foil  Bohr: quantum theory  Schrodinger: charge- cloud

 1808  English  Two very important tenets ◦ Law of Conservation of Mass ◦ Law of Definite Proportions

 All matter is made up of atoms.  All atoms of an element are identical in size, mass, and chemical properties.  Atoms of an element are different from atoms of all other elements.  Atoms may neither be divided nor destroyed.  Atoms may be combined, separated, and rearranged to form new compounds.  Atoms of different elements combine in whole-number ratios to form compounds.

 1911  Rutherford’s Gold Foil Experiment  Actually conducted by Rutherford’s assistant  Hugely advanced for time period

 Bohr’s model related structure and position of electrons to energy levels  Ground state  Excited state  Line-emission spectrum  Continuous spectrum  Only worked consistently for hydrogen

 Based on Bohr’s ideas with one main difference  Electrons are not traveling in fixed positions  Use of orbitals rather than orbits  Areas where electrons are most likely to be found  For example, an electron will never be expected to be found in the atom’s nucleus

 Protons have a positive charge. ◦ Mass of 1 unit ◦ Located in nucleus ◦ Identity of an atom  Neutrons have no charge ◦ Mass of 1 unit ◦ Located in nucleus ◦ Help to stabilize nucleus

 Located outside of the nucleus ◦ Negative charge ◦ Mass is insignificant to mass of whole atom ◦ Responsible for chemical reactivity of an atom  Though mass is tiny, equal but opposite charge ◦ Can change position ◦ Relate to energy of an atom

ProtonsNeutrons  Positive charge  Located in nucleus  Determine properties of an atom  Unique to each element  No charge (neutral)  Located in nucleus  Thought to provide stability to nucleus  Numbers may vary (isotopes) Negative charge Located in energy levels around nucleus Responsible for chemical reactivity of an atom Electrons Sub-Atomic Particles

 All mass measurements are comparisons based on standards  All atoms based on mass of carbon-12 atom  Weighted average of all known isotopes based on measured frequency  Combination of protons and neutrons

C 6 Atomic Mass represents combined masses of protons and neutrons Atomic Number Represents number of protons; in a neutral atom represents number of electrons

 While all atoms of an element have the same number of protons, numbers of neutrons may differ  These atoms still retain the basic properties of the element  Small differences in behavior  Atoms of the same element with differing numbers of neutrons  Indicated by symbol with number to indicate number of neutrons or mass

 Mendeleev ( ); Russian; 1880’s ◦ listed approximately 73 known elements in columns in order of increasing atomic mass ◦ left blank spaces for unknown elements  Moseley ( ); British; worked w/ Rutherford; 1913 ◦ determined atomic number of atoms of known elements ◦ arranged elements by atomic number

 Periods ◦ Horizontal ◦ 7 ◦ Periodic Law ◦ Properties change as you go from left to right on the Periodic Table.  Atomic mass increases  Atomic number increases

 Group or Family  Vertical  Similar physical and chemical properties  Significance of Group Number ◦ For main group elements represents number of valence electrons

 Approximately 80% of known elements  Alkali: Group I ◦ Highly reactive  Alkaline Earth: Group 2 ◦ Highly reactive  Transition, inner transition  Properties ◦ malleable, ductile, lustrous ◦ solids at room temp except Hg ◦ good conductors of heat and electricity

 Not lustrous  Poor conductors of electricity and heat  Halogens ◦ Group 7  Noble gases ◦ Group 8  Right side of Periodic Table

 Zig zag line  Properties of both metals and nonmetals  Used in computer chips and solar cells; semiconductors

Source: Jefferson Labs

 Historical perspective ◦ Models of atoms based on behavior rather than a visual analysis  All atoms of an element have same basic properties ◦ Isotopes  Atoms of same element with differing # of neutrons  Identity of an atom based on protons  Chemical reactivity of an atom based on electrons  Periodic Table of the Elements ◦ Organization and Patterns