Atomic structure Structure of an atom is based

Atomic structure • Structure of an atom is based on theories • Theories are based on experimental and physical evidence • History of the development of atom model (structure) began with Greeks – Democrutus • Stated all materials were formed from a small unique indestructible unit called an atomus unit

• First modern theories were proposed by Dalton in early 1800’s • Utilized three laws to postulate his Atomic Theories – Law of Conservation of mass-energy – Law of Definite proportions – Law of Multiple proportions

Dalton’s Atomic Theories • All matter consists of small, indestructible and indivisible spheres called atoms • Atoms of the same element are all identical(same mass) • Atoms of different elements are different (different mass) • Atoms of elements can rearrange themselves into compounds that contain simple whole number ratios of the elements to each other

• Four basic theories still true with some modifications – Examples • Average mass not just mass • Indestructible in ordinary chemical reactions •

• Invention of battery – Positive electrode (anode) – Negative electrode (cathode) • Development of cathode ray tube and discovery of cathode rays • Used by J. J. Thomson – Discovered rays were bent by magnets and opposite charges – Discovered rays were particles • Called electrons e– Negatively charged particles • Also discovered protons p+ – Positively charged particles

• Isotopes – Atoms of same element with different masses – Since number protons in neutral atom equals the number of electrons, isotopes must have different numbers of neutrons – Neutrons existence were not proven until 1934 by James Chadwick using Mass Spectroscopy – Thomson called his atom model the “Plum Pudding” model

• Discovery of Radioactive elements – Three types of radiation • Alpha- positively charged He nuclei a • Beta- high energy electrons b • Gamma- high energy waves g • Used by Rutherford to determine a new atomic model • Gold foil experiment

• Rutherford Model of Atom – Atoms consist of mostly empty space – Dense compact nucleus consisting of the protons – e- orbit nucleus like planets

Bohr-Rutherford Model • Based on Bohr’s observations of radiant(light) energy or spectroscopy • Noted specific spectral patterns from atoms of substances when exposed to exciting energy

• There were always the 7 primary colors • ROYGBIV • Led Bohr to determine that atoms had all their e- on 7 principle levels of energy • KLMNOPQ • Each level held a specific # of e- according to the equation 2 n 2 • Levels were changed to Energy Levels 1 -7 • Energy level 1 2(1)2=2 e-

• Energy level occupied by an e- based on its Potential(positional) energy • Electrons always occupy lowest energy levels first • Normal level occupied called its ground state • When e- gains energy jumps to a higher energy level(whole number level gain) • Called its excited state • The e- fall back down to ground state and emit energy in form of light ROYGBIV • All e- move at a constant velocity and therefore do not lose energy

• This explains the e- problems encountered by the Rutherford model • Still has problems • What are they • If all e- are in an energy level they should all have the same energy in that level • Since the jump in whole quanta values(one whole energy level) then there should only be a maximum of 7 colored lines: one of each color • What did you observe in the Spectra Lab?

• Multiple lines of same color • Represent different energies being gained and lost • How does this happen • Stay tuned for the next exciting adventure into the structure of the atom • Now the nucleus

• How do p+ stay together • Explained when neutron is discovered • Development of quark and gluon theory • Proof of protons was provided by Henry Mosley using X ray diffraction to determine atomic number • • Atomic Particles Proton 1. 0 amu +1 charge p+ Electron 0 amu -1 charge e. Neutron 1. 0 amu 0 charge no