1 Ionization of Transition Metals  K  Ca  Sr  Ti  V  Cr  Mn  Fe

2 Oxidation Numbers: Examples  H 2 O  CH 4  NH 4 Cl  NaH  CaH 2  KCl  RbNO 3  SrSO 4  CaBr 2  CO  CO 2  Mg 3 N 2  P 4 O 10  (NH 4 ) 2 S  BeF 2  SO 2

3 CHAPTER 7 Chemical Bonding

4 Chemical Bonds Attractive forces that hold atoms together in compounds are called chemical bonds. There are two main types of chemical bonds Ionic bonds – resulting from electrostatic attraction between cations and anions Covalent bonds – resulting from sharing of one or more electron pairs between two atoms

5 Lewis Dot Formulas Valence electrons Electrons which are involved in chemical bonding These are usually the outermost electrons These electrons are most important chemically Schematic representations of valence electrons in atoms

6 Lewis Dot Formulas: Single Atoms We show only electrons in the outermost occupied shell An electron pair is represented as a pair of dots An unpaired electron is represented as a single dot Li Na F Cl

7 Formation of Ionic Compounds Consider reaction between metallic sodium and gaseous chlorine Electron configurations of the elements Na Cl Sodium atom has low ionization energy and easily looses the only 3s electron forming a cation of Na + Chlorine atom has highly negative electron affinity and readily gains an electron becoming an anion of Cl –

8 Formation of Ionic Compounds Na – e –  Na + Cl + e –  Cl – Na + Cl  Na + + Cl – Na + cations and Cl – anions are electrostatically attracted to each other resulting in an extended ionic lattice The high energy of the lattice overcomes all other factors involved in the formation of NaCl from elemental sodium and chlorine

9 Formation of Ionic Compounds Na + Cl  Na + Cl – We can write this equation using Lewis dot formulas The complete equation is 2 Na + Cl 2  2 Na + Cl – It can also be written as

10 Alkali Metals + Halogens Reaction with halogens leading to the formation of ionic halides M + X – is a general chemical property of alkali metals It is also a general chemical property of halogens 2M(s) + X 2  2MX(s)

11 Ionic Bonding Electrostatic interaction Non-directional The central ion attempts to maximize the number of interactions with the ions of opposite charge Formation of an ionic compound involves loss of electrons by metal (oxidation) and gain of electrons by nonmetal (reduction)

12 Alkali Earth Metals + Halogens The remainder of the IIA metals and VIIA nonmetals react similarly: M(s) + X 2  M 2+ (X – ) 2 (s) Ca + F 2 

13 Formation of Ionic Compounds Li + O 2  The remainder of the IA metals and VIA nonmetals react similarly: 4M(s) + O 2 (g)  2(M + 2 O 2– ) (s) 2M(s) + X(s)  M + 2 X 2– (s) X = S, Se, Te, Po

14 Formation of Ionic Compounds Mg + O 2  The remainder of the IIA metals and VIA nonmetals react similarly: 2M(s) + O 2 (g)  2(M 2+ O 2– ) (s) M(s) + X(s)  M 2+ X 2– (s) X = S, Se, Te, Po

15 Example Write the reaction between calcium and nitrogen. Show what happens to valence electrons using Lewis dot formulas. Learn Table 7-2

16 Covalent Bonding If the difference in electronegativity of two elements is not large enough, an electron cannot be transferred completely from one atom to the other It becomes shared between both atoms and a covalent bond is formed

17 Formation of H 2 Molecule When two H atoms are indefinitely far from each other, they do not interact If the separation decreases to a certain distance, the 1s electron of each H atom is attracted by the nucleus of the other H atom, as well as by its own nucleus If electrons from different atoms can occupy the same orbital, they will form a covalent bond

18 H 2 Molecule We can use Lewis dot formulas to show covalent bond formation The covalently bonded atoms are held at a distance corresponding to the lowest total energy

19 Covalent Bond We say that the covalent bond is formed by the overlap of atomic orbitals The covalently bonded atoms are held together by a pair of shared electons The distance between their nuclei corresponds to the lowest total energy Below this equilibrium distance the nucleus-nucleus and electron-electron repulsions become too large, pushing the nuclei back to the equilibrium distance

20 HF Molecule F is more electronegative than H In this molecule the electron pair will be shifted towards the F atom

21 F 2 Molecule

22 H 2 O Molecule

23 NH 3 Molecule

24 NH 4 + Ion Lewis formulas can also be drawn for polyatomic ions

25 Bonding & Nonbonding Electrons Representative elements usually attain stable noble gas electron configurations in most of their compounds Electrons which are shared among two atoms are called bonding electrons Unshared electrons are called lone pairs or nonbonding electrons Lewis dot formulas are based on the octet rule

26 The Octet Rule S = N - A S = total number of electrons shared in bonds N = total number of electrons needed to achieve a noble gas configuration 8 for representative elements 2 for H atoms A = total number of electrons available in valence shells of the atoms A is equal to the periodic group number for each element A-S = number of electrons in lone pairs

27 Examples F 2 H 2 O CH 4 CO 2

28 Covalent Bonding Covalent bonds are formed when atoms share electrons If the atoms share 2 electrons a single covalent bond is formed If the atoms share 4 electrons a double covalent bond is formed If the atoms share 6 electrons a triple covalent bond is formed

29 For ions we must adjust the number of electrons available, A: Add one e - to A for each negative charge Subtract one e - from A for each positive charge The Octet Rule Example: NH 4 +

30 Assignments & Reminders Go through the lecture notes Read Sections 7-1 through 7-5 Read Sections 4-5 & 4-6 of Chapter 4 Homework #3 due by Oct. 10 Monday (10/10) and Tuesday (10/11) – lecture quiz #3 based on Chapters 5&6