Atoms Unit

Atoms Unit History of the Atomic Theory Democritus

His theory: Matter could not bedivided into smallerand smaller pieces forever. There was a piecethat was indivisible.

Daltons TheoryHe deduced that all elements are composed of atoms. Atoms are indivisible and indestructible particles.Atoms of the same element are exactly alike.Atoms of different elements are different.Compounds are formed by the joining of atoms of two or more elements.

Thomsons Plum Pudding Model

In 1897, the English scientist J.J. Thomson proved an atom is not solid and has even smaller particles. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding

Thomson Experiment

Thomson studied the passage of an electric current through a gas.

As the current passed through the gas, it gave off rays of negatively charged particles.

Rutherfords Gold Foil Experiment

In 1908, the English physicist Ernest Rutherford was working on an experiment that involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick). He assumed that his + particles would go right through the gold foil, since there was not any actual positive particles in Thomsons model of the atom.

Atoms werent a pudding filled with a positively charged material.Rutherford concluded that an atom must have a small, dense, positively charged center that repelled his positively charged bullets. Rutherford named this center the nucleus

Bohr Model

According to Bohrs atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus.Bohr

The show Homer is watching, When Dinosaurs Get Drunk, is replaced with another called The Boring World of Niels Bohr. The Wave Model

According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.They move in certain wavelike pathways described by complicated equations.

Erwin SchroedingerWave Model

IndivisibleElectronNucleusOrbitElectron CloudGreek XDalton XThomson XRutherford X XBohr X X XWave X X XEnd Atomic HistoryAtomic Structure Electron Cloud Model

Sub atomic particleRelative massRelative chargeLocation in the atomProton1 amu+1Inside the nucleusNeutron1 amu0Inside the nucleusElectron1/1840 amu-1Outside the nucleusWhat defines an element?Atomic number the number of protons in the nucleus of an atomThis is the same for every atom of a certain element.

What is the mass number of an element?Mass number the sum of the number ofprotons and the number of neutrons in thenucleus of an atom. (Electrons are too small tocontribute to the mass.) It is the mass of thatatom.There can be different Mass numbers for different atoms of an element.

IsotopesAtoms of the same element with a different number of neutrons are called isotopes.Many elements have one or more isotopes.

Average atomic mass

So why is the number 6.941 written as the mass for Lithium?This is the weighted average of the mass of the isotopes of Lithium.The isotopes are not equally represented.92.5% of all lithium atoms are the Li-7 isotope; 7.5% are the Li-6 isotope. The other isotope is very rare.How is it calculated?92.5% Li-7 7.5% Li-6Percent abundance of each isotope.

[0.925 (7)] + [0.075(6)] = 6.475 + 0.45 = 6.925

SymbolAtomic NumberMass NumberNumber of ProtonsNumber of ElectronsNumber of NeutronsNa-23K4019383852F10204120505072131I26Mg109474736SEnd Atomic Structure General Electron PropertiesAtoms and LightAbout 100 years ago some scientists noticed that certain elements emitted different colors of visible light when heated in a flame.

More research helped them connect this light to the arrangement of each elements electrons.

LIGHTTo understand the structure of the atom we need to learn about light and how it can act like a wave and a particle at the same time .

Simply stated, light is nature's way of transferring energy through space , light is energy.Light can behave like a wave

wave pattern

Light as Waves The Electromagnetic Spectrum

Visible light ROYGBIVLow frequencyLong wavelengthLow energyHigh frequencyShort wavelengthHigh energy

400nm700nmMake sure to note that the longer the wave the less energy and the shorter the wave the higher energy. 27Light behaves like a particle

photoelectric effectPhotoelectric effect colorado

PhotonsLight is absorbed by electrons in chunks or particles.A particle of light is a photon.

Photon absorption and emissionAbsorption at the quantum level happens when an individual photon has the exact energy that corresponds to an energy gap between two possible energy states.The type of energy gap corresponds directly to the frequency of the photon

Putting it all togetherAtomic emission spectra each element has a unique spectrum of colors that it produces as the electrons absorb energy. A spectrum is like the elements fingerprint.Spectra

Not all spectra are visible; some are too high frequency; some are too low frequency

Visible Hydrogen spectrumEnd Atoms Unit