Atomic Structure Historical look at the Atom 440 BC 1897 1803 1911 1922 1932 Democritus John Dalton...
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Transcript of Atomic Structure Historical look at the Atom 440 BC 1897 1803 1911 1922 1932 Democritus John Dalton...
Atomic StructureHistorical look at the Atom
440 BC
1897
1803
1911
1922
1932
Democritus
John Dalton
Joseph John (J.J.) Thomson
Ernest Rutherford
Niels Bohr
James Chadwick
Democritus 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.
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
Established in the 1760’s by Antoine Lavoisier in any chemical reaction, mass is neither created nor
destroyed (reactants will have the same mass as products in a chemical change)
Ex. C6H12O6 + O2 CO2 + H2O 180 grams 70 grams 110 grams 140 grams
Ex. CO2 + H2O C6H12O6 + O2 110 grams 140 grams 180 grams 70 grams
Law of Definite Proportions
Established in 1799 by the French Chemist Joseph Louis Proust
States that in a pure compound, the elements are always present in the same definite proportion by mass.
Ex. H2O2.0 g of hydrogen will combine with 16g of oxygen to
produce 18 g of H2O and
4 g of hydrogen will combine with 32 g of oxygen to produce 36 g of H2O
Ratio of H:O is always 2:16 or 1:8 (by mass)
Law of Multiple Proportions 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.
57.1 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 1st Subatomic ParticleGeneral Info
William CrookesWilliam 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
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)
Also felt there was a positive balance to this negative charge
negative charged particle
positive charged matter
Robert Millikan
American Physicist ~ 1909 determined the charge of an electron and
the mass of an electron Oil drop experiment Used Thomson’s charge to mass ratio (e-/m)
1.602 x 10-19 coulombs (charge of electron) 9.109 x 10-28 g (mass of electron)(100 years later …within 1% of the known value)
Sir Ernest RutherfordPositively a great scientistStudent of J.J. Thomson
1909 conducted the alpha scattering experiment (better known as the ‘gold foil exp.’)
used a beam of high speed alpha particles to study the interaction with metal foils (gold mostly)
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 LIGHTCharacteristics of energy as waves
Wavelength … symbol lambda Distance between any point on a wave to a corresponding point on the next wave Expressed in meters (m)… as the wave shortens nm (10-9)
Amplitude Represents the height of the crest (or depth of a trough) Represents the intensity of the radiation (ie. visible light)
Frequency … symbol nu Number of cycles a wave passes a given point every second Expressed as.. 1 or waves or sec-1 or Hertz
sec sec Speed … symbol c
Represents the speed of light in a vacuum 3.00 x 108 m
sec
EM Spectrum
Radio, Microwave, Infrared, Visible light, Ultraviolet, X-Rays, Gamma rays
Emission spectra represents energy released by an electron as it
returns to ground state from excited state Absorption spectra
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
Ground state – lowest energy level for an electron when an atom is in its most stable energy state
Excited state – a higher energy state of an electron (energy is absorbed by electron)
Electromagnetic spectrum – range of energy that exhibits wave-like behavior as it travels through space. Consists of both electric and magnetic field
components. classified according to the frequency of its wave.
Bohr ModelQuestion: 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
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Parts of Atom Electrons
negatively charged subatomic particle mass = 9.11 x 10 -28 g
Protons positively charge subatomic particle mass = 1.67 x 10 -24 g
Neutron – Discovered by James Chadwick (~ 1932) Subatomic particle with no charge mass = 1.67 x 10 -24 g
Terminology
Atomic number number of protons in the nucleus due to atom’s electrical neutrality, also indicates the number of electrons 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