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Chemical Bonds

Chemical Bonds. Chemical bond is the attractive force that holds atoms or ions together. An atom with an unfilled outer electron shell is likely to bond with another atom. Noble gases have filled outer shells. They are unlikely to form bonds readily. Stability and Bonding.

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Chemical Bonds

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  1. Chemical Bonds • Chemical bond is the attractive force that holds atoms or ions together. • An atom with an unfilled outer electron shell is likely to bond with another atom. • Noble gases have filled outer shells. They are unlikely to form bonds readily. Mullis

  2. Stability and Bonding • Matter in lowest energy state is more stable than higher energy state. • More stable = less likely to change. • Filled outer shell = more stable. • How can an atom fill its unfilled outer shell? • With electrons from another atom Mullis

  3. Three Kinds of Bonds • Ionic • Electrons transferred from atom to atom • Example: NaCl • Type of bonds in ionic compounds • Covalent • Electrons are shared. • Usually 2 atoms share a pair of electrons. • Example: C6H12O6 • Type of bonds in molecular compounds. • Metallic • Electrons are shared between many atoms. • Many atoms share many electrons. • Example: Pure Ag Mullis

  4. Metallic Bonds • The valence electrons make up a “sea” of electrons. • Valence electrons do not belong to individual atoms, so charge is positive. (It’s like living in a commune.) • Metals have high density because lattice is tightly packed atoms. • Metals conduct electricity because electrons move freely. Mullis

  5. Covalent Bonds • shared pair of electrons • Between 2 or more nonmetals • Nonmetals have outer shells that are at least ½ full. • Molecules are formed with covalent bonds ( …molecular compounds). Mullis

  6. Ionic Bonding • Like loaning your friend your extra baseball glove if you want to play ball: • The friend is using your glove and you are not, but • Both of you benefit. • Ionic bonds: • One atom uses the electron from another atom. • Both benefit because both are more stable. Mullis

  7. More on Ionic bonds • The atom that gives up the electron = positive ion. • The atom that accepts the electron = negative ion. • The ions are attracted to each other because they have opposite charges. • AN IONIC BOND IS AN ELECTROSTATIC ATTRACTION BETWEEN OPPOSITELY CHARGED IONS. Mullis

  8. Na Ions • Alkali metals form ions with + 1 charge since they tend to lose an electron. • Halogens tend to form ions with –1 charge since they tend to gain an electron. • Positive ions are smaller than atoms of the same element. • Nucleus holds on to the remaining electrons (existing happily in their filled outer shell). • Negative ions are larger than atoms of the same element. • More electrons means more repulsion . • Cl- has radius of almost 2x the radius of Cl atom. - 1 electron Na+ + 1 electron Cl Cl- Mullis

  9. Example of Ionic Bonding Chlorine and sodium Sodium atom Chlorine atom 11 protons = 11+ 17 protons = 17+ 11 electrons = 11 - 17 electrons = 17- Charge 0 Charge 0 Sodium ion Chloride ion 11 protons = 11+ 17 protons = 17+ 11 electrons = 10 - 17 electrons = 18- Charge 1 + Charge 1- Together, Na and Cl are attracted to each other and they are electrically neutral. Mullis

  10. Ionic Bonding • 2Na + Cl2  2NaCl ∆H°f = -410.0 kJ/mol • This is violently exothermic! • Therefore, NaCl is more stable than Na and Cl2. • Each element has adjusted its number of electrons to achieve the same electron configuration as the nearest noble gas. • Na = 11 e- Na+ = 10 e- (same as Ne) • Cl = 17 e- Cl- = 18 e- (same as Ar) Mullis

  11. Ions, Continued • When an ionic compound dissolves in water, each ion is surrounded by water molecules. • Living things take up the ions dissolved in water to use as nutrients. • Water softeners replace Ca and Mg ions in hard water with Na ions. Mullis

  12. The Crystal Lattice • 3-dimensional pattern that repeats itself over and over again. • Each ion is bonded with all oppositely charged ions that directly surround it. • NaCl forms a cube shape, called a body-centered-cubic structure. • There are 7 crystal shapes, determined by how the ions are arranged in the lattice. Mullis

  13. Crystal Growth • Crystals grow by adding ions to all sides. • They grow equally in all directions from the outside. • Crystals form in 2 ways: • Solution containing a dissolved ionic compound evaporates. • An ionic solid is heated until it melts, then liquid is cooled. (Igneous rocks) Mullis

  14. Energetics of Ionic Bond Formation • Recall that heat of formation of NaCl was exothermic (∆H°f = -410.9kJ/mol) • Separation of NaCl is endothermic: • (∆H = +788 kJ/mol) • The energy required to separate 1 mol of ions in an ionic lattice into gaseous ions is called lattice energy, ∆Hlattice . • Lattice energy depends on the charge on the ions and the size of the ions. Mullis

  15. Lattice Energy (Ionic bonds) • Lattice energy depends on the charge on the ions and the size of the ions. • The stability of the compound comes from the attraction between ions of unlike charge. • The specific relationship is given by Coulomb’s equation: • E = kQ1Q2 Q is the charge on the particles, d is the distancedbetween their centers and k is aconstant. • As Q1 and Q2 increase, E increases and as d increases, E decreases. Mullis

  16. Transition Metal Ions • Lattice energies compensate for the loss of up to 3 electrons. • Most transition metals cannot achieve a noble gas configuration since most have more than 3 electrons past a noble gas core. • Transition metals tend to lose the valence shell electrons 1st, and then as many d electrons as required to reach the desired charge on the ion. • Electrons are removed from 4s before 3d, etc. Mullis

  17. Ions • Alkali metals form ions with + 1 charge since they tend to lose an electron. • Halogens tend to form ions with –1 charge since they tend to gain an electron. • Positive ions are smaller than atoms of the same element. • Nucleus holds on to the remaining electrons (existing happily in their filled outer shell). • Negative ions are larger than atoms of the same element. • More electrons means more repulsion . • Cl- has radius of almost 2x the radius of Cl atom. Mullis

  18. Predicting Ionic Size • Ionic size is important: • In predicting lattice energy • In determining the way in which ions pack in a solid. • Just as atomic size is periodic, ionic size is also periodic. • For ions of the same charge, size increases down a group. • All members of an isoelectronic series have the same number of electrons. • As nuclear charge increases in the isoelectronic series, the ions become smaller: • O2- > F- > Na+ > Mg 2+ > Al 3+ Mullis

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