2 Introduction to Bonding Chemical bond: an interaction between atoms or ions that results in a reduction of the potential energy of the system, thereby becoming more stableThree types of bonds: ionic, metallic, and covalentThe bond type depends on the atom's electronegativitites
3 MoreIf the atoms have very different electronegativities, then ionic bonding occursIf they both have high electronegativities, then covalent bonding occursIf they both have low electronegativities, then metallic bonding occurs
4 Practice: What Kind of Bond? Na and ClSr and OC and ONi and FeN and OLi and NTi and CrIonic Covalent Metallic metallic
5 Valence ElectronsValence electrons are the electrons in the outermost energy level, which is the highest occupied energy levelThey are the electrons responsible for the chemical properties of atomsCore electrons – are those in the energy levels below.
6 Keeping Track of Electrons Atoms in the same group have the same outer electronic structure and therefore the same number of valence electrons.The number of valence electrons is easily determined. It is the group number for a short group elementGroup 2: Be, Mg, Ca, etc.Each has 2 valence electrons
7 Electron Dot (Lewis Dot)diagrams A way of showing & keeping track of valence electrons.Write the symbol - it represents the nucleus and inner (core) electronsPut one dot for each valence electron (8 maximum)They don’t pair up until they have to (Hund’s rule)X
8 The Electron Dot Diagram (Lewis Structure) for Nitrogen Nitrogen has 5 valence electrons to show.First we write the symbol.NThen add 1 electron at a time to each side.Now they are forced to pair up as one side has two electrons
9 The Octet Rule The noble gases are unreactive in chemical reactions In 1916, Gilbert Lewis used this fact to explain why atoms form certain kinds of ions and moleculesThe Octet Rule: in forming compounds, atoms tend to achieve a noble gas structure; 8 in the outer level is stableEach noble gas (except He, which has 2) has 8 electrons in the outer level
10 Formation of CationsMetals lose electrons (are oxidized) to attain a noble gas structure.They make positive ions (cations)If we look at the electronic structure, it makes sense to lose electrons:Na: 1s22s22p63s1 shows 1 valence electronNa1+: 1s22s22p6 This is a noble gas structure with 8 electrons in the outer level.
11 Electron Dots For Cations Metals have few valence electrons (usually 3 or less); calcium has only 2 valence electronsCa
12 Electron Dots For Cations Metals will lose the valence electronsCa
13 Electron Dots For Cations Form positive ionsCa2+This is named the “calcium ion”.No dots are now shown for the cation.
14 Electron Configurations: Anions Nonmetals gain electrons to attain noble gas electronic structures.They make negative ions (anions)S = 1s22s22p63s23p4 = 6 valence electronsS2- = 1s22s22p63s23p6 = noble gas structure.Halide ions are ions from chlorine or other halogens that gain electrons
15 Electron Dots For Anions Nonmetals will have many valence electrons (usually 5 or more)They will gain electrons to fill outer shell.3-P(This is called the “phosphide ion”, and should show dots)
16 Stable Electron Configurations All atoms react to try and achieve a noble gas structure.Noble gases have 8 valence electrons and so are already stableThis is the octet rule (8 in the outer level is particularly stable).Ar
17 Ionic BondingAnions and cations are held together by opposite charges (+ and -)Simplest ratio of elements in an ionic compound is called the formula unit (also called the empirical formula).The bond is formed through the transfer of electrons (lose and gain)Electrons are transferred to achieve noble gas structure.
18 Ionic BondingNaClThe metal (sodium) tends to lose its one electron from the outer level.The nonmetal (chlorine) needs to gain one more to fill its outer level, and will accept the one electron that sodium is going to lose.
19 Ionic BondingNa+Cl -Note: Remember that no dots are now shown for the cation
20 Ionic Bond Negative charges are attracted to positive charges. Negative anions are attracted to positive cations.The result is an ionic bond.A three-dimensional crystal lattice of anions and cations is formed.
21 Ionic CompoundsThe ionic substance is held together by strong electrostatic attractions between all ions in all three dimensionsNo molecules presentAn ionic lattice is formedThis gives them distinct physical properties
22 Preserve Electroneutrality When ions combine, electroneutrality must be preserved.In the formation of magnesium chloride,2 Cl- ions must balance a Mg2+ ion:Mg Cl- → MgCl2
24 Please go to the “Naming ions, compounds and molecules presentation.
25 Properites of Ionic Compounds Hard, brittle crystalline solidsRelatively high melting and boiling pointsDo not conduct electricity when solid, but do when molten or in aqueous solutionAre more soluble in water than other solvents
26 - Page 198The ions are free to move when they are molten (or in aqueous solution), and thus they are able to conduct the electric current.
27 Predicting Ionic Charges Group 1A:Lose 1 electron to form 1+ ionsH+Li+Na+K+Rb+
28 Predicting Ionic Charges Group 2:Loses 2 electrons to form 2+ ionsBe2+Mg2+Ca2+Sr2+Ba2+
29 Predicting Ionic Charges Group 3:Loses 3electrons to form3+ ionsB3+Al3+Ga3+
30 Predicting Ionic Charges NitrideGroup 5:Gains 3electrons to form3- ionsP3-PhosphideAs3-Arsenide
31 Predicting Ionic Charges OxideGains 2electrons to form2- ionsGroup 6:S2-SulfideSe2-Selenide
32 Predicting Ionic Charges Gains 1 electron to form1- ionsGroup 7:F-FluorideBr-BromideCl-ChlorideI-Iodide
33 Predicting Ionic Charges Stable noble gases do not form ions!Group 0:
34 Predicting Ionic Charges Many transition elementshave more than one possible oxidation state.Note the use of Roman numerals to show chargesIron (II) = Fe2+Iron (III) = Fe3+