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ATOMS & THE PERIODIC TABLE OF ELEMENTS Chapter 3 CLASSIFYING MATTER • What is matter made of? • The ancient Greeks thought that everything in the universe was made up of a combination of 4 “elements”: air, fire, water, and earth. People believed this for many centuries! • In the late 1600s, early chemists began to discover that this was not the case, that there are more than 4 elements and they are not what the Greeks thought they were. • Now we know that all matter in the universe is made up of over 100 different kinds of elements. ELEMENTS • An element is a substance that cannot be broken down into any other substance by physical or chemical means (methods or ways). • Like all matter, elements are identified by their physical and chemical properties. THE ELEMENTS • Name some elements! Remember, these are substances that cannot be broken down into other substances. • Elements are abbreviated by one or two letter symbols • Examples: oxygen is O, calcium is Ca, carbon is C, aluminum is Al) • Link ATOMS • What would happen if you tore a piece of aluminum foil into smaller and smaller pieces? What is the smallest possible piece of a substance? • All matter is made up of atoms. • An atom is the particle out of which all elements are made. MOLECULES • Atoms can combine and form a chemical bond, or a force of attraction between 2 or more atoms. • A molecule is a group of 2 or more atoms held together by chemical bonds • A molecule can be made of the same atoms or several different atoms • Examples: O2, H20, CO2, NH3 • How can oxygen (O2) be both an element and a molecule? • It is two or more atoms but they are the same element COMPOUNDS • A compound is a substance made of 2 or more elements that are chemically combined in a certain ratio. • For example, carbon dioxide (CO2) is 1atom of carbon and 2 atoms of oxygen. Carbon monoxide (CO) is 1 atom of carbon and 1 atom of oxygen. • Compounds are made of molecules. • Table sugar has the chemical formula C12H22O11. What is the ratio of carbon to oxygen in this compound? • 12:11 • Is H2O the same as H2O2? Would you expect them to have the same properties? • No, they are different substances (H2O2 has one more oxygen) so they would have different properties. BELLRINGER •Without using a periodic table, what elements are represented by the symbols O, C, H, and Cl? •The properties of compounds are different that those of the elements they are made of. •Example: sodium (Na) + chloride (Cl) = salt (NaCl) •Why?? •Because Na and Cl are different substances. WHAT IS AN ATOM MADE OF? • Positive center (nucleus) with a cloud of negatively charged particles around it • Proton: positive (+), in nucleus • Neutron: neutral (0), in nucleus • Electron: negative (-), “orbits” nucleus in energy levels SUBATOMIC PARTICLES • The proton of an atom of gold is the same as a proton of oxygen. Same for electrons and neutrons! All protons, neutrons, and electrons are the same. What makes elements different is the relative numbers of each. ATOMIC NUMBER • # of protons = atomic number • Defines each element • How the elements are arranged on the periodic table • Number of protons equals number of electrons (unless it is an ion, or atom that is charged) • Atoms of one element are different from atoms of another element because of their number of protons (atomic number) • Draw an atom of helium. Helium has 2 electrons, 2 protons, and 2 neutrons. ISOTOPES • Isotopes are atoms with a same number of protons but different number of neutrons • Isotopes are identified by their mass number (sum of protons + neutrons) • Electrons have very little mass • With a partner, on a blank sheet of paper, write the names of as many elements as you can think of (do not use a periodic table!) • Now organize them into groups based on their characteristics. Every group must have more than one element in it. THE PERIODIC TABLE • Russian chemist Dmitri Mendeleev organized the elements into the Periodic Table in the 1870s • Puzzle of the Periodic Table • He did this by studying each element’s melting point, density, color, and atomic mass (average mass of all isotopes of that element) • He predicted the existence of many elements that were discovered later! THE PERIODIC TABLE • Most periodic tables include the atomic number, chemical symbol, name, and atomic mass of each element • Where did all the elements come from? • All elements up to iron were created in stars, elements heavier than iron come from supernovas • How did they get their names? • Names of scientists, places, Latin/Greek, etc. • Element Song THE PERIODIC TABLE • Although the Periodic Table is arranged by increasing atomic number, it is the arrangement of each element’s electrons that determines its properties • There are 7 possible energy levels for electrons and 7 periods (rows) on the periodic table • The periods go in order of increasing atomic # • Elements in a group (column) have the same number of outermost (valence) electrons and react similarly • Noble gases are unreactive because their valence orbitals are full • Metals are on the left of the table, nonmetals on the right, and metalloids in between • You can predict the properties of an element by knowing where it is on the Periodic Table! 11/24/14 •Bellringer: How many electrons does this element have? How many neutrons does it have? METALS • Most elements on the Periodic Table • Have physical properties like luster (shininess), malleability (can be hammered flat), ductility (can be made into wires), conductivity (good conductors of heat & electricity) • Metals usually react by losing electrons • When metals deteriorate due to chemical reactions with substances in the environment (like oxygen or water) it is called corrosion. • Example of corrosion: rust METALS METALS • Alkali metals (group 1) are the most reactive metals • Are never found uncombined (alone) in nature • Alkaline earth metals (group 2) are harder, denser, and melt at higher temps, and are less reactive than the alkali metals • Transition metals (group 3-12) are hard, shiny, with high melting pts and densities, less reactive NON-METALS • Lack the properties of metals • Poor conductors, dull, brittle • Many are gases at room temp; some are solids (C, S, I), bromine is the only liquid • On the right side of Periodic Table (except Hydrogen) • Usually gain or share electrons when they react NON-METALS • Halogens (group 17), like chlorine, are the most reactive nonmetals • Noble gases (group 18), like neon, are usually nonreactive METALLOIDS • Metalloids have some properties of metals and some properties of nonmetals • Examples are silicon & arsenic RADIOACTIVE DECAY • Remember what an isotope is.. • An atom that has a different # of neutrons than usual • Some isotopes are unstable, meaning they undergo radioactive decay RADIOACTIVE DECAY • Radioactive decay is when a nucleus gives off alpha or beta particles and/or energy in the form of gamma rays • This is a nuclear reaction • This changes the identity of the atom (because if protons are lost, the atomic # changes); it becomes a new element • Something is radioactive if it does radioactive decay RADIOACTIVE DECAY • Radioactive dating is using isotopes to determine how old something is • This is done by knowing the half life of the isotopes, or how long it takes for half of it to decay • Isotopes are also used as tracers (see where things go)