BONDING

1. BONDING

2. Introduction to Bonding • Atoms are generally found in nature in combination held together by chemical bonds. • A chemical bond is a mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together. • There are two types of chemical bonds: ionic, and covalent.

4. Introduction to Bonding • What determines the type of bond that forms? • The valence electrons of the two atoms involved are redistributed to the most stable arrangement. • The interaction and rearrangement of the valence electrons determines which type of bond that forms. • Before bonding the atoms are at their highest possible potential energy

5. Introduction to Bonding • There are 2 philosophies of atom to atom interaction • One understanding of the formation of a chemical bond deals with balancing the opposing forces ofrepulsion and attraction • Repulsion occurs between thenegative e-clouds of each atom • Attraction occurs between thepositive nucleiand the negative electron clouds

6. Introduction to Bonding • When two atoms approach each other closely enough for their electron clouds to begin to overlap • The electrons of one atom begin to repel the electrons of the other atom • And repulsion occurs between the nuclei of the two atoms

7. Introduction to Bonding • As the optimum distance is achieved that balances these forces, there is a release of potential energy • The atoms vibrate within the window of maximum attraction/minimum repulsion • The more energy released the stronger the connecting bond between the atoms

10. Introduction to Bonding • Another understanding of the form-ation of a chemical bond between two atoms centers on achieving the most stable arrangement of the atoms’ valence electrons • By rearranging the electrons so that each atom achieves a noble gas-like arrangement of its electrons creates a pair of stable atoms (only occurs when bonded)

11. Ionic Bonding Covalent Bonding Introduction to Bonding • Sometimes to establish this arrange-ment one or more valence electrons are transferred between two atoms • Basis for ionic bonding • Sometimes valence electrons are shared between two atoms • Basis for covalent bonding

12. Introduction to Bonding • A good predictor for which type of bonding will develop between a set of atoms is the difference in their electronegativities. • Remember, electronegativity is a measure of the attraction an atom has for e-s after developing a bond • The more extreme the difference between the two atoms, the less equal the exchange of electrons

14. Introduction to Bonding • Let’s consider the compound Cesium Fluoride, CsF. • The electronegativity value (EV) for Cs is .70; the EV for F is 4.00. • The difference between the two is 3.30, which falls within the scale of ionic character. • When the electronegativity difference between two atoms is greater than 1.9 the bond is mostly ionic.

16. Introduction to Bonding • The take home lesson on electro-negativity and bonding is this: • The closer together the atoms are on the P.T., the more evenly their e- interact, and are therefore more likely to form a covalent bond • The farther apart they are on the P.T., the less evenly their e- interact, and are therefore more likely to form an ionic bond. metal w/nonmetal = ionic nonmetal w/nonmetal = covalent

17. Introduction to Covalent Bonding • In a co-valent bond: • The electronegativity difference between the atoms involved is not extreme • So the interaction between the involved electrons is more like a sharing relationship • It may not be an equal sharing relationship, but at least the electrons are being “shared”.

18. Cl Cl Cl Cl Shared Electrons Covalent Bonds Lets look at the molecule Cl2 +

19. Shared electrons are counted with both atoms Cl 2 each atom must have 8 valence e's Cl Cl Notice 8 e- in each valence shell!!!

20. H Cl Cl H 2.1 3.0 Covalent Bonds How about the molecule HCl? + (Polar Covalent) shared, but not evenly

21. So what’s the bottom line? To be stable the two atoms involved in the covalent bond share their electrons in order to achieve the arrangement of a noble gas.

23. Introduction to Ionic Bonding • In an ion - ic bond: • The electronegativity difference is extreme, • So the atom with the stronger pull doesn’t really share the electron • Instead the electron is essentially transferred from the atom with the least attraction to the atom with the most attraction

24. + - - - - - - - - - - - - - - - - - - - - - - - - - - - - An electron is transferred from the sodium atom to the chlorine atom + Na Cl

25. + - - - - - - - - - - - - - - - - - + - - - - - - - - - - - Both atoms are happy, they both achieve the electron arrangement of a noble gas. Notice 8 e- in each valence shell!!! -1 +1 Na Cl

26. Very Strong Electrostatic attraction established… IONIC BONDS

28. So what’s the bottom line? To be stable the two atoms involved in the ionic bond will either lose or gain their valence electrons in order to achieve a stable arrangement of electrons.

30. Introduction to Metallic Bonding • In a metallic bond: • The resulting bond is a cross between covalent and ionic bonding • Valence electrons are transferred from one metal atom to the surrounding metal atoms • But none of the involved metal atoms want the electrons from the original atom, nor their own so they pass them on

31. Introduction to Metallic Bonding • What results is a sharing/transfer of valence electrons that none of the atoms in the collection own the valence electrons • It resembles a collection of positive ions floating around in a sea of electrons • It’s this interaction that leads to the properties unique to metals • Conductive • Malleable and Ductile

32. Sea of Electrons

35. Bond Energies and Bonding • As we’ve learned so far ionic com-pounds are formed by the transfer of electrons from ametal to a nonmetal • The ionic compound is held together by the strong electrostatic attraction between oppositely charged ions. • There is a tremendous amount ofenergystored in thebondsformed in an ionic compound.

36. Bond Energies and Bonding • It takes a lot of energy (A.K.A. bond energy) to pull the two ions apart once they have established their stable arrangement through bonding • Energy can bereleasedorabsorbedwhen ions form • Removing electrons from atoms requires aninputof energy • Remember from last chapter this energy is calledionization energy

37. Energy and Ionic Bonding • On the other hand adding electrons to atomsreleasesenergyinto the environment • Remember this has to do with the atomsaffinityfor electrons • Sometimes this energy is used to help remove the electron from another atom • The ionization energy to remove 1 e- from each atom in a mole of Na atoms is495.8kJ

38. Energy and Ionic Bonding • A mol of Cl atoms releases 348.6 kJ when an e- is added to the atom • Notice that it takes more energy to remove Na’s e- than the amount released from the Cl atoms. • Forming an ionic bond is a multi-step process • The final step releases a substantial amount of energy (a.k.a. the driving force)

39. Cl Na 2 +110.8 kJ/mol Na Crystal Formation At the beginning there is solid sodium and chlorine gas. Na(s) & Cl2(g) A mol of sodium is converted from a solid to a gas: Na(s) + energy  Na(g) Step #1 ENERGY IN

40. + Na Na +495.8 kJ/mol e +100.8 kJ/mol Cl 2 One electron is then removed from each sodium atom of form a sodium cation Na(g) + energy  Na+(g) + e- Step #2 ENERGY IN Energy is required to break the bond holding 0.5mol of Cl2 molecules together to form a mole of chlorine atoms Cl2(g) + energy  2Cl(g) Step #3 ENERGY IN

41. Cl - Cl e The next step involves adding an electron to each chlorine atom to form a chloride anion: Cl(g) + e- Cl-(g) + energy Step #4 +348.6 kJ/mol ENERGY OUT The final step provides the driving force for the reaction. Na+(g) + Cl-(g)  NaCl(s) + energy Step #5 +787.5 kJ/mol ENERGY OUT

42. Crystal Formation • Energy released in the final step is called the lattice energy • Energy released when the crystal lattice of an ionic solid is formed • For NaCl, the lattice energy is 787.5 kJ/mol, which is greater than the input of energy in the previous steps • The lattice energy provides enough energy to allow for the formation of the sodium ion

43. Crystal Formation • We can use the lattice energy as a method for measuring the strength of thebond in ionic compounds. • The amount of energy necessary to break a bond is called bond energy. • This energy is equal to the lattice energy, but • Bond energy movesinto the system • Lattice energy movesout of the system

44. Bond energy Lattice energy Compound kJ/mol (out) kJ/mol (in) LiCl -861.3 +861.3 LiBr -817.9 +817.9 LiI -759.0 +759.0 NaCl -787.5 +787.5 NaBr -751.4 +751.4 NaI -700.1 +700.1 CaF2 -2634.7 +2634.7 MgO -3760.2 +3760.2

45. Hydrate Formation • In the construction of a crystal lattice, depending on the ions involved there can be small “pores” develop between ions in the ionic crystal. • Some ionic compnds have enough space between the ions that water molecules can get trapped in between the ions • Ionic compounds that absorb water into their pores form a special type of ionic compound called a hydrate.

46. Trapped Water Molecules Hydrated Crystal

47. Hydrate Formation • Hydrates typically have different properties than their dry versions - A.K.A.anhydrides • Anhydrous CuSO4 is nearlycolorless • CuSO4•5 H2O is a brightblue color • WhenCopper (II) Sulfateis fully hydrated there are5 water moleculespresent for every Copper ion. • The hydrated name would be Copper (II) Sulfate Pentahydrate

48. Hydrate Formation • Have you ever bought a new purse or camera and found a small packet of crystals labeled – do not eat? • These crystals are there to absorb water that might lead to mildew or mold • The formula of a hydrate is XAYB • Z H2O(Z is a coefficient indicating how many waters are present per formula unit)

49. Percent Composition • The FDA requires manufacturers to provide nutritional information on the labels of processed food products. • Dietary guidelines are based on the percent of calories that the average person should consume from fats, carbohydrates, proteins, etc.

50. Percent Composition • percent composition in a compnd can be determined in 2 ways • The 1st is by calculating the percent composition by mass from a chemical formula. • The 2nd is a lab scenario where an unknown compound is chemically broken up into its individual components and percent compo-sition is determined by analyzing the results.