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Never trust an atom THEY MAKE EVERYTHING UP How small can you get? Atoms are the smallest particle of a material that can exist and still have its properties Properties: the characteristics of a material that can be seen when that material changes. Examples: melting point, boiling point, flammable, reactiveness to other materials If you break an atom apart, it looses all its properties and is no longer that material There are 118 types of atoms known on earth and in the universe They are called the elements History of the atom Atoms were first predicted by Greek philosopher Democritus about 2000 years ago He hypothesized that everything was made up of a basic thing that could not be broken down any more He called it an atomos, Greek for “unable to divide” Beginnings of Atomic Theory 1800s – scientists began discovering basic elements and identifying their properties John Dalton made several important observations about atoms He turned his observations into the first atomic theory Dalton’s atomic theory: All matter is made of atoms which are indivisible and indestructible All atoms of the same element have identical properties Combining two or more different atoms make compounds A chemical reaction is when atoms are rearranged Changes to atomic theory J. J. Thomson discovered that atoms were not solid but were made of electrons Electrons: negatively electrically-charged particles Thought the electrons floated in a positively charged soup Ernest Rutherford discovered atoms had a nucleus made of protons Nucleus: dense mass at the center of an atom Proton: positively electrically-charged particle James Chadwick discovered that there were neutrons in the nucleus Neutron: neutrally charged particle with the same mass as a proton Niels Bohr discovered that electrons orbit the nucleus like the planets around the sun in specific orbitals Current atomic theory All matter is made of atoms Atoms are made of: A large nucleus made of protons (positive charge) and neutrons (no charge) Electrons (negative charge) which travel around the nucleus in set paths called orbitals All atoms of the same element have the same properties Different atoms can be combined to make compounds Atoms can be rearranged using a chemical reaction How big is an atom? Atoms are about 1,000,000 times smaller than a human hair Normal ways of measuring cannot be used on something so small Atomic mass and atomic mass units or AMU are used to measure atoms Protons and neutrons are the same size and comparatively large 1 proton = 1 neutron = 1 AMU Electrons are about .0005 AMU Atomic mass: the combined AMUs of all protons, neutrons, and electrons in an atom Example: Helium (2 protons, 2 neutrons, 2 electrons) 2 + 2 + (.0005 x 2) = 4.001 AMU (actual size 4.003 AMU)
