2 ElementsAn element is a pure substance of one type of atom that cannot be broken down into other substancesPeriodic Table of ElementsOver 100 elements known, but only about 2 dozen commonly found in living systems
3 Chemical CompoundsSubstance formed by chemical combination of 2 or more elements in definite proportionsChemical formulas used to write compoundsExample: H2O, NaClCompounds have different properties than the elements that form themExample: H = gas, O = gas; H2O = liquid
5 Atoms Smallest unit of matter Composed of subatomic particles: Protons – positively (+) chargedNeutrons – not chargedElectrons – negatively (-) charged
6 Neutrons and protons have about the same mass and pack together to form the atomic nucleus at the center of the atom (nucleus is thus + charged)Electrons are in constant motion; are attracted to the + charged nucleus but are outside of the nucleusAtoms have equal numbers of protons (+) and electrons (-) keeping the atom’s overall charge neutral
7 Atomic NumberAtomic number = # protons and is the top number on most Periodic Tables (also, since there are equal numbers of protons and electrons in an atom, the atomic number also tells the # electrons)6Atomic NumberCCarbon12.011
8 Atomic MassAtomic mass = total mass of an atom (protons + neutrons) and is the bottom number on most Periodic Tables66CCCarbonCarbon12.01112.011Atomic Mass
9 Number of NeutronsFrom the Mass number, we can figure out the # neutrons(atomic weight – atomic number = # neutrons)6CCarbon12.011
10 Atomic Number, Mass, and Mass Number Atomic number = 6 (6 protons, 6 electrons)Atomic mass =6 neutrons (atomic mass – atomic number)6CCarbon12.011Go to Section:
11 Energy Levels of Electrons Only electrons are involved in chemical reactionsMore distant an electron is from the nucleus, the greater the energy possible in the atom
12 Energy Levels of Electrons (Continued) Different states of energy are called energy levels or electron shells1st shell is closest to the nucleus, has the lowest energy, and holds only 2 electrons2nd shell has a little more energy and holds 8 electrons3rd shell has even more energy, holding 8 electrons, etc.
13 Valence ElectronsChemical behavior of an atom depends on the number of electrons in outermost shell = valence electronsElectron configuration – where electrons are within the atomExample: Oxygen – atomic number of 8, with 6 valence electrons
14 Isotopes Elements with different numbers of neutrons Example: isotopes of carbon can have 6, 7, or 8 neutronsNonradioactive carbon-12Nonradioactive carbon-13Radioactive carbon-146 electrons6 protons6 neutrons6 electrons6 protons7 neutrons6 electrons6 protons8 neutrons
15 Isotopes (continued)Isotopes identified by mass numbers – weighted averages of the masses of an element’s isotope = atomic mass“Weighted” means abundance of each isotope in nature is considered when average is calculatedCarbon-12 is most abundant, thus Carbon’s atomic mass is
16 Radioactive IsotopesSome isotopes have unstable nuclei that break down at a constant rate over time – this “break down” can give off radiationUse of radioactive isotopes:Radioactive dating of rocks and fossilsTreat cancerKill bacteriaMedical tracers
Your consent to our cookies if you continue to use this website.