Atoms and Moles

Atomic Models Atomic TheoriesAtomic Theory A Short HistoryFifth Century, BCEDemocritusBelieved matter was composed of very small, individual particles that were indestructibleHe called them atomos (meaning uncuttable)His ideas persisted for centuries even though there was no experimental proof

Atomic TheoriesJOHN DALTON - 1808Revised early Greek ideasinto testable scientific theory

Based his Atomic Theory onthree important concepts:1. Law of Conservation of Mass2. Law of Multiple Proportions3. Law of Definite Proportions

Atomic TheoriesLaw of Conservation of MassStates that mass cannot be created or destroyed the mass of the reactants equals the mass of the products

Atomic TheoriesDaltons Principles All matter is composed of extremely small particles called atoms which cannot be subdivided, created or destroyed

Atoms of a given element are identical in their physical and chemical properties Example: all water molecules freeze at 0 deg C and react with explosively with sodium

Atomic TheoriesDaltons Principles (continued)3. All atoms of one element are different from those of any other element 4. Atoms combine in simple, whole-numbered ratios to form compoundsBased on the Laws of Definite and Multiple Proportions

Atomic TheoriesDaltons Principles (continued)5. In chemical reactions, atoms are combined, separated or rearranged but NEVER created, destroyed or changedBased on The Law of Conservation of Mass

Atomic TheoriesDalton, however, did all this work in the early 1800s without ever knowing about subatomic particles!(Protons, Neutrons, Electrons)

Atomic ModelsThe Adventures of J.J. Thomson, Plum Pudding and the Electron!

MMMMM..That plum pudding looks delicious!Atomic ModelsJJ Thomson:

English Physicist Experimented with cathode raysWas able to determine that the mass of the particles in the ray was much smaller than the mass of a hydrogen atomThe particle must be a smaller than an atom!

I LOVE plum pudding!Atomic ModelsJJ Thomsons Cathode Ray Tube (CRT)

Anode attached to thepositive terminal of thevoltage sourceCathode attached to theNegative end of the voltagesourceAtomic ModelsA magnet placed near the tube deflected the beam, proving it was negatively charged.

A small paddlewheel in the tube turned when hit by the beam, meaning the particles had mass.

Atomic ModelsThompson called the negatively charged particles in the beam corpuscles

This name was later changed to electrons by one of Thompsons associates.

Atomic ModelsThompsons Conclusion:An atom is a sphere of positive charge with electrons embedded in it

Plum-Pudding Model of the AtomAtomic ModelsIn 1909, one of his students, Ernest Rutherford, disproved the Plum Pudding model by doing is famous Gold Foil experiment.

Atomic ModelsRutherfords experiment

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf

Atomic ModelsWhat conclusions were drawn from Rutherfords Experiment?

Atomic ModelsRutherfords Model of the Atom:

Electrons orbit the nucleus just as planetsorbit the sunDid not explain why the negatively charged electrons did not crash into the positively charged nucleus.

Atomic ModelsTwo years later, Danish physicist, Niels Bohr, proposed the Bohr Model of the atom

Atomic ModelsBohrs Model Electrons are located certain distances from the nucleus

Each distance is a certain quantity of energy that the electron can have

Atomic ModelsElectrons closest to the nucleus have the lowest energy, while the ones further away are in higher energy levels

Atomic ModelsThe difference between two energy levels is called a quantum of energy.Electrons can be only in an energy level, NOT between levels.

Atomic ModelsElectrons and Light (pg. 92) Basic Information:Light travels in wavesEach wave has a certain wavelength (distance between two consecutive peaks of a wave)

Atomic ModelsEach wavelength hasa certain frequency (the # of waves thatpass through a specific point in onesecond)

Atomic ModelLight and the Visible Electromagnetic SpectrumWe can only see a small amount of the electromagnetic spectrumBohr found that if you pass a high voltage through a gas and look at it through a prism, it will have a distinctive pattern of colored lines called line-emission spectrum

Atomic ModelsBohr calculated that the line-emission spectrum corresponded to energy levels in the atom (which give off a different wavelength of visible light)The colors are caused when an electron falls from its excited state to its ground state and gives off energy in the form of light! (pg. 94)

Atomic ModelsGreat video on line-emission spectrum

http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/linesp16.swf

Spectrum of a Fluorescent Light

Spectrum