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The History of the Modern Periodic Table See separate slide show for Periodic Table History Periodic Law • When elements are arranged in order of increasing atomic #, elements with similar properties appear at regular intervals. Atomic Radius (pm) 250 200 150 100 50 0 0 5 10 Atomic Number 15 20 Chemical Reactivity Families Similar valence e- within a group result in similar chemical properties 1 2 3 4 5 6 7 •Alkali Metals •Alkaline Earth Metals •Transition Metals •Halogens •Noble Gases Periodic Table Reveals Periodic Trends • Effective Nuclear charge • Reactivity • atomic size or radius • bonding characteristics • ionization energy • crystal configurations • electron affinity • acidic properties • electronegativity • densities • metallic character • Melting/Boiling points Electron screening or shielding • Electrons are attracted to the nucleus • Electrons are repulsed by other electrons • Electrons would be bound more tightly if other electrons weren’t present. • The net nuclear charge felt by an electron is called the effective nuclear charge ( Zeff ). Quantum Mechanical Model Zeff is lower than actual nuclear charge. Zeff increases toward nucleus ns > np > nd > nf This explains certain periodic changes observed. Effective Nuclear Charge ( Zeff) • The effective nuclear charge acting on an electron equals the number of protons in the nucleus, Z, minus the average number of electrons, S that are between the nucleus and the electron in question. Zeff = Z S Zeff = attractive forces repulsive forces Zeff = # protons # shielding electrons For Example, Lithium vs. Carbon Li Zeff = 3 2 = 1 C Zeff = 6 2 = 4 When moving across a row: The greater the Zeff value, the smaller the atom’s radius. So, carbon has a much smaller atomic radius compared to lithium: Rcarbon =77 pm Rlithium = 152 pm Trend #1 Atomic Radii Increases to Left and Down 1 2 3 4 5 6 7 •Why larger going down? •Higher energy levels have larger orbitals •Shielding - core e- block the attraction between the nucleus and the valence e•Why smaller to the right? • Increased nuclear charge without additional shielding pulls e- in tighter Atomic radii The Periodic Table & Radii Periodic Trend is Due to Effective Nuclear Charge Atomic Radii vs. Zeff: Trends in Ionic Radii • Using your knowledge of Zeff, how would the size of a cation compare to neutral atom? Anion? Trends in Ionic Radii • The cation of an atom decreases in size. • The more positive an ion is, the smaller it is because Zeff increases • The anion of an atom increases in size. • The more negative an ion, the larger it is because Zeff decreases. Cations lose electrons, become smaller Anions gain electrons, become bigger Ion Radii Increases down 1 2 3 4 5 6 7 Increases moving across, but depends if cation OR anion +3 +4 -3 -2 -1 Ions and Ionic Radii Trend in Ionization Energy • Ionization NRG is the NRG required to remove an electron from an atom Successive Ionization NRG • Ionization energy increases for successive electrons from the same atom. Why do you think there is such a big jump for Mg3+? *Notice the large jump in ionization energy when a core e is removed. • The smaller the atom, the higher the ionization energy due to Zeff • Bigger atoms have lower ionization NRG due to the fact that the electrons are further away from the nucleus and therefore easier to remove. Decreases Increases Electron Affinity • The energy change associated with the addition of an electron • Tends to increase across a period • Tends to decrease as you go down a group • Abbreviation is Eea, it has units of kJ/mol. Values are generally negative because energy is released. • Value of Eea results from interplay of nucleus electron attraction, and electron–electron repulsion. Electron Affinity Trends in Electronegativity • tendency for an atom to attract electrons when it is chemically combined with another atom. • decreases as you move down a group • increases as you go across a period from left to right. Trend #5 Metallic Character • The metallic character of atoms can be related to the desire to lose electrons. • The lower an atom’s ionizatoin energy, the greater its metallic character will be. • On the periodic table, the metallic character of the atoms increase down a family and decreases from left to right across a period. Metallic Character Increases moving down and across to the left 1 2 3 4 5 6 7 Rb Cs Ba Fr Ra Lower left corner -- elements most likely to lose their valence electrons # 6 Melting/Boiling Points • Highest in the middle of a period (generally). 1 2 3 4 5 6 7 Some Important Properties of Alkali Metals • Easily lose valence electrons (Reducing Agents) – React with halogens to form salts – React violently with water • Large Hydration NRG – Positive ionic charge makes ions attractive to polar water molecules • Radius and Ionization NRG follow expected trends.