Chem 314 Supplement material Beauchamp 1 The structure of atoms…provides a basis for the structure of molecules What is the relative volume of an electron cloud (Ve) compared to volume of a nucleus (Vn)? eval p,n ecore Ve Vn = 4π 3 re rn 3 = (1.33)(3.14)(100,000) 3 = 4 x 1015 = 4,000,000,000,000,000 1 Electron clouds determine the overall volume of atoms. The size of an atom is the size of its electron cloud. What is the relative mass of the electrons compared to the mass of nuclear particles? d=1 d = 100,000 mass protons (or neutrons) mass electrons 1840 1 Protons and neutrons determine the mass of an atom. The mass of an atom is mostly the mass of its incredibly small nucleus. p = protons = This number is constant for a particular type of atom and defines an element. If there are six protons, the element has to be carbon. n = neutrons = This number can vary in an element; it defines an isotope. Some isotopes are stable and some are radioactive. Carbon-12 has six protons and six neutrons and is stable. Carbon-13 has six protons and seven neutrons and is stable. Carbon-14 has six protons and eight neutrons and is unstable (and radioactive) with a half life of almost 6000 years. e = electrons = The number of electrons can vary. It can increase or decrease, depending on an atom's position in the periodic table. If electrons = protons? (same number of electrons and protons) associated term neutral atom If electrons < protons? (deficiency of electrons) posistively charged cation If electrons > protons? (excess of electrons) negatively charged anion Valence electrons (eval) are the outermost layer of electrons. They determine the bonding patterns of an atom and the usual goal is to attain a Noble gas configuration. This is accomplished by losing electrons (becoming cations) or gaining electrons (becoming anions) or sharing electrons in covalent bonds (in neutral molecules or in complex ions). Core electrons (ecore) are electrons in completely filled inner-shells. They are held too tightly for bonding (sharing with another atom) and they are not usually considered in the bonding picture. They are important because they cancel a portion of the nuclear charge (called shielding) so that the valence electrons only see an effective nuclear charge, Zeffective. The effective nuclear charge, Zeffective, is the net positive charge felt by the valence electrons (bonding and lone pair electrons). It can be estimated by subtracting the number of core electrons from the total nuclear charge. approximate positive charge Zeffective = (Ztotal) - (# core electrons) = felt by the valence electrons Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc Chem 314 Supplement material Beauchamp 2 s and p atomic orbitals (…most important to organic students) +z +z 1s atomic orbital has a spherical shape, no nodes. This is the orbital used by hydrogen in bonding. a single 2p atomic orbital, artificially separated from the other two 2p orbitals, dumbell shape at 90o angle to the other two p orbitals, there is a single node at the nucleus 2s atomic orbital spherical shape, one node all 2p atomic orbitals together, 2px, 2py, 2pz, when completely filled the 2p orbitals have spherical symmetry node = a region in space where the probability of finding electron density goes to zero. d orbitals look more complicated (…only occasionally invoked in organic chemistry) dyz z z dxz y x y x lobes in the yz plane z dxy dx2-y2 z dz2 y x y lobes in the xy plane y x x lobes in the xz plane z lobes in the xy plane along axes lobes along the z axis We rarely have occasion to discuss d orbitals, but we will use them briefly in discussions of sulfur, phosphorous and a few transition metals. Even in those discussions we will mainly look at them as bigger versions of p-like orbitals. Atomic Configuration p d s n=3 p s n=2 s +Z n=1 +Z = total nuclear charge = number protons Zeff = residual positive charge felt by the outer most valence electrons = (Ztotal - core electrons) 1. Aufbau Principle - Orbitals are filled with electrons in order of increasing energy, from the innermost orbitals (lower energy) towards the outermost orbitals (higher energy). 2. Pauli Exclusion Principle - Only two electrons may occupy any orbital and those electrons must have opposite spins (applicable to s, p, d and f orbitals). 3. Hund's Rule - Electrons entering a subshell containing more than one orbital will spread themselves out over all of the available orbitals with their spins in the same direction, until the subshell is over half filled. Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc Chem 314 Supplement material Beauchamp 3 1. Ionization potential as a measure of an atom’s electron attracting power. ionization potential (kcal/mole) electron is lost Atom Atom + electron Atom efinal energy greater PE (less stable) Potential Energy Ionization always has an energy cost to strip an electron from an atom. ∆ Energy = Atom starting energy lower PE (more stable) Energy to ionize an electron from neutral atoms = IP1 (units are kcal/mole). Compare rows and compare columns. Group 1A H Li Na K +314 +124 +118 +99 Zeff = +1 Group 2A Group 3A Be +215 Mg +177 Ca +141 B +192 Al +138 Ga +138 Zeff = +3 Zeff = +2 H H C H H H H C C H H Group 6A C +261 Si +189 Ge +182 N +335 P +242 As +226 Zeff = +5 O +315 S +239 Se +225 Zeff = +4 electron ionized +260 electron ionized +229 electron ionized +201 N C H H C H N H H electron ionized +335 electron ionized N F Cl Br Zeff = +6 +402 +300 +273 Zeff = +7 fluorine H N +261 electron ionized O +315 electron ionized H O +296 H H H H C Group 8A (He) +568 ( Ne) +499 (Ar) +363 (Kr) +323 Zeff = +8 Group 7A oxygen nitrogen carbon C Group5A Group 4A H C H C H H H O electron ionized +285 electron ionized O F +402 H O H H C O H How does the ionization potential of the halogen atoms vary down a column? The entire halogen family has a constant Zeffective of +7. F electron ionized +402 F + e- Cl electron ionized +298 Cl + e- Br electron ionized +272 Br + e- I electron ionized +241 I + e- Larger ionization potential Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc F Chem 314 Supplement material Beauchamp 4 Electronegativity defines the relative attraction an atom has for electrons in chemical bonds with other atoms. Mulliken Model of Electronegativity ≈ Average of ionization potential (IP) and electron affinity (EA). χ≈ attracting power of atoms for (IP + EA) = electrons in chemical bonds 2 Electronegativity will determine nonpolar, polar and ionic characteristics of bonds, and when shapes are included it determines the same attributes in molecules. Electronegativity = Table of electronegativities. Group 1A H Li Na K Group 2A 2.2 1.0 0.9 0.8 Be Mg Ca Zeff = +1 H 1.5 1.2 1.0 B Al Ga Zeff = +2 1.8 1.5 1.6 C 2.5 Si 1.9 Ge 1.9 Zeff = +3 H N 3.0 P 2. 2 As 2.0 Zeff = +4 O hydrogen gas, a single bond Group5A Group 4A Group 3A Group 6A O S Se Zeff = +5 3.5 2.6 2.4 Zeff = +6 O N Group 7A F 4.0 Cl 3.2 Br 3.0 I 2.7 Zeff = +7 Group 8A (He) (Ne) (Ar) (Kr) - Zeff = +8 N nitrogen gas, a triple bond oxygen gas, a double bond χa symbolizes the electronegativity of atom a. bond atom a χa ∆χ atom b χb = χa _ χb ∆χ = χa _ χb ≤ 0.4 ∆χ = χa _ χb < 1.4-2.0 classify bond as polar covalent ∆χ = χa _ χb ≥ 1.4-2.0 classify bond as nonpolar covalent classify bond as ionic The symbols + and - represent qualitative charge separation forming a bond dipole. Alternatively, an arrow can be drawn pointing towards the negative end of the dipole and a positive charge written at the positive end of the dipole. Two qualitative pictures of a bond dipole. B is assumed to be more electronegative than A. µ δδ+ or A B A B ........... d µ= amount of charge separated d x distance between charges in cm = units of Debye (D = 10-18 esu-cm) µ = (e)(d) = dipole moment e = electrostatic charge (sometimes written as q) The abosolute value of a unit charge on an electron or proton is 4.8x10-10 esu d = distance between the opposite charges This is often given in angstrums, but converted to cm for use in calculations (1A = 10-8 cm, A = angstrum) Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc Chem 314 Supplement material Beauchamp 5 Ionic bonding "explosion?" with transfer of electrons Na Cl Cl (curved arrows Na show electron movement) Cl2 (gas) mp = -101oC bp = -35oC Na (metal) mp = 98oC bp = 883oC Na Cl Cl Na + Our arbitrary rules classify this difference in electronegativity as ionic. + + + + + + + + + + + + + χa _ χb = 3.2 - 0.9 = 2.3 = + + + ∆χ NaCl (salt) mp = 801oC bp = 1413oC + + table salt lattic structure, both Na and Cl ions attaine a Noble gas configuration + Lattice structure - depends on the size and charge of the ions. + Each ion is surrounded on many sides by oppositely charged ions. To introduce the disorder of a liquid (melt) or a gas (boil) requires a very large input of energy (mp indicates the amount of energy required to breakdown the ordered lattice structure and boiling point indicates the amount of energy required to completely remove an ion pair from the influence of the lattice structure). Ionic bonds (ionic attractions on all sides) can only be broken at a great expense in energy. Examples Melting point (oC) Boiling point (oC) ∆χ NaCl 801 1465 2.3 1275 sub 2.5 Na2O NaOH 318 1390 2.5? >2200 1.4* AlN 674 sub 1.4* FeCl2 306 315 dec 1.2* FeCl3 MgCl2 714 1412 1.9* MgO 2800 3600 2.1 1184 2.1? Mg3(PO4)2 * = exception to our electronegativity rules about bond polarity sub = sublimation Notice the very high melting points and boiling points of ionic substances. Covalent Bonding (Molecules) H H H If two hydrogen atoms should find one another, they would from a diatomic molecule with a tremendous release of energy, about 104 kcal/mole. H The line symbolizes a two-electron, purecovalent bond based on the calculation below. ∆χ = χa _ χb = 2.2 - 2.2 = 0 Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc Chem 314 Supplement material Beauchamp 6 H H C H H N H O H H F H H polar covalent bond based on the calculation below pure covalent bond based on the calculation below polar covalent bond based on the calculation below polar covalent bond based on the calculation below _ ∆ χ = χa χb _ ∆ χ = χa χb _ ∆ χ = χa χb _ ∆ χ = χa χb ∆ χ = 2.5 - 2.2 = 0.3 ∆ χ = 3.0 - 2.2 = 0.8 ∆ χ = 3.5 - 2.2 = 1.3 ∆ χ = 4.0 - 2.2 = 1.8 single bonds H H H H H H C H H H H H H C C H H H H N N H H H H C N H H H H H C O H H H C F H H O N H H O O H F N H O F F F F double bonds H H C H H C C H H C N H H O N H H N N O O O H3C H two double bonds H H H C C C C triple bonds C N C C H H H H H H C H H C H C C H H C N H C H C N H CH2 H2C H2C H2 C H2C C H2 CH2 H C C H2C O NH CH2 H2C N H rings H2 C O C H H C O O H C C H2C C C H2 H Z:\classes\314\314 Special Handouts\1 recent updates\1. 314 supplement at orbs, IP, bonds.doc C H H N