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Chapter 8 Chemical Bonding Section 1 Electrons • A chemical bond is the joining of atoms to form new substances with new properties – Compounds will not have the same properties as the elements that went into them 1. Electrons are the key as chemical bonding involves the interaction of certain electrons (valence). 2. Atoms are most stable when their outer energy level is full. Which group has elements that already have a full outer energy levels? Valence Electrons • Valence electrons refer to the electrons on the outer-most electron level. • These are the only electrons involved in a chemical reaction. • Atoms will be most stable if they have a full outer energy level. • Some atoms will lose electrons,… Na 8 8 2 Sodium 11 protons Valence Electrons • Valence electrons refer to the electrons on the outer-most electron level. • These are the only electrons involved in a chemical reaction. • Atoms will be most stable if they have a full outer energy level. • Some atoms will lose electrons, other will gain electrons. S 8 8 2 Sulfur 16 protons 3. Only valence electrons are involved in chemical bonding. • Valence electrons are the outer-most electrons – There are only 8 electrons that are valence. Group 1 = 1 valence electron Group 2 = 2 valence electrons Group 18 Group 17 Group 16 Group 15 Group 14 Transition Metals Group 13 Group 2 Group 1 Groups 3-12 Group 13 = 3 valence electrons Group 14 = 4 valence electrons Group 15 = 5 valence electrons Group 16 = 6 valence electrons Group 17 = 7 valence electrons Group 18 = 8 valence electrons (full) There is no rule for the valence electrons of Transition Metals. Some have one, others two or three. Electron Dot Diagrams • Electron Dot Diagrams can be used to represent how many valence electrons there are for each atom • Follow the pattern 1 5 8 2 E 4 7 3 6 • For example… B N O • Special Exceptions… He C Types of Chemical Bonds: 1. Ionic Bonds • Ionic bond is the force of attraction between oppositely charged ions – Ions are atoms that have lost or gained electrons in a chemical reaction. – The positive ion and the negative ion form a neutral compound. • Examples of ionic bonds – Salt (NaCl) – Plaster (cast) – Seashells (calcium carbonate) • Ions – Through chemical reactions atoms may gain or lose electrons – Ions are charged particles • Positive ions have lost one or more electrons • Negative ions have gained one or more electrons Lithium ion (Li+) has lost one electron (should have three) 3 positive protons 2 negative electrons Oxygen ion (O-2) has gained two electrons (should have eight) 8 positive protons When an alkali metal reacts with a halogen it forms a salt 10 negative electron For example • Ions are identified by their oxidation numbers Li+ +2 Ca +3 Al -3 N -2 S Br Oxidation numbers represent the number of electrons lost or electrons gained, written above and to the right of the symbol Lost one electron Lost two electrons Lost three electrons Gained three electrons Gained two electrons Gained one electron Positive Ions Group Name Valence Electrons Gain/lose Oxidation number Group 1 Alkali Metal 1 Lose +1 Group 2 Alkaline Earth Metal 2 Lose +2 Group 13 Boron 3 Lose +3 Losing Electrons in chemical reactions is Endothermic. Making Positive Ions • Positive ion (metals) – Ions where electrons have been lost Sodium is an alkali metal, having one valence electron In a chemical reaction it will lose that electron. – Example Sodium + Na Na Sodium becomes a positive ion after a chemical reaction because it has more protons than electrons. Notice: P CHEMICAL 11+ REACTION Atoms require extra energy to remove electrons (losing 10 - electrons electrons is endothermic) 11 + protons +++++++++++ ----------- • Example aluminum Notice: +3 Al Al • Aluminum is a group 13 atom, having 3 valence electrons. 13 + protons 10 - electrons P CHEMICAL 13+ REACTION • During a chemical reaction aluminum will lose 3 electrons • Aluminum becomes a positive ion because it has more protons than electrons. Negative Ions Group Name Valence Electrons Gain/lose Oxidation number Group 15 Nitrogen 5 Gain -3 Group 16 Oxygen 6 Gain -2 Group 17 Halogen 7 Gain -1 Gaining Electrons in chemical reactions is Exothermic. Making Negative Ions • Negative ions (nonmetals) – Ions where electrons have been gained – Add an “ide” ending to show which atoms have gained electrons – Example chlorine Notice: 17 + protons Cl Cl 18 - electrons CHEMICAL P REACTION • Chlorine is a halogen (group 17) , having 7 valence electrons. • Chlorine, a nonmetal, will gain electrons in a chemical reaction. • It will become a negative ion because it has more electrons than protons after a chemical reaction. +++++++++++++++++ • Gaining electrons releases - - - - - energy -exothermic). - - (-gaining - - -electrons - - - is- - - 17+ Chlorine becomes chloride • Oxygen is a group 16 atom, having 6 valence electrons. – Example oxygen • Oxygen, a nonmetal, will gain 2 electrons in a chemical reaction. Notice: 8 + protons -2 OO 10 - electrons CHEMICAL P REACTION • It will become a negative ion because it has more electrons than protons after a chemical reaction. 8+ Oxygen becomes oxide Metals lose electrons in chemical reactions Metals become positive ions in chemical reactions +1 +2 +1 +2 +1 +2 +1 +2 +1 +2 +1 +2 +3 +3 +3 +3 +3 +3 Nonmetals gain electrons in chemical reactions Nonmetals become negative ions in chemical reactions +2 +2 +2 +2 +2 +2 +3 +3 +3 +3 +3 +3 -4 -3 -2 -1 -3 -2 -1 -2 -1 +4 -1 +4 -1 +4 • • Positive and negative ions will attract each other Ionic bonds have the following properties 1. High melting/boiling points (strong bonds) 2. Form crystal lattices 3. Brittle Na+ Cl- Covalent Bonds • A covalent bond involves the sharing of electrons to form molecules – Electrons are not exchanged (ions are not formed) – Nonmetals – Properties 1. Low melting and boiling points 2. brittle when solid • Examples – – – – Water (H2O) Carbon dioxide (CO2) Sugar (C6H12O6) Diatomic molecules • Elements that are found in pairs when not part of a compound, never alone • There are 8 of them (H2, N2, O2, Group 17) No, really….. •DIATOMIC I2 Br2 H2 At2 N2 O2 Cl2 F2 • • • • Hydrogen Nitrogen Oxygen Group 17 – – – – – Fluorine Chlorine Bromine Iodine Astatine Diatomic molecules are formed because certain elements react so easily they automatically combine with each other when they are in their gaseous state. H2 H + H H H HHH HHH Cl2 Cl + Cl Cl Cl Cl Cl Cl ClCl Other Covalent Molecules Water (H2O) H O O H OH H + - Metallic Bonds • Metallic bond is the force of attraction between positive metal ions and the loose valence electrons in a metal – Valence electrons can easily travel from one atom to the next because of overlapping electron rings Fe + Fe (Iron i (Iron ion) + Fe (Iron ion) • The properties of metals are explained by metallic bonds – Malleabililty/ductility because the atoms are not locked to one single atom – Conducting heat/electricity because electrons can move freely or can be forced to move in one direction Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au • Electrons can be made to travel in one direction WIRE Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au Au WIRE