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Atomic Structure/Atomic Theory Atomic Structure: Unit 2 The atom o The building block of matter o Each element is a unique type of atom Areas of the Atom: o Nucleus Center of the atoms Contains almost all of the mass but very little of the atoms space Contains Protons and Neutrons o Electron Cloud Area around the nucleus Contains up to 7 energy levels where electrons can be found Each energy level can hold only so many electrons 1st energy level holds 2 electrons 2nd energy level holds 8 electrons 3 rd energy level holds 18 electrons Particles that make up the atom o Proton Positive charged particle in the nucleus Has a mass of 1 atomic mass unit (u) The number of protons determines the type of element o Neutron Neutral (no charge) particle in the nucleus Has a mass of 1 atomic mass unit (u) o Electron Negative charged particle found in the electron cloud Electrons circle the nucleus Electrons contain almost no mass. There atomic mass is zero (u) In stable atom, the number of electrons will be the same as the number of protons Valence electrons are electrons in an atoms outermost electron energy level and determine how the atom reacts with other atoms Elements: The number of protons in an atom determines which type of element the atom is. The atomic number on the periodic table identifies the number of protons: example H = atomic number 1 and it has one proton; Carbon is atomic number 6 and it has six protons. Atomic Structure/Atomic Theory Isotopes of an element have different numbers of neutrons Example Carbon-14 Still has 6 protons, but it has 8 neutrons. Carbon-14 has this name because its mass is 14 Ions – When an atom loses or gains an electron, then the atom has a positive or negative charge. Example: if Carbon loses an electron, then it will have 6 protons and only 5 electrons, giving it a positive + 1 charge overall (6 positives protons and 5 negative electrons) Development of the Atomic Theory Democritus (430 BC) Greek Philosopher Proposed existence of atoms (atoma) = Greek for indivisible John Dalton (1803) British Chemist Compounds form from specific amounts of elements Atoms cannot be created, divided or destroyed Each atom of the same element is identical, but different from the atoms of other elements Solid Sphere (Bowling ball) model J.J. Thomson (1897) British Chemist Discovered 1st subatomic particle (electrons) Plum pudding model – positive sphere with negative particles Ernest Rutherford (1912) New Zealand Physicist Discovered the nucleus Nuclear model – small, dense, positive charged particles in the nucleus = protons Electrons travel around nucleus Niels Bohr (1913) Danish Physicist Discovered energy levels Bohr model (planetary model) Electrons travel in fixed paths, but can jump from one to another Erwin Schrodinger (1924) Austrian Physicist Developed the electron cloud model Exact path of electrons cannot be predicted James Chadwick (1932) English Physicist Discovered neutrons (no charge) Mass of neutrons nearly equal to mass of protons (atomic mass unit)