2 ObjectivesModel two types of compound formation: ionic and covalent at the atomic level.Demonstrate how and why atoms achieve chemical stability by bonding.Compare, using examples the effect of covalent and ionic bonding on the physical properties of compounds.
3 How Elements Form Compounds In the 19th Century chemists tried to picture how atoms combine.Early models included atoms with “hooks” that allowed them to attached to one another.
4 When Atoms Collide Atoms collide when they react These collisions determine what kind of compounds are formed.When atoms collide it is the electron clouds that interact with one another.To be more precise it is the valence electrons that interact.Remember: valence electrons are responsible for the chemical properties of elements
5 Valence Electrons and Bonds To understand how valence electrons affect the bonds of compounds let look at elements with unusual properties….the noble gases
6 Noble Chemical Stability Noble Gases are located in Group 18They are inert or unreactiveThis lack of reactivity makes them useful as incandescent light bulbs (argon or krypton) and neon lights (orange-neon, blue-argon, yellow-helium)Occur naturally in the environment
7 Noble GasesNot a single compound of these elements has ever been found naturally in the environment.In 1960 chemists were able to react fluorine with krypton and xenon under high temperature and pressure.Since then a few additional compounds of xenon and krypton have been synthesized, but none with helium, neon or argon.
8 The Octet RuleLack of reactivity of the noble gases must mean that they are stable.Elements in the same group have similar valence arrangements.Noble gases have 8 valence electrons, except for helium that has 2.Because the electron arrangement determines chemical properties unreactive nature of noble gases
9 Octet Rule Atoms combine because they become more stable by doing so. Modern model of bonding is based on the fact that the stability of noble gases is a result of their valence electron arrangements.Octet Rule- atoms can become stable by having eight electrons in their outer energy level (or two electrons in the case of some of the smaller atoms).
10 Noble Gas Configuration Atoms obtain stability by achieving the same configuration of valence electron as found in noble gases; most stable configuration.
11 Valence Electron Review How many valence electrons do you find in Group 1, 2, 13, 15, 16 and17?
13 Ways to Achieve a Stable Outer Energy Level 4.2 ContinuedWays to Achieve a Stable Outer Energy Level
14 Forming CompoundsWhen atoms collide with enough force their outer electrons may rearrange to achieve a stable octet of valence electrons.When it occurs they achieve a noble gas configuration and the atoms form compounds
15 How do the electrons rearrange? Electrons are transferred from one atom to anotherElectrons are shared between atoms
16 Electrons Can Be Transferred Example Sodium (Na) and Chlorine (Cl)Na has one valence electronCl has seven valence electronsNa transfers its 1 to Cl 7 Na now has 8 electrons in its outer shell and Cl now has 8 in its outer shellNa now has 1 less electron has +1 chargeCl now has 1 more electron has a -1 charge
17 IonsAtom or group of atoms that has a charge b/c they have either lost or gained electronsIons form when valence electrons rearrange by transferCompounds composed on ions are ionic compounds
19 Ions Attract Each Other Opposites attractPositive sodium is attracted to the negative chloride ion.Positive ions are called cationsNames stays the sameNegative ions are called anionsAdd –ide to the endSulfur Sulfide ion
20 Ionic bond Strong attractive force between ions of opposite charge Hold ions together in an ionic compoundCrystal is a regular, repeating arrangement of atoms, ions or molecules
21 The Results of Ionic Attraction b/c of the strong attractive forces and the degree of organization solid at room tempRaising temp causes particles to move faster particles overcome attractive forces and the crystal organization breaks down
22 Representing Compounds with Formulas Sodium ChlorideFormula: NaClWrite cation first followed by the anionAlways in the smallest whole number ratio
23 Review What is a negatively charged ion called? What is a positively charged ion called?What is the noble gas configuration?
24 Electrons Can Be Shared What happens when hydrogen and oxygen collide?Oxygen has 6 valence electronsHydrogen has 1 valence electronCan hydrogen lose its one valence electrons?
25 Colliding atomsTransfer electrons only when one atom has a stronger attraction for valence electronsIn the case of sodium and chlorine, chlorine attracts sodium’s valence electrons strongly and sodium is holding its electrons weaklyIn the case of hydrogen and oxygen neither atom attracts strongly enough to take electrons from the other.
26 Hydrogen and Oxygen They will share their electrons When atoms collide with enough energy to react, but neither atom attracts electrons strongly enough to take electrons from the other, the atoms combine by sharing valence electrons.
28 Formation of Compounds Like the formation of sodium chloride all the components present before the reaction are still present after the reactionsThe valence electrons no longer reside in the same position.They have rearranged
29 Electrons Sharing Produces Molecules Covalent bond- the attraction of two atoms for a shared pair of electronsElectrons are sharedNeither atom has an ionic chargeCovalent compound- compound whose atoms are held together by a covalent bond, also called molecular compoundsWater is an exampleMolecules-uncharged group of two or more atoms held together by covalent bonds (water molecules)
30 More Than Two Electrons Can Be Shared Let’s look at Carbon DioxideCarbon has 4 valence electronsOxygen has 6 valence electrons
31 Covalent and Ionic bonds Two nonmetallic elements usually form molecular or covalent compounds by sharing electronsNonmetallic and a metallic element usually form ionic compounds
32 Review What is a covalent bond? What is the definition for a molecular compound?Give me an example of a molecular compound?
33 Review What is a covalent bond? What is the definition of a molecular compounds?What are two examples of a molecular compound?
35 BondingWhen elements combine they either form ions or molecules…there are no other possibilities.When particles change dramatically, when they change from sodium to sodium ions or from hydrogen and oxygen to water molecules.These changes explain why compound have different properties from the elements they are composed of.
36 Explaining the Properties of Ionic Compounds Physical properties are a result of the well-organized, tightly bound ions.Ions forms strong, three-dimensional crystals
37 Properties of Ionic Compounds Generally ionic compounds are crystalline solids at room temperature.Generally hard, rough and brittleHigh melting points and boiling pointsThis is because of the strong attraction between ionsUsually dissolve in water
38 Properties of Ionic Compounds Solutions will conduct electricityLiquid state (melted state) will conduct electricityThey are electrolytes- any compound that conducts electricity when melted or dissolved in waterIn order to conduct electricity the ions must be free to move. Must take on or give up electronsSolid state does not conduct electricity. Why?
39 Explaining the Properties of Covalent Compounds Composed of moleculesHeld together by covalent bonds-make the molecule a stable unitMolecules have no ionic chargeAttractive forces between molecules are usually weakInterparticle forces- forces between particles that make up a substance
40 Properties of Covalent Compounds Many are liquids or gases at room temperatureLow melting points and boiling pointsDo not conduct electricitySome do not dissolve in waterVegetable oil and gasolineIn general they are less soluble in water than ionic compounds
41 Ionic Vs. CovalentIt is the strength between interparticle forces in covalent compounds compared to the strong attractive forces of ions in ionic compounds is what explains many of the differences in their physical propertiesBecause covalent compounds do not have ions you would expect that they don’t conduct electricityIonic compounds are more soluble in water because ions are attracted by the water molecules, while molecular compounds are not
42 Ionic Vs. Covalent Compounds High melting pointHigh boiling pointSolid at room temperatureSoluble in waterExcellent conductor of electricityMany crystallineLow melting pointLow boiling pointLiquid or gas at room temperatureLess soluble in waterPoor conductor of electricity