2007, Prentice Hall Chemistry: A Molecular Approach, 1 st Ed. Nivaldo Tro Roy Kennedy Massachusetts Bay Community College Wellesley Hills, MA

Mendeleev order elements by atomic mass saw a repeating pattern of properties Periodic Law – When the elements are arranged in order of increasing atomic mass, certain sets of properties recur periodically put elements with similar properties in the same column used pattern to predict properties of undiscovered elements where atomic mass order did not fit other properties, he re-ordered by other properties Te & I Tro, Chemistry: A Molecular Approach2

Periodic Pattern Tro, Chemistry: A Molecular Approach3 H nm H 2 O a/b 1 H 2 Li m Li 2 O b 7 LiH Na m Na 2 O b 23 NaH Be m/nm BeO a/b 9 BeH 2 m MgO b 24 MgH 2 Mg nm B 2 O 3 a 11 ( BH 3 ) n B m Al 2 O 3 a/b 27 (AlH 3 ) Al nm CO 2 a 12 CH 4 C nm/m SiO 2 a 28 SiH 4 Si nm N 2 O 5 a 14 NH 3 N nm P 4 O 10 a 31 PH 3 P nm O 2 16 H 2 O O nm SO 3 a 32 H 2 S S nm Cl 2 O 7 a 35.5 HCl Cl nm 19 HF F a = acidic oxide, b = basic oxide, a/b = amphoteric oxide m = metal, nm = nonmetal, m/nm = metalloid

Mendeleev's Predictions Tro, Chemistry: A Molecular Approach4

What vs. Why Mendeleev’s Periodic Law allows us to predict what the properties of an element will be based on its position on the table it doesn’t explain why the pattern exists Quantum Mechanics is a theory that explains why the periodic trends in the properties exist Tro, Chemistry: A Molecular Approach5

Electron Spin experiments by Stern and Gerlach showed a beam of silver atoms is split in two by a magnetic field the experiment reveals that the electrons spin on their axis as they spin, they generate a magnetic field spinning charged particles generate a magnetic field if there is an even number of electrons, about half the atoms will have a net magnetic field pointing “North” and the other half will have a net magnetic field pointing “South” Tro, Chemistry: A Molecular Approach6

Electron Spin Experiment Tro, Chemistry: A Molecular Approach7

Spin Quantum Number, m s spin quantum number describes how the electron spins on its axis clockwise or counterclockwise spin up or spin down spins must cancel in an orbital paired m s can have values of ±½ Tro, Chemistry: A Molecular Approach8

Pauli Exclusion Principle no two electrons in an atom may have the same set of 4 quantum numbers therefore no orbital may have more than 2 electrons, and they must have with opposite spins knowing the number orbitals in a sublevel allows us to determine the maximum number of electrons in the sublevel s sublevel has 1 orbital, therefore it can hold 2 electrons p sublevel has 3 orbitals, therefore it can hold 6 electrons d sublevel has 5 orbitals, therefore it can hold 10 electrons f sublevel has 7 orbitals, therefore it can hold 14 electrons Tro, Chemistry: A Molecular Approach9

10 Allowed Quantum Numbers

Quantum Numbers of Helium’s Electrons helium has two electrons both electrons are in the first energy level both electrons are in the s orbital of the first energy level since they are in the same orbital, they must have opposite spins nlmlml msms first electron 100+½+½ second electron 100-½-½ Tro, Chemistry: A Molecular Approach11

Electron Configurations the ground state of the electron is the lowest energy orbital it can occupy the distribution of electrons into the various orbitals in an atom in its ground state is called its electron configuration the number designates the principal energy level the letter designates the sublevel and type of orbital the superscript designates the number of electrons in that sublevel He = 1s 2 Tro, Chemistry: A Molecular Approach12

Orbital Diagrams we often represent an orbital as a square and the electrons in that orbital as arrows the direction of the arrow represents the spin of the electron Tro, Chemistry: A Molecular Approach13 orbital with 1 electron unoccupied orbital orbital with 2 electrons

Sublevel Splitting in Multielectron Atoms the sublevels in each principal energy level of Hydrogen all have the same energy – we call orbitals with the same energy degenerate or other single electron systems for multielectron atoms, the energies of the sublevels are split caused by electron-electron repulsion the lower the value of the l quantum number, the less energy the sublevel has s (l = 0) < p (l = 1) < d (l = 2) < f (l = 3) Tro, Chemistry: A Molecular Approach14

Penetrating and Shielding the radial distribution function shows that the 2s orbital penetrates more deeply into the 1s orbital than does the 2p the weaker penetration of the 2p sublevel means that electrons in the 2p sublevel experience more repulsive force, they are more shielded from the attractive force of the nucleus the deeper penetration of the 2s electrons means electrons in the 2s sublevel experience a greater attractive force to the nucleus and are not shielded as effectively the result is that the electrons in the 2s sublevel are lower in energy than the electrons in the 2p Tro, Chemistry: A Molecular Approach15

Penetration & Shielding Tro, Chemistry: A Molecular Approach16

Energy 1s 7s 2s 2p 3s 3p 3d 6s 6p 6d 4s 4p 4d 4f 5s 5p 5d 5f Notice the following: 1.because of penetration, sublevels within an energy level are not degenerate 2.penetration of the 4 th and higher energy levels is so strong that their s sublevel is lower in energy than the d sublevel of the previous energy level 3.the energy difference between levels becomes smaller for higher energy levels

Order of Subshell Filling in Ground State Electron Configurations Tro, Chemistry: A Molecular Approach18 1s1s 2s2s2p2p 3s3s3p3p3d3d 4s4s4p4p4d4d4f4f 5s5s5p5p5d5d5f5f 6s6s6p6p6d6d 7s7s start by drawing a diagram putting each energy shell on a row and listing the subshells, (s, p, d, f), for that shell in order of energy, (left-to-right) next, draw arrows through the diagonals, looping back to the next diagonal each time

Filling the Orbitals with Electrons energy shells fill from lowest energy to high subshells fill from lowest energy to high s → p → d → f Aufbau Principle orbitals that are in the same subshell have the same energy no more than 2 electrons per orbital Pauli Exclusion Principle when filling orbitals that have the same energy, place one electron in each before completing pairs Hund’s Rule Tro, Chemistry: A Molecular Approach19

Example 8.1 – Write the Ground State Electron Configuration and Orbital Diagram and of Magnesium. 1. Determine the atomic number of the element from the Periodic Table This gives the number of protons and electrons in the atom Mg Z = 12, so Mg has 12 protons and 12 electrons Tro, Chemistry: A Molecular Approach20

Example Write the Ground State Electron Configuration and Orbital Diagram and of the following element Lithium Magnesium Sulfur

Valence Electrons the electrons in all the subshells with the highest principal energy shell are called the valence electrons electrons in lower energy shells are called core electrons chemists have observed that one of the most important factors in the way an atom behaves, both chemically and physically, is the number of valence electrons Tro, Chemistry: A Molecular Approach22

Electron Configuration of Atoms in their Ground State Kr = 36 electrons 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 there are 28 core electrons and 8 valence electrons Rb = 37 electrons 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 5s 1 [Kr]5s 1 for the 5s 1 electron in Rb the set of quantum numbers is n = 5, l = 0, m l = 0, m s = +½ for an electron in the 2p sublevel, the set of quantum numbers is n = 2, l = 1, m l = -1 or (0,+1), and m s = - ½ or (+½) Tro, Chemistry: A Molecular Approach23

Examples Write an electron configuration for phosphorous and sodium then identify the valence electrons and core electrons

Electron Configurations Tro, Chemistry: A Molecular Approach25

Electron Configuration & the Periodic Table the Group number corresponds to the number of valence electrons the length of each “block” is the maximum number of electrons the sublevel can hold the Period number corresponds to the principal energy level of the valence electrons Tro, Chemistry: A Molecular Approach26

Tro, Chemistry: A Molecular Approach27 s1s1 s2s2 d 1 d 2 d 3 d 4 d 5 d 6 d 7 d 8 d 9 d 10 s2s2 p 1 p 2 p 3 p 4 p 5 p6p6 f 2 f 3 f 4 f 5 f 6 f 7 f 8 f 9 f 10 f 11 f 12 f 13 f 14 f 14 d

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Transition Elements for the d block metals, the principal energy level is one less than valence shell one less than the Period number sometimes s electron “promoted” to d sublevel 29 4s3d Zn Z = 30, Period 4, Group 2B [Ar]4s 2 3d 10 for the f block metals, the principal energy level is two less than valence shell two less than the Period number they really belong to sometimes d electron in configuration Eu Z = 63, Period 6 [Xe]6s 2 4f 7 6s4f

Practice – Use the Periodic Table to write the short electron configuration and orbital diagram for each of the following Na (at. no. 11) Te (at. no. 52) Tc (at. no. 43) Tro, Chemistry: A Molecular Approach30

Properties & Electron Configuration elements in the same column have similar chemical and physical properties because they have the same number of valence electrons in the same kinds of orbitals Tro, Chemistry: A Molecular Approach31

Electron Configuration & Element Properties the number of valence electrons largely determines the behavior of an element chemical and some physical since the number of valence electrons follows a Periodic pattern, the properties of the elements should also be periodic quantum mechanical calculations show that 8 valence electrons should result in a very unreactive atom, an atom that is very stable – and the noble gases, that have 8 valence electrons are all very stable and unreactive conversely, elements that have either one more or one less electron should be very reactive – and the halogens are the most reactive nonmetals and alkali metals the most reactive metals as a group Tro, Chemistry: A Molecular Approach32

Tro, Chemistry: A Molecular Approach33 Electron Configuration & Ion Charge we have seen that many metals and nonmetals form one ion, and that the charge on that ion is predictable based on its position on the Periodic Table Group 1A = +1, Group 2A = +2, Group 7A = -1, Group 6A = -2, etc. these atoms form ions that will result in an electron configuration that is the same as the nearest noble gas

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Tro, Chemistry: A Molecular Approach35 Electron Configuration of Anions in their Ground State anions are formed when atoms gain enough electrons to have 8 valence electrons filling the s and p sublevels of the valence shell the sulfur atom has 6 valence electrons S atom = 1s 2 2s 2 2p 6 3s 2 3p 4 in order to have 8 valence electrons, it must gain 2 more S 2- anion = 1s 2 2s 2 2p 6 3s 2 3p 6

Tro, Chemistry: A Molecular Approach36 Electron Configuration of Cations in their Ground State cations are formed when an atom loses all its valence electrons resulting in a new lower energy level valence shell however the process is always endothermic the magnesium atom has 2 valence electrons Mg atom = 1s 2 2s 2 2p 6 3s 2 when it forms a cation, it loses its valence electrons Mg 2+ cation = 1s 2 2s 2 2p 6

Examples Write the electron configuration and orbital diagram for each of the following ions and predict whether the ion will be paramagnetic or diamagnetic Al 3+ N 3- Fe 2+