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2/25/2014 “An expert is a person who has made all the mistakes that can be made in a very narrow field.” —Niels Bohr “The only real mistake is the one from which we learn nothing.” — John Powell Today: ◦ Periodic Properties: Electron Shielding & Effective Nuclear Charge Atomic Radii Ionic Radii Ionization Energy Electron Affinity ◦ Electron Configurations & the OCTECT Rule Midterm EXAM 1: ◦ Scores posted to gradebook on canvas ◦ Please see me if your score is < 70 % Next Monday: ◦ Read Chp. 3, Sections 3.93.14, pp. 118-129 iClicker Participation Question: Review of VALENCE Electrons vs. CORE electrons Write the electron configuration for the VALENCE electrons of a neutral Selenium atom. A. 1s22s22p63s23p64s23d104p4 B. 1s22s22p63s23p6 C. 4s23d104p4 D. 4s24p4 Valence Electrons: e- in outer most s & p orbitals. • • If the d orbitals are fully filled, they become CORE electrons If the d orbitals are only PARTIALLY filled, they are still considered valence electrons E. 4p4 1 2/25/2014 Core Electrons & Valence Electrons: fully filled shells of electrons buried deep within the atom. Electrons in the outmost shell on the surface of the atom. These are the electrons involved in bonding and chemical reactions. Summary of Quantum Mechanics: Electrons exist in orbitals around nuclei as MATTER WAVES: ◦ The orbitals are filled in layers or SHELLS with different shapes & energies Electrons PAIR UP in atomic orbitals… The INNER SHELLS are composed of CORE ELECTRONS. These do not participate in chemical reactions. The OUTER SHELL is composed of VALENCE ELECTRONS. These are more exposed & participate in chemical reactions. ◦ The number of valence electrons corresponds to the group number (i.e. 1A for Na, 2A for Mg, etc.) Pure Substances Elements: Compounds: Only one type of atom combinations of elements Molecular: Molecular: Atomic: Single atoms Ionic: Atoms of multiple Atoms of only one Atoms of multiple element connected elements connected elements locked into into a molecule into a molecule a periodic lattice GOAL: Understand how complex substances behave based on their basic ELECTRONIC & ATOMIC STRUCTURE. 2 2/25/2014 Atomic Radius (Size): The size of an atom is determined by the electron orbitals The atomic radius of an element can be defined as half the distance between the nuclei of a diatomic molecule of the element. For example: the atomic radius of carbon is 77 picometers (7.7 x 10-11 m). Radius determination for a metal atom (Al) Nanoparticles visualized with Transmission Electron Microscope (TEM) 3 2/25/2014 ATOMIC RADIUS (pm) We might expect the atomic radii of elements to increase as we move from LEFT to RIGHT across the periodic table. However, experimentally, this is not observed. Why not? ATOMIC NUMBER (Z) Increasing Effective Nuclear Charge (Zeff) Effective nuclear charge (Zeff) and Shielding Zeffective is the average shielded charge felt by an electron. in an atom is the “blocking” or canceling of positive charge by core electrons. Shielding 4 2/25/2014 we move from left right on the periodic table, shielding of the nucleus by inner (core) electrons remains constant. (Shielding by other outer electrons is minimal.) As However, as the number of protons increases, the electrons in the valence (outer) shell experience a greater attraction for the nucleus – a higher Zeffective. Because of the greater pull on the outer electrons as we move across the periodic table, atomic radius generally DECREASES from left right within a period. Atomic Radii (pm) As we move from top bottom on the periodic table, we would expect the atomic radius to INCREASE, and it does, because the energy level of the outer e-’s increases: 1s→2s→3s→4s→etc. 5 2/25/2014 iClicker Participation Question: Trends in Atomic Size Rank the atoms below from smallest to largest atomic radius. A. Cl < F < S < Ba B. F < Cl < S < Ba C. Ba < S < Cl < F D. F < Cl < Ba < S E. None of the above are correct Periodic Properties in the Periodic Table: The chemical properties of elements are largely determined by The Noble gases all have 8 valence electrons FULLY FILLING the outermost shell. This is called an OCTET. Elements will react to lose or gain enough electrons to arrive at a Noble gas electron configuration with a complete OCTET. The Alkali metals all have 1 valence electron in their neutral form Chemical Reactions based on Electron Configurations: Elements will react to lose or gain enough electrons to arrive at a Noble gas electron configuration with a complete OCTET. Metals LOSE electrons to reach a complete OCTET with a Noble gas configuration. Nonmetals GAIN electrons to reach a complete OCTET with a Noble gas configuration. 6 2/25/2014 Ionization Energy First Ionization Energy – the energy required to remove the highest energy electron from an isolated atom in the gaseous state. Second Ionization Energy – the energy required to remove the second highest energy electron from the +1 ion. Third, Fourth, Fifth, etc. Ionization energies are positive: they refer to the energy ABSORBED by the atom (system) to remove an electron. A negative energy indicates energy is RELEASED. 1st Ionization Energy Why does oxygen have an anomalously low ionization energy? Or sulfur for that matter? ATOMIC NUMBER (Z) LR trend: 1st ionization energy INCREASES, because the pull on the outermost electron increases: Zeff increases. TB trend: 1st ionization energy DECREASES, because the outermost electron is at a higher energy level. 2nd Ionization Energy and higher… Beryllium 1st ionization: Beryllium 2nd ionization: Beryllium 3rd ionization: Be (1s22s2) → Be+ (1s22s1 ) + 1eBe+ (1s22s1) → Be2+ (1s2) + 1eBe2+ (1s2) → Be2+ (1s1) + 1e- A LARGE JUMP between subsequent ionization energies is observed when core electrons start to be removed. (in kJ/mol) 7 2/25/2014 Ionic Radii Reported Ionic radii are determined from the measured distances between nuclei in ionic compounds. Cations are always smaller than the atom they are derived from. In many cases, the outer energy level of the cation is one energy level lower than that of the atom. Anions are always larger than the atom they are derived from, primarily due to repulsive forces of electrons in the outer energy level. Size Change in Ion Formation Size Change in Ion Formation 8 2/25/2014 ANIONS CATIONS Ionic Radii (pm) The LR trend for ionic radius is DECREASING for the same type of ion – cation or anion. The TB trend is INCREASING. iClicker Participation Question: Trends in the Size of IONS Rank the ions below from smallest to largest IONIC radius. A. S2- < Cl- < Na+ < Mg2+ B. Cl- < S2- < Mg2+ < Na+ C. Na+ < Mg2+ < S2- < ClD. Mg2+ < Na+ < Cl- < S2E. None of the above are correct Electron Affinity The energy absorbed (+) or released (-) when an isolated gaseous atom ACCEPTS an electron to become a negative ion. LR trend: Generally increasing, because the attraction for the outermost electrons increases from left to right. TB trend: Generally decreasing, but trend is not highly predictable from top to bottom. 9 2/25/2014 Table of Electron Affinities • Why does Nitrogen have an anomalously low electron affinity? 10