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Periodic Trends (Section 4-3, Pgs Periodic Trends (Section 4-3, pgs. 132 – 141) • Trend – predictable change in a particular direction (allows scientists to predict how substances will react). • Can be explained in terms of electron configuration. Reactivity of Metals decreases least metallic increases most metallic • Metals give up electrons when reacting • Elements that require the least energy to give up an electron have the highest reactivity (most “metallic” behavior) Electron shielding – a decrease in the attractive force of the nucleus for its valence electrons due to the presence of electrons in inner energy levels: • Across a period: constant – electrons added to same outer energy level – # of electrons in inner E levels constant • Down a group: – electrons added to higher energy level increases – # of electrons in inner E levels increases • Explains other trends Shielding electrons shown in red: Li (1s22s1) Ne (1s22s2 2p6) same shielding Rb has much greater shielding Rb (1s22s22p63s23p64s23d104p65s1) Effective Nuclear Charge (ENC) – effectiveness of the positive pull of the nucleus on the electrons of an atom: • Across a period: – no change in shielding increases – increase in # of positive charges (protons) in nucleus increases pulling force • Down a group: – increased shielding – increased # protons in constant nucleus counteracted by increased shielding • Explains other trends Effective Nuclear Charge: Shielding reduces the charge Z that the valence electrons feel to Zeff, which is called the effective nuclear charge. The innermost (core) electrons screen or “shield” the nucleus from the view of the outer (valence) electrons. The valence electrons in effect see a charge (Zeff) that is less than the actual charge on the nucleus, Z. Atomic radius • reflects size of the atom • depends on the volume occupied by the electron cloud • no clear-cut edge • measured by bond radius – half the distance between the nuclei of two bonded atoms Trends in Atomic Radii • Across a period: – electrons added to decreases same E level • shielding is constant – ENC increases • outer electrons held more tightly • Down a group: – # of energy levels increases • outer e- further from increases nucleus – shielding increases • outer electrons held less tightly Periodic Trends in Atomic Radii Ionization Energy – the energy required to remove an electron from an atom • Example: Na• + energy Na+ + e- Valence Trends in Ionization Energy • Across a period: – shielding is constant – ENC increases increases – outer (valence) electrons held more tightly – more E needed to remove valence electrons • Down a group: – electrons added to higher E levels further from the nucleus decreases – shielding increases – outer electrons held less tightly – less E needed to remove valence electrons Trends in First Ionization Energies Trends in Ionic Size Atoms form ions when they gain or lose electrons. • Across a period decreases – increased ENC – shielding constant • Down a group – increased shielding – ENC constant increases Comparing the size of an ion to its parent atom: Positive Ions (cations): Negative Ions (anions): • smaller than parent atom • larger than parent atom • elements < 4 valence e- • elements > 4 valence e- • metals • nonmetals • increased ENC • decreased ENC Ionic Size and Charge Ionic Size Across Period 3 2.12 3-P Electronegativity – a measure of the ability of an atom in a chemical compound to attract the electrons in a chemical bond • atoms of elements with Cl is more electronegative than Na: a high electronegativity will pull bonding electrons closer to their nucleus • bonding electrons will be further from the nucleus of an atom with O is more electronegative than H: a low electronegativity Trends in Electronegativity • Across a Period – shielding is constant increases – ENC increases – pulling force of nucleus increases • Down a Group – shielding increases – distance from valence decreases e- to nucleus increases – pulling force of F (fluorine) is the most nucleus decreases electronegative element in the periodic table. Another look at Electronegativity Trends Electron affinity – the energy change that occurs when a neutral atom gains an electron (opposite of ionization energy) • Metals – small energy change (not favored) Na + e Na • Nonmetals – large energy change (favored) F + e F Trends in Electon Affinity are the same as trends in Ionizaton Energy • Across a Period increases – ENC increases • Down a Group – shielding increases decreases Trends in Melting and Boiling Points • Stronger bonding forces result in higher melting and boiling points. • Unpaired electrons in outermost subshells are most often involved in bonding. Melting and Boiling Points s-block # valence e- increases p-block d-block increases decreases increases decreases peak peak • # of • # of • # of • # of unpaired e- paired e- unpaired e- paired e- in d-orbitals in d-orbitals in p-orbitals in p-orbitals increases increases increases increases • results in • results in • results in • results in stronger weaker stronger weaker metallic metallic bonding bonding bonds bonds forces forces Summary of Several Periodic Trends and electronegativity ionic radius ionic and general and electronegativity and and general ionic radius.
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