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

-

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