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First 9 weeks Study Guide 8th Grade Read this MULTIPLE times to prepare for the exam! Elements and Compounds 6.5ABC Key Concepts • • An element is a pure substance that cannot be broken down into simpler substances. A substance that consists of two or more different elements is a compound. Living matter is made up mostly of oxygen, carbon, hydrogen, nitrogen, calcium, and phosphorus which form organic compounds. Elements Molecules Compounds Carbon O2 Oxygen C6H12O6 Hydrogen Oxygen H2 Hydrogen H2O CH4 Glucose water Methane Protons, Neutrons, and Electrons the subatomic particles that make up atoms Atoms Atoms are the smallest part of an element. All atoms of the same element have the same number of protons. Elements Made up of atoms with the same number of protons, arranged on the periodic table. Elements can be metals, metalloids, and non-metals. Compounds two or more different elements H2O C6H12O6 Molecules made by two of the same elment such as O2 1 Atomic Structure/Properties 8.2C/8.3BD/8.5AB Key Concepts • An atom is the smallest unit of an element that upholds all of the properties of that element. • Atoms are made up of subatomic particles called protons, neutrons, and electrons. Atomic Number Element Name Symbol Atomic Mass A tomic number M ass number = Protons =Electrons - Atomic number=Neutrons The protons give you an elements identity. The protons give you the atomic number which is like the address for that element on the periodic table. The atomic mass is the sum (the total) of the protons and neutrons. Protons + •Positive charge •Identity of the atom •In the nucleus •1 amu Neutrons Atom Electrons _ •Neutral charge •In the nucleus •1 amu •Negative charge •orbit the nucleus •are in the electron cloud • way almost nothing •valence electrons are farthest from the nucleus 2 Atoms Protons Valence Electrons Postive Charge Negative Charge Located in the nucleus Outer most shell Determines an elements identity Determines an atoms reactivity Always equal to the atomic number Same group, same # of Valence electrons 3 Periods Groups Metals • • Metalloids Non Metals Elements of the periodic table are Metals, Non-Metals or Metalloids Metals have four characteristics that are very important: o o o o Conductivity – ability to transfer heat and electricity Luster – refers to the level of “shininess” or light reflection Malleable – ability to be shaped without breaking like a shield Ductile – ability to be hammered thin or turn into wire: Copper has high ductility. Periods Groups Increasing Atomic Number from left to right Same valence electrons Decreasing Atomic Radius or Atomic Size from left to right Similar chemical properties Decreasing Metallic properties from left to right Similar reactivity 4 Important HIGHLY REACTIVE Groups to remember Group 2 has 2 valence electron Group 1 has 1 valence electron 8 valence electrons a full shell 8 valence electrons a full shell Group 16 has 6 valence electrons Group 17 has 7 valence electrons Nobel Gases have a full shell so they are stable and will not react with other element’s atoms. The have 8 valence electrons. Chemical Formulas, Reactions, Equations 8.2C/8.3BCD/8.5DEF Key Concepts • • • During a chemical reaction, the atoms of substances rearrange themselves into a new configuration, forming new substances. The reactants combine to produce new products. A chemical formula is the combination of all of the elemental symbols found within a substance. The atom numbers of each element are identified by subscripts to the right of the elemental symbol. Chemical equation shows the atom numbers and molecules making up the reactants and products of a chemical reaction. A number, or coefficient, in the front of the molecule’s chemical formula represents the molecule number in each reaction. 2H2 + O2 2 H2O The co-efficient tells how many molecules There are two water molecules. 2 H2 means 4 hydrogen The subscript tells how many atoms of that element there are. O2 means 2 Oxygen How many: 3 C6H12O6 Elements: 3 Carbon, Hydrogen, and Oxygen Atoms: 3x6+3x12+3x6 = 72 atoms Reactants Products Molecules: 3 (the coefficient tells us) In a chemical reaction two elements or compounds come together and yield something new. All of the reactants must be accounted for (balanced) and should show up in the products. 5 A chemical equation is balanced when the reactants and products have the same number of each atom on each side in a chemical equation No new atoms can appear on the product side and no atoms can be lost. The Law of Conservation of Mass means we can rearrange the atoms like stacks of coins, but the total must be the same. The ways we know a chemical reaction has taken place when we see: o o o o o Balanced Change in temperature (heat is produced, or heat is absorbed) Light is produced (welders’ torch) Production of bubbles which means gas (remember the vinegar and baking soda made CO2) Formation of a precipitant (solids formed in a liquid) Change in color (remember the cabbage) Fe + Cl2 Fe = 1 Cl = 2 2 Fe + 3 Cl2 Fe = 2 Cl = 6 FeCl3 Fe = 1 Cl=3 2 FeCl3 Fe = 2 Cl=6 6