History of the Atom & Atomic Structure

History of the Atom & Atomic Structure

History of the Atom & Atomic StructureAtomic Structure

Chemistry Ms. PielaHistory of the AtomDemocritus (460 BC 360 BC)Ancient Greek philosopherNo experiments performed!Major Contribution: The AtomHe proposed that everything was made of these atoms and they were all indivisibleWas his theory correct?No! There are subatomic particles!

History of the AtomJohn Dalton (1766-1844)Major Contribution: Atomic Theory (1808)This began the modern era of chemistryFour Principles:Elements are composed of indivisible particles called atoms.All atoms of a given element are identical.

History of the AtomJohn DaltonCompounds are composed of atoms of one or more elements, and will form only in whole-number ratios.This is called the Law of Multiple Proportionsi.e. H2O exists, while H2.35O does notA chemical reaction involves the combination, separation, or rearrangement of atoms, not their creation or destructionThis is called the Law of Conservation of MassWas his theory correct?Mostly! Parts 1 & 2 have problems!John DaltonWhen we attempt to conceive the number of particles in an atmosphere [gas], it is somewhat like attempting to conceive the number of stars in the universe; we are confounded with the thought. But if we limit the subject, by taking a given volume of any gas, we seem persuaded that the number of particles must be finite

- John Dalton on his approach to the theory of atoms, 1808

History of the AtomJ.J. Thomson (1856-1940)Major Contribution: The ElectronCathode Ray Tube Experiment (1897)Nobel prize (1906)

History of the AtomJ.J. Thomson

History of the AtomThomsons Atomic Model

Also known as the Plum Pudding ModelWas his theory correct?No! Missing parts of atom!

History of the AtomErnest Rutherford (1871-1937)Two Major Contributions:The nucleusThe atom is mostly empty spaceGold Foil Experiment (1910)Nobel prize in Chemistry (1908)

History of the AtomErnest Rutherfords Gold Foil Experiment

History of the AtomRutherfords Atomic Model

Was his theory correct?Mostly! Missing neutrons and location of electrons!

History of the AtomNiels Bohr (1885-1962)Major Contribution: Planetary Model of the AtomNobel Prize in Physics (1922) for spectrum of hydrogenAtomic Line SpectraBohr observed that when light was given off from an atom, there were only single lines visibleBohr proposed that each line represented an electron in a different orbit

History of the AtomAtomic Line Spectra

History of the AtomBohrs Atomic Model

ElectronsNucleusHistory of the AtomCurrent Theory of the AtomMany scientists contributed to developing quantum mechanics, which is the current model of the atomKnown as the electron cloud modelThe cloud is an area of probability where the electron is foundThese electrons, moving at extremely high speeds, effectively occupy the entire area of the cloud, the same way that moving fan blades effectively occupy the entire area through which they pass.History of the AtomCurrent Model of the Atom:

Probability cloud where electrons foundNucleusAtomic StructureParts of the AtomProtonPositiveNucleusNeutronNeutralNucleusElectronNegativeOrbitting Nucleus

Atoms and the Periodic TableAtomic NumberAtoms are identified by their number of protonsThis is referred to as their atomic numberThink of atomic # like a social security card for each element

Atoms & ChargesIn atoms that have a neutral charge, the numbers of electrons equals the number of protonsWhen an atom gains a charge, it is called an ion

Atoms & ChargesAtoms and the Periodic TableMass NumberThe mass of an atom is the number of protons plus the number of neutronsThis is referred to as mass numberThe mass of protons and neutrons are equal

Electrons and Mass of AtomWhy is the electron not part of the mass?It takes roughly 1800 electrons to equal the mass of 1 proton, so it is left out.If we wanted the mass of an elephant, would we weigh the flies buzzing around it? Doesnt count!

Mass Number and Periodic TableThe periodic table does not give the mass number, but always the atomic numberFor simplicity, we round the number on the periodic table to get the mass number

Round this number to whole numberIsotopesIsotopes are elements that have the same number of protons, but contains a different number of neutronsCompare: carbon-12 vs. carbon-14The number indicates the mass numberBoth contain the same number of protons (6), so carbon-14 must have two extra neutrons

Isotopes & Periodic TableThe masses given on the periodic table are an average of all the isotopes on the planetWe refer to the masses on the periodic table as the average atomic mass of an elementThis explains why the atomic masses are not whole numbers it is an average!