 Atom: Smallest unit of an element that maintains the properties of that element.  Democritus: Believed that the atom could not be cut smaller. Aristotle believed that you would never end up with a particle that could not be cut. Disagreed with Democritus.  Dalton’s Atomic Theory: All substances are made of atoms. Atoms are small particles that cannot be created, divided or destroyed. Atoms of the same element are exactly alike, and atoms of different elements are different. Atoms join with other atoms to make new substances.

 Atoms can be divided into even smaller particles  Plum Pudding Model  Electrons: Negatively charged subatomic particle Bohr said that electrons move around the nucleus in certain paths, or energy levels. Nucleus:  An atoms central region, which is made of protons and neutrons

Rutherford  1909-Gold foil experiment  Findings from this experiment led him to revise the atomic theory  Atoms had a positive nucleus that is surrounded by electrons

 Electrons do not travel in definite paths  Cannot predict the path of an electron  Electron Clouds: Region around the nucleus of an atom where electrons are likely to be found Schrodinger and Heisenberg

 How Small is an atom? 20,000,000,000,000,000,000,000 atoms of copper and zinc in a penny -20 thousand billion billion  What is an atom made of? Protons:  Positively charged particles in the nucleus Atomic Mass Unit:  Used to express the masses of particles in atoms  Each proton has a mass of about 1amu Neutron:  Particles of the nucleus that have no electrical charge Charges of protons and electrons are opposite but equal, so there charges cancel out. Atoms have no overall charge  Unless the number of electrons and protons are unequal Ion:  Charged particle formed when electrons and protons are not equal More electrons / negatively charged ion More protons / positively charged ion

 All atoms contain protons and electrons  Most atoms contain neutrons, but not all  An atom does not have to have an = number of protons and neutrons  Atomic Number: The # of protons in the nucleus

 Isotope: Have the same number of protons but have different numbers of neutrons Atoms that are isotopes of each other are always the same element because isotopes always have the same number of protons Have different # of neutrons which gives them different masses

 Some isotopes are unstable  An unstable atom is an atom with a nucleus that will change over time – radioactive  Radioactive atoms spontaneously fall apart over time  Telling Isotopes Apart: You can tell isotopes of an element apart by its mass number  protons + neutrons =mass number (figure 5 page 92) + H ionH atom- H ion Electrons are not included in the mass number because they are so small that they have little effect on the elements atomic mass. Atomic Mass:  the mass of an atom expressed in atomic mass units  The weighted average of the masses of all the naturally occurring isotopes of that element.

 Each atom has forces that act between the particles Gravitational force Electromagnetic force Strong force Weak force (page 94 figure 7)