Introduction to Bonding

Key Terms:Chemical BondIonic BondCovalent BondElectronegativityMetallic BondIntroduction to BondingWhy Form Bonds?Single atoms are not how most of the elements in nature exist Atoms seek the lowest energy state possibleIn the dissociated stated, aka individual atom state, most atoms are at a higher energyWhen the atoms combine it lowers the total energy of the system, which means it is more favorable

Chemical BondsA chemical bond forms when 2 or more atoms rearrange valence electrons to increase stabilityRemember the term valence electron, what was considered a magic number of valence electrons?Most atoms prefer 8 valence electrons to fill their s- and p-orbitalsTwo ways for atoms to share electrons:Completely transfer Share the electron(s)

Octet RuleA chemical rule of thumb in which atoms that combine seek to have 8 electrons as their valence electronsAn atom that is surrounded by 8 electrons that only contain s- and p-orbitals as valence orbitals will be stable

Ionic BondingIonic bonding forms when valence electrons are transferred from one atom to anotherWhen an ionic bond is present the compound is considered an ionic compoundOne of the most common examples of an ionic compound is that of common table salt, NaCl

Ionic Bonding

Anions and CationsIonic compounds are comprised of:Cations- An atom or molecule that loses an electron to become positively chargedAnions-An atom or molecule that gains an electron to become negatively charged

Properties of Ionic CompoundsIonic Compounds are arranged in crystal lattice structures to put the atoms in the lowest energy state possibleStrong forces hold the ions togetherHigh melting and boiling pointsHard- not easily crushedConduct electricity when melted or dissolved because ions are free to move

Pure Water vs Salt WaterIf you are sitting in a pool of pure water and someone drops a plugged in toaster in with you. What will happen to you?NothingNow, if someone adds salt to the pool before you get in what would happen once the plugged in toaster is added? Zap!

Covalent BondA covalent bond occurs when electrons are shared when forming moleculesFormation of molecules causes increased stabilityCovalent compound have weaker forces holding them together compared to that of ionic bonds

Properties of Covalent BondsCovalent compounds have:Lower melting and boiling pointsMany covalent compounds are volatile liquids or gasesSoft- easier to crushAre not conductors of electricityEx. Methane is a covalently bound molecule that is a gas or a volatile liquid

Electronegativity Electronegativity is a property that tells how strong an atoms attraction is for electronsElectronegativity is a property of an atom that can be found using a chart or tableWhat do you think happens when one atom has a stronger pull on an electron than another?Think of a large dog and a smaller dog playing tug of warUnequal sharing

Polar Covalent BondsPolar covalent bonds occur when electrons are shared unequally giving these molecules charged ends or polesSince oxygen has a higher electronegativity than hydrogen, oxygen holds onto shared electrons more, giving the oxygen a partial negative charge and the hydrogen a partial positive chargeThis type of unequal sharing is denoted by a + or - to say slightly more positive or negative at that atom

Nonpolar Covalent BondsNonpolar covalent bonds occur when electrons are shared equally because the atoms have the same or close electronegativitiesNonpolar compounds tend to have weaker interactions between molecules while polar compounds have stronger interaction between moleculesThis idea of molecules interacting with each other will be covered in a later lesson

Determining Bond TypeElectronegativity DifferenceType of BondGreater than or equal to 1.7Ionic bondBetween 1.7 and 0.3 Polar CovalentLess than or equal to 0.3Nonpolar covalent Metallic BondMetallic Bonds form in metals because the electrons are delocalized, giving metals a sea of electronsDue to this sea metals have the following propertiesLusterConductorsMalleableDuctile

Practice ProblemsDetermine the type of bonds for the following compounds:Mg and F4.0 1.2 = 2.8 ionicS and O3.5 2.5 = 1.0 polar covalentNa and Cl3.0 0.9 = 2.1 ionicN and O3.5 3.0 = 0.5(slightly) polar covalentB and F4.0 2.0 = 2.0ionic