Atomic Structure Historical look at the Atom 440 BC Democritus John Dalton Joseph John (J.J.) Thomson Ernest Rutherford Niels Bohr James Chadwick

Democritus F DISCONTINUOUS THEORY OF MATTER All matter is composed of atoms, which are bits of matter too small to be seen. These atoms CANNOT be further split into smaller portions. F Aristotle  Continuous theory of Matter Democritus Where does the term atom come from? In Greek, the prefix "a" means "not" and the word "tomos" means cut. Our word atom therefore comes from atomos, a Greek word meaning uncuttable.

o the Law of Conservation of Mass (Lavoisier) o the Law of Definite Proportions (or Law of Constant Composition) (Proust) o the Law of Multiple Proportion (his own) proposed the first theory of the nature of matter in stating that all matter was composed of atoms. Dalton based his theory on three scientific principles: John Dalton (~1803)

Law of Conservation of Mass u Established in the 1760’s by Antoine Lavoisier u in any chemical reaction, mass is neither created nor destroyed (reactants will have the same mass as products in a chemical change) Ex. C 6 H 12 O 6 + O 2  CO 2 + H 2 O 180 grams 70 grams 110 grams 140 grams Ex. CO 2 + H 2 O  C 6 H 12 O 6 + O grams 140 grams 180 grams 70 grams

Law of Definite Proportions n Established in 1799 by the French Chemist Joseph Louis Proust n States that in a pure compound, the elements are always present in the same definite proportion by mass. n Ex. H 2 O 2.0 g of hydrogen will combine with 16g of oxygen to produce 18 g of H 2 O and 4 g of hydrogen will combine with 32 g of oxygen to produce 36 g of H 2 O n Ratio of H:O is always 2:16 or 1:8 (by mass)

Law of Multiple Proportions n When two or more compounds are formed from the same pair of elements, the masses of one element combines with a fixed mass of the other element to form simple whole-number ratios of those elements

Example: Two different compounds are formed by the elements carbon and oxygen. The first compound contains 42.9% by mass carbon and 57.1% by mass oxygen. The second compound contains 27.3% by mass carbon and 72.7% by mass oxygen g O / 42.9 g C = 1.33 g O per g C 72.7 g O / 27.3 g C = 2.66 g O per g C Dividing the mass O per g C of the second (larger value) compound: 2.66 / 1.33 = 2

Four basic ideas in Dalton’s chemical atomic theory: (Postulates of Atomic Theory) Based upon 3 Laws:  Matter is composed of tiny indivisible particles  all atoms of the same element are identical in mass, but differ from atoms of other elements  atoms only combine in definite fixed numerical ratios such as 1:1, 1:2, 2:1  chemical change consists of a reshuffling of atoms, the individual atoms themselves remain intact John Dalton cont. John Dalton

Electrode ==> substance that allows electrons to enter or exit Anode ==> positively charged electrode Cathode ==> negatively charged electrode The 1 st Subatomic Particle General Info

William Crookes William Crookes  Determined the flow of current was in a straight path  The flow always originated from the cathode (negative electrode) and flowed to the anode (positive electrode)  Called these “rays” cathode rays

Z 1897, used magnetic and electric fields to alter the cathodes path which showed the “rays” must be negatively charged  Determined the charge to mass ratio of this negative particle ( e - /m ) Z Also felt there was a positive balance to this negative charge

negative charged particle positive charged matter

Robert Millikan n American Physicist n ~ 1909 determined the charge of an electron and the mass of an electron n Oil drop experiment n Used Thomson’s charge to mass ratio (e - /m) u x coulombs (charge of electron) u x g (mass of electron) (100 years later …within 1% of the known value)

Sir Ernest Rutherford Positively a great scientist Student of J.J. Thomson n 1909 conducted the alpha scattering experiment (better known as the ‘gold foil exp.’) n used a beam of high speed alpha particles to study the interaction with metal foils (gold mostly) n found that over 99% of the alpha particles went through the foil determined the atom was mostly empty space with a positively charge core (nucleus)

Deflections were a result of electrical repulsion Determined nearly all the mass of an atom is located in the nucleus (“little nut”) This positive charged core only occupied a small portion of the atoms volume

DUAL NATURE OF LIGHT Characteristics of energy as waves n Wavelength …symbol  lambda u Distance between any point on a wave to a corresponding point on the next wave u Expressed in meters (m)… as the wave shortens  nm (10 -9 ) n Amplitude u Represents the height of the crest (or depth of a trough) u Represents the intensity of the radiation (ie. visible light) n Frequency … symbol  nu u Number of cycles a wave passes a given point every second u Expressed as.. 1 or waves or sec -1 or Hertz sec n Speed … symbol  c u Represents the speed of light in a vacuum u 3.00 x 10 8 m sec

EM Spectrum n Radio, Microwave, Infrared, Visible light, Ultraviolet, X-Rays, Gamma rays n Emission spectra u represents energy released by an electron as it returns to ground state from excited state n Absorption spectra u represents energy absorbed by the electron as an electron moves to an excited state

Formulas you need  = c and E = h x   Wavelength = speed of light Energy = Planck’s constant x freq. frequency

Terminology n Ground state – lowest energy level for an electron when an atom is in its most stable energy state n Excited state – a higher energy state of an electron (energy is absorbed by electron) n Electromagnetic spectrum – range of energy that exhibits wave-like behavior as it travels through space. u Consists of both electric and magnetic field components. u classified according to the frequency of its wave.

Bohr Model Question: Why didn’t the atom collapse on itself? electrons move within defined “orbits” electrons have a “fixed” energy within the orbit and do not radiate energy as they move (DUE TO UNDERSTANDING OF THE ENERGY ASSOCIATED TO THE ELECTROMAGNETIC SPECTRUM) electrons can move from orbit to orbit, but can not be between orbits the orbits are considered “energy levels” the Bohr model was a precursor to the “quantum mechanical model” ANSWER:

Bohr Model + ORBITS (CONCENTRIC ENERGY LEVELS AROUND THE NUCLEUS) NUCLEUS -

Parts of Atom n Electrons u negatively charged subatomic particle u mass = 9.11 x g n Protons u positively charge subatomic particle u mass = 1.67 x g n Neutron – Discovered by James Chadwick (~ 1932) u Subatomic particle with no charge u mass = 1.67 x g

Terminology Atomic number u number of protons in the nucleus u due to atom’s electrical neutrality, also indicates the number of electrons u on the periodic table above or below the symbol of the atom Mass number  number of neutrons and protons in the nucleus  mass number - atomic number = neutrons Atomic mass  weighted average of all the isotopes of a particular element  on the periodic table above or below the symbol of the atom Isotope  atoms with the same number of protons but with different number of neutrons