7.1 Development of the Periodic Table Abundance of Elements Presently 118 elements known 92 occur naturally rest are made in labs Most abundant element in Earths crust, oceans, and atmosphere: Oxygen 49.2% Silicon 25.7% Aluminum 7.5% Discovery of elements is ongoing process since ancient times. Gold appears in nature in elemental form= discovered thousands of years ago Technetium radioactive and unstable= wasnt discovered until 20 th century bc of development of technology Majority of elements readily form compounds not found in nature in elemental form= scientist where unaware of existence 19 th century= advances in chemistry made it easier to isolate elements 31 in 1800 to 63 by 1865 Organizing the Elements As # of elements increased, scientists investigated ways to classify them: 1869 Dmitri Mendeleev (Russian chemist) organized the known elements in order of increasing atomic weights - noted similar chemical and physical properties Left spaces in pt to keep elements properly lined up according to chemical properties = able to predict the properties and the mass #s of new elements that had not been discovered yet. ctenImage/10_sc_periodicTable4.PNG Improving the Periodic Table 1913 2 yrs after Rutherford proposed nuclear model of atom Henry G.J. Moseley- developed concept of atomic numbers How? observed spectra of 38 different elements that were bombarded with high energy e- emitted different frequencies of x-rays (each element was unique) observed frequencies increased as atomic mass increased arranged x-ray freq in order of increasing nuclear charge (or number of protons in nucleus) Led to modern def of atomic number and organization of pt by atomic number Clarified some problems in pt which was based on atomic weights ex: Argon mass (atomic number 18) is greater than Potassium mass (atomic number 19) yet Ar has chemical and physical properties more like Ne and Kr than Na and Rb Mosley's studies made it possible to identify holes in periodic table lead to discovery on previously unknown elements Result 7.2 Effective Nuclear Charge e- are attracted to nucleus, but repelled by other e- Thus e- shield other e- from nucleus This shielding reduces the full nuclear charge to an effective nuclear charge (Z eff ) the nuclear charge an e- actually experiences Z= # protons, S=screening constant (# of core e-) Coulombs law strength of 2 electrical charges depends on magnitude(size) of charge and distance between them force of attraction increases as charge increases and distance from nucleus decreases Periodic Trends Z eff increases as move across any row(period) on pt Z eff increases slightly as go down family bc larger e- cores are less able to screen the outer e- from nuclear charge Groups- columns of elements that have similar chemical properties ex: Group 1 Alkali Metals Rows- horizontal periods of elements that increase in atomic number Metals- physical properties include hard, shiny, solid, good conductor Nonmetals- physical properties include gas or brittle solid, not malleable or ductile, poor conductors ex: Oxygen Metalloids : elements that share properties metals and some nonmetals ex: Boron Protons positively charged particle found in nucleus= atomic number ex: Li 3 Neutron- neutral charged particle found in nucleus ex: Li 4 Electron- negative charge particle found around in electron cloud Ex: Li 3 Valence Electrons- electrons in the outer energy level; each group has the same number ex: Noble gases have 8 Homework pg Bonding Atomic Radius the radius of an atom; defined as distance separating it from other atoms which it is chemically bonded 7.3 Sizes of Atoms Knowing atomic radii allows us to estimate the bond lengths between different elements in molecules Using radii for C, S, and H from Fig 7.7 predict the lengths of the C-S,C-H, and S-H bonds in the molecule methyl mercaptan CH3SH: C-S = Radius of C + Radius of S = 0.77 A A = 1.79 A (E=1.82) C-H= 0.77 A A = 1.14 A (E=1.10) S-H= 1.02 A A = 1.39 A (E=1.33) Bond Lengths in a Molecule Predict which will be greater, the P-Br bond in PBr3 or the As-Cl bond in AsCl3. As-Cl = 2.18 A P-Br = 2.2A Practice Exercise Imagine Argon gas atoms colliding with each other in the course of motions ricochet apart like billiard balls Why? When e- clouds collide they cannot penetrate each other to any significant extent Nonbonding atomic radius the closest distances separating the nuclei during collisions determine the apparent radii of the Ar atoms What about Nobel Gases? Periodic Trends in Atomic Radii Arrange the following in order of increasing atomic radius: Na, Be, Mg BeK + >Ca 2+ Arrange the ions K +,Cl -,Ca 2+, S 2- in decreasing order: def. minimum energy required to remove an electron from the ground state of the isolated gaseous atom or ion energy must be added to remove an e- 7.4 Ionization Energy Every element exhibits a large increase in IE when e- are removed from noble-gas core Supports idea that sharing and transfer of e- give rise to chemical bonding and reactions- inner e- too tightly bounds to nucleus lowest: Al highest: C Which will have the lowest/highest 1 st IE: B, Al, C, Si ? When e- removed from atom to form cation e- removed 1 st from occupied orbitals having the largest principal quantum number Electron Configurations of Ions If there is more than one occupied subshell for a given value of n, then e- are first removed from the orbital with the highest l value. Ex: tin atom loses its 5p electrons before losing its 5s electrons Anion- e- are added to empty or partial filled orbital having lowest n value [Ar] [Ar] 4s 2 3d 1 Write EC for Ca 2+, S 2-, Cr 3+ def. the energy change that occurs when an e- is added to a gaseous atom or ion measure of attraction of atom for the added e- energy is released when e- added the more negative the EA, the greater the attraction of the atom for an e- ex: Cl(g) + e- Cl-(g) E= -349kJ/mol 7.5 Electron Affinities For some elements (ex Noble gases) EA is + = the anion is higher energy than are the separated atom and e- Ar(g) + e- Ar-(g) E>0 +EA means at e- will not attach itself to an Ar atom Ar- unstable and does NOT form Electron Affinity > 0 pg 289 7.17 7.55 odd Homework