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Unit 2 – Chapter 11 The Atom Atomic Theory • Democritus – 440 BCE – Idea that cutting something until it cannot be cut anymore • Aristotle – Disagreed with Democritus • Atom – Smallest particle an element can be divided and still be the same substance Atomic Theory • John Dalton – 1803 – British chemist and teacher – Elements combine in certain proportions because they are made of single atoms John Dalton’s Atomic Theory • All substances are made of atoms • Atoms of the same element are exactly alike, and atoms of different elements are different • Atoms join with other atoms to make new substances JJ Thomson • 1897 • JJ Thomson discovered a mistake in Dalton’s theory • There are actually smaller particles INSIDE the atom • Cathode ray tube JJ Thomson • Passed an electrically charged beam through a metal plate • The positively charged plate attracted the beam’s particles • This meant that there are negatively charged particles as well • Electrons – negatively charged particles in an atom JJ Thomson • Proposed a model of an atom • Plum pudding model – Electrons mixed throughout the atom Ernest Rutherford • Student of JJ Thomson • 1909 • Sent positively charged particles through thin sheets of gold foil • Could see where the particles went after they went through the foil Gold Foil Experiment • Most particles passed right through • Some were deflected slightly • Some were deflected right back Rutherford’s Atomic Theory • Nucleus: Tiny, dense, positively charged center of atom • Positive repels positive • Empty space between particles Niels Bohr • 1913 • Danish scientist who worked with Rutherford • Proposed that electrons move around the nucleus in paths • Electrons can jump from path to path Modern Atomic Theory • Erwin Schrödinger (Schrödinger’s cat) • Werner Heisenberg • Discovered electrons do not travel in definite paths or orbits • Most likely within regions • Electron Clouds: Regions within an atom where electrons are found The Atom • How small is an atom? – One penny contains about 2x1022 atoms, or 20,000,000,000,000,000,000,000 atoms – One Al (aluminum) atom has a diameter (how large across the entire atom) of 0.00000003 cm – Three one hundred millionths of a centimeter Subatomic Particles • Electrons – Negatively charged – VERY tiny!!!!! – Found in electron clouds – Mass of 9.1x10-31kg – Because the mass is so small, we ignore it and call it zero Subatomic Particles • Protons – Positively charged – Found in nucleus – Mass of 1.67x10-27kg – Mass of 1 AMU (atomic mass unit) Subatomic Particles • Neutron – No charge (neutral) – Found in nucleus – Mass of 1.67x10-27kg – Mass of 1 AMU Nucleus • • • • Proton and neutron together make up nucleus Has a positive charge EXTREMELY DENSE!!! If the nucleus were 1 cubic meter in size, its mass would be over 100,000,000,000,000 tons • Makes up 99.99% of mass of atom Outside the nucleus • Electrons in electron clouds • Negative charge – cancels out positive charge of nucleus, so overall charge of atom is neutral • Atoms can gain or lose electrons – These are called ions – Can be positive or negative – Positive – lose electrons (less negative) – Negative – gain electrons (more negative) How do elements differ? • About 119 elements overall • Protons and electrons found in all atoms of every element • Number of protons = number of electrons • Number of neutrons can differ, C-12 and C-14 • Atomic number: Number of protons in the nucleus of an atom – This is what makes that atom that specific element – Carbon has atomic number of 6, Oxygen is 8, etc – Every atom of that element has same atomic number Isotopes • Atoms that have same number of protons, but different number of neutrons – Carbon-12 has 6 protons and 6 neutrons – Carbon-14 has 6 protons and 8 neutrons • Isotopes can be unstable – Nucleus will DECAY over time – This is called radioactivity – Spontaneous – Gives off energy and particles Isotopes • Mass number: Total of protons + neutrons • To calculate, add protons plus neutrons • To name an isotope: – Write name of element, followed by mass number – C-12 – O-18 • The isotope must be given to you to find the number of neutrons, or vice versa Mass of element • Most elements contain a mixture of two or more isotopes • Copper contains Cu-63 and Cu-65 atoms • Atomic mass: – Average mass of all naturally occurring isotopes – Shown as a decimal on periodic table Forces • There are four main forces at work in the universe – Gravitational – Electromagnetic – Strong Nuclear – Weak Nuclear Gravitational • Gravitational force depends on size and mass of objects • Pulls other objects towards the gravitational center of object • You toward center of Earth • Earth toward Sun • Sun toward middle of Milky Way galaxy • Considered weakest force (short range) Weak nuclear force • Responsible for decay of subatomic particles • Can turn neutrons into protons and/or electrons • Plays a part in radioactivity Electromagnetic • Same charges repel (positive repels positive, negative repels negative) • Opposite charges attract (like magnets) • Electrons are held in place around the nucleus due to electromagnetic force • Magnetic field around Earth • Larger range (planetary) Strong nuclear force • Holds protons and neutrons together in nucleus • Must be larger than electromagnetic force or protons would fly away from each other in nucleus (positive repels positive) • Considered strongest force in universe at short distances (inside nucleus) Strongest? • What happens if electromagnetic force is larger than strong nuclear force? • Can gravitational force be the weakest AND the strongest force in the universe??