1. Atomic Theory CMS Science 4.0

2. Development of Atomic Models Atom - the smallest particle of an element Our atomic theory has grown as models were tested and altered based on observations and experimental results, which led to changes and new ideas.

3. Ancient Greek Atomos – uncuttable Smallest possible pieces All matter is composed of atoms which are too small to see. There is a void, or empty space, between atoms. – Atoms are solid with no internal structure. – Different materials have atoms of different shapes and sizes. Aristotle (384-322 BC) Democritus (c460-371 BC)

4. John Dalton 1776 – 1844 1803 All elements are made of atoms that cannot be divided. Similar to hard balls. All atoms of the same element are exactly alike. Atoms cannot be changed into atoms of another element. Compounds are formed by joining atoms of two or more elements in constant proportions.

5. Sir John Thompson 1856 – 1940 1897 Discovered the electron, common to all elements. Atoms have positive and negatively charged parts. – Plum pudding (Muffin) model – negative electrons in a positively charged ball.

6. Ernest Rutherford 1871 – 1937 1911 Nucleus is a tiny dense positively charged mass Peach pit model Protons -positively charged particles in the nucleus

7. Niels Bohr 1885 - 1962 1913 Electrons are found certain distances away from the nucleus with specific amounts or quantities of energy. Orbits further from the nucleus have more energy. Orbits are called energy levels or electron shells. – Solar system model, planets orbiting the sun.

8. The Bohr Model

9. Electron cloud 1920’s Electron’s location and movement are related to the energy level but their exact location is never known Werner Heisenberg 1901 – 1976 1925 The Heisenberg Uncertainty Principle Electrons cannot be pinpointed at any given moment: Travel near the speed of light The electron cloud is huge

10. Discovered the neutron. Neutral – no electrical charge Mass slightly greater than a proton Isotopes are atoms with the same atomic number, number of protons, but a different number of neutrons. When comparing Hydrogen atoms and Helium atoms, Chadwick found that Hydrogen has one proton and Helium has two. When massing the atoms, H was found to have a mass of 1 and Helium a mass of 4. What accounted for the extra two mass units? 2 Neutrons. James Chadwick 1891 – 1974 1932

11. The Particles of the Atom Particle symbol charge mass, kg mass, (amu) electrone- -1 9.10953×10-31 0.000548 protonp++11.67265×10-271.007276 neutronn 0 0 1.67495×10-27 1.008665 THE MASS OF THE NEUTRON IS 1839 times greater than an electron. Composition of the Nucleus: nuclei are composed of "nucleons": protons and neutrons atomic mass units: 1 amu = exactly 1/12 the mass of a carbon-12 nucleus

12. Modern Atomic Model Nucleus contains Protons (+) and Neutrons - almost all of the mass of the atom - tiny compared to size of the atom 1 cm1 km Electrons (–) moving in a cloudlike region around the nucleus

13. Modern Atomic Model The protons equal the electrons number and equal but opposite charge - neutral charge on the atom - number of protons is the atomic number All atoms of an element have the same number of protons Isotopes are atoms of the same element with different numbers of neutrons

14. New Particles - Quarks Leptons: "Lightweight" elementary particles not composed of quarks. Electron Muon Tau Neutrinos Hadrons: "Massive" particles composed of quarks. –Baryons: Composed of three quarks. Found in "everyday" matter. Proton: quarks = up + up + down Neutron: quarks = down + down + up –Mesons: Composed of a quark and an antiquark. Found in cosmic rays. Pion: quarks = up + down Bosons: The exchange of these "force carriers" between fundamental particles allows the formation of Composite Particles. –Photon - massless, no charge, electromagnetic force carrier. –Gluon - massless, no charge, strong nuclear force carrier. –W and Z - massless, no charge, weak nuclear force carriers. –Graviton - massless, no charge, gravity force carrier. –The graviton is the only force-carrying particle that has not been observed, either directly or indirectly.