Why do atoms bond?

1. Why do atoms bond?

2. Electronegativity Electronegativity

3. 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 outer electrons of different atoms that binds the atoms together so that they behave as one unit. • There are two types of chemical bonds: ionic, and covalent.

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

7. Introduction to Bonding • There are 2 philosophies of atom to atom interaction • One deals with balancing the opposing forces ofrepulsion and attraction • As the atoms approach repulsion occurs between thenegative e-clouds of each atom • And attraction occurs between thepositive nucleiand the negative electron clouds

8. 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 chemical bond philosophy 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. Determining the Type • A good predictor for which type of bonding will develop between a set of atoms is the difference in their electronegativities. • The greater the difference between the electronegativities, the less equal the exchange of electrons between two atoms • This leaves us with three different levels of interaction: pure covalent (nonpolar covalent), polar covalent, and ionic

14. Determining the Type • Let’s consider the compound Cesium Fluoride, CsF. • The electronegativity value (EV) for Cs is .70; the EV for F is 4.00. • This is a difference of 3.30, which falls within a scale of ionic character. • The scale is a continuous spectrum of electronegativity differences • Since it is continuous it is hard to deter-mine where one type of bond ends and another starts.

16. Determining the Type • So how can we determine the bond type? • We will define it as…when the electro-negativity difference between two atoms is greater than 2.0 the bond is ionic. • When the difference is less than 0.3the bond is considered pure (nonpolar) covalent. • When the difference is between 0.3 and 1.6the bond is considered polarcovalent.

17. Determining the Type • So what about the range between 1.6 and 2.0? • If one of the atoms is a metal we will define it as ionic. • If both atoms are nonmetals we will define it as polar covalent. • These ranges aren’t conclusive, they are our attempts to explain observed patterns in nature

18. Determining the Type • 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 so are more likely to form a pure 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. • In between the extremes exists varying degrees of polar covalent interactions

19. Rule of Thumb metal w/nonmetal = usually ionic nonmetal w/nonmetal = usually covalent

20. Your Turn • Rank each of the following in order of increasing electronegativity Na, O, Os, Zn, Sr, Cl, N, Br, Xe, Al, B • Decide the type of bonding each of the following would exhibit (nonpolar covalent, polar covalent, or ionic) Al-Br, S-Cl, Ge-F, Si-F, Ti-Cl, Ga-Br, Si-O • Which bond in each of the following will be the most polar? • C-H, Si-H, Sn-H b) C-O, Si-O, N-O

21. 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 relation-ship, but at least the electrons are being “shared”. • There can be as many as 6 electrons shared between two different atoms

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

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

24. O O O N N O Covalent Bonds There can be as many as 6 e-s being shared between 2 atoms + N N + The more e-s shared the stronger the covalent bond.

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

27. Properties of Covalent Bonding • The properties of covalent bonded molecules are extremely varied. • Some are low melting points some are high melting points, some are hard, some are soft, some are gases, some are liquids • The properties are determined by how the strongly the molecules interact with each other • The interaction is dependent on molecu-lar size and intermolecular forces

28. Properties of Covalent Bonding • Intermolecular forces (IMF) hold molecules together • Covalent bonds hold a water molecule together • Intermolecular forces hold a group of water molecules together • There are different kinds of intermolecular forces • The type of IMF is dependent on how polar the bond is

29. Properties of Covalent Bonding The stronger the IMFs are the stronger the hold the more dramatic the properties.

30. Properties of Covalent Bonding Example Bond Energy Basis of Attraction

31. Introduction to Ionic Bonding • In an ion - ic bond: • The electronegativity difference between the two atoms 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

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

33. + - - - - - - - - - - - - - - - - - + - - - - - - - - - - - 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

34. Very Strong Electrostatic attraction established… IONIC BONDS

35. 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.

36. Properties of Ionic Bonding • Ionic compounds tend to be crystalline solids. • The crystals are all different colors and textures, but they all tend to be hard and brittle • They can be broken to leave a clean smooth surface • Ionic compounds tend to have high melting points • But when melted, they conduct electricity

37. Properties of Ionic Bonding • Many ionic solids are soluble in water where they are dissolved into separate ions. • Dissolved ions in water make water conductive • These are called electrolytes. • Most covalent compnds are not electrolytes (acids are an exception)

39. Discussion Question • Identify each of the following as an electrolyte (ionic) or a nonelectrolyte (covalent) MgCl2 CO2 C6H12O6 CaO

40. Hydrate vs. Anhydrous • 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.

41. Trapped Water Molecules Hydrated Crystal

42. Hydrate Formation • Hydrates typically have somewhat different properties than their “dry” versions - A.K.A.anhydrate or anhydrous • Anhydrous copper sulfate is nearlycolorless • The hydrated version is a brightblue color • WhenCopper (II) Sulfateis fully hydrated there are5 water moleculestrapped for every Copper sulfate.

43. Hydrate Formation • These hydrate-able ionic compounds are sometimes used to indicate the presence of water. • For example, Cobalt Chloride is a compound that is blue in its anhydrous version, and magenta when it is hydrated.