2 Review: bond types, bond energy 30/09/99We have been talking about atomic structure, now we are going to focus on moleculesThere are 2 types of bonding: ionic, covalentFrom 3A: ionic = stealing of electrons to form +ve and –ve ions. +ve and –ve then attractCovalent = sharing of electronsWe will see that there is no clear dividing line.What causes atoms to form molecules?Basically, all things that happen spontaneously are energetically favorable (a book dropping)You never see a book spontaneously riseSomething must be energetically favorable about atoms coming together as moleculesWe will look at ionic first and then covalent
3 Ionic bonding Ionic bonding involves 3 steps (3 energies) 30/09/99Ionic bonding involves 3 steps (3 energies)1) loss of an electron(s) by one element,2) gain of electron(s) by a second element,3) attraction between positive and negativeIonization energy+Nae–Na+Electron affinityCle–+Cl–+Cl–Na+Lattice energy
4 Ionic bonding: energies 30/09/99Let’s keep track of numerical energy valuesBy convention, a requirement for energy is given a + sign (we have to put energy in) and is called endothermic, a release of energy is given a – sign and is called exothermic.
5 Ionic bonding Ionic bonding involves 3 steps (3 energies) 30/09/99Ionic bonding involves 3 steps (3 energies)1) loss of an electron(s) by one element,2) gain of electron(s) by a second element,3) attraction between positive and negativeIonization energy+Nae–Na++ 496Electron affinityCle–+Cl–– 349+Cl–Na+Lattice energy– 766
6 Ionic bonding: energies 30/09/99Let’s keep track of numerical energy valuesBy convention, a requirement for energy is given a + sign (we have to put energy in) and is called endothermic, a release of energy is given a – sign and is called exothermic.Problem: the sum is A spontaneous change must involve a net lowering of energySolution: the lattice energy provides the energy neededNote that although we represent this as a three step process it actual occurs all at once
7 7.1 questions (pg. 225 – 8)30/09/99A metal + non-metal gives what kind of bond?What is necessary for any stable compound to form from its elements?Define cation. Define anion.List the 3 energies involved in forming an ionic bondWhat term describes a release of energy?Is breaking a bond endothermic or exothermic?Define lattice energy.Explain why metals form cations and non-metals form anions.Explain why calcium exists as Ca2+ but not as Ca3+ in ionic compounds.Explain why most transition metals form a 2+ ion.
8 7.1 Answers Ionic There must be a net lowering of energy 30/09/99IonicThere must be a net lowering of energyCation: a positively charged ion, Anion: a negatively charged ionIonization energy, electron affinity, lattice energyExothermicEndothermicThe energy released by the imaginary process in which isolated ions come together to form a crystal of an ionic compound
9 Metals form cations because they have small IEs (and EA), non-metals form anions because they have large EAs (and IE). These trends are energetically favorable.The first two electrons from the 4s subshell are easily lost (they can be made up for by the lattice energy). Losing a third electron is not energetically favorable because of the large third IE for Ca (the energy required to remove the third electron can not be made up by the lattice energy.The loss of two electrons from the s subshell accounts for the typical 2+ charge of the transition elements.30/09/99
10 Covalent bonding30/09/99Just as with ionic bonds, covalent bonds must involve a net lowering of energyWe can explain this net lowering of energy in two ways:1) visualizing the combination of attraction as two atoms approach each other2) drawing and combining orbital diagrams
11 As atoms approach30/09/99+–Recall that EA for all atoms, except the noble gasses is negativeIn other words we have no trouble adding electrons to atoms–+–The attraction for electrons is not limited to free electrons, but also involves electrons that are part of other atoms.Thus, atoms are pulled toward each otherHow far they are pulled together will depend on a balance of attraction (nucleus to electrons) and repulsion (nucleus to nucleus and electrons to electrons)
12 For more lessons, visit www.chalkbored.com Reducing energy30/09/99The energy associated with moving atoms closer and farther is shown in 7.3 (pg. 231)Energy of separated atomsIt requires lots of energy to push nuclei close together (top). The atoms bond where energy is most favorable (lowest) similar to position marble would find on a trackFor more lessons, visit