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Lecture 18

Atomic Structure. Chemical Bonds. Electronic Shells Chemical Bonds Chemical Equations. Lecture 18. Quantum Numbers. Three quantum numbers determine the size and shape of the probability cloud of an atomic electron. n  the principal quantum number

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Lecture 18

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  1. Atomic Structure. Chemical Bonds. Electronic Shells Chemical Bonds Chemical Equations Lecture 18

  2. Quantum Numbers Three quantum numbers determine the size and shape of the probability cloud of an atomic electron. n  the principal quantum number l  the orbital quantum number ml  the magnetic quantum number The fourth quantum number is ms spin magnetic quantum number. Only 1 electron can exist in each quantum state of an atom. In an unexcited hydrogen atom, the electron is in its quantum state of the lowest energy.

  3. Shells and Subshells Electrons with the same quantum number n are about the same distance from the nucleus, move in about the same electric field, and have similar energies. Such electrons occupy the same atomic shell. The energy of an electron depends also on the quantum number l. The higher the value of l, the higher the energy. Electrons with the same value of l in a shell occupy the same subshell.

  4. Shells and Subshells The subshells in a shell of given n can have any value of l from 0 to n1. The n=1 shell has only the single subshell l=0. The n=2 shell has 2 subshells l=0 and l=1. The n=3 shell has 3 subshells l=0, l=1, and l=2. The exclusion principle limits the number of electrons in a shell or a subshell. A shell or subshell with the maximum number of allowed electrons is called closed.

  5. The larger the l, the more electrons can occupy the corresponding subshell. Number of Electrons in Closed Shells If l = 0, then the maximum number of electrons is 2. For l = 1, it is 6. For l = 2, it is 10. Thus, the total allowed number of electrons in a closed shell is: n = 1 may have 2 electrons with l = 0 n = 2 may have 2 l =0 electrons and 6 l = 1 electrons

  6. Chemical Properties of Elements Inert (Noble) Gases: Outer shells are closed the electrons are tightly bound to the atom  chemically passive Hydrogen and Alkali Metals: Single outer electrons, which see only +e charge and are loosely bound to the nucleus,  lose the outermost electrons in chemical reactions, have similar behavior, and are chemically active

  7. Chemical Properties of Elements Halogen Atoms: Outer shells lack 1 electron  tendency to pick up such an electron through the strong attraction of the poorly shielded nuclear charge. Metals and Nonmetals: Metals have 1 or several electrons outside the closed shells  combine chemically by losing these electrons to nonmetals Nonmetals lack 1 or several electrons in the outermost shell  combine by picking up electrons from metals or by sharing electrons with other nonmetals

  8. Chemical Bonds Atoms are held together by electric forces Three types of chemical bonds: Covalent bond one or more pairs of electrons are shared by atoms (H2). Polar covalent bond one part of the molecule attracts shared electron pairs stronger (HCl). Ionic bond  one or more electrons from one atom shift to another atom in a molecule (NaCl).

  9. Atom Groups Some groups of atoms remain together during chemical reactions. Examples: SO4 (charge 2)  Na2SO4, CuSO4 NO3 (charge 1)  Ca (NO3)2 Conventions about naming compounds: NaCl – sodium chloride (only 2 elements) Ba(OH)2 – barium hydroxide (contains the OH ion) Na2SO4 – sodium sulfate (oxygen and 2 more elements) CO – carbon monoxide, CO2  dioxide

  10. Chemical Equations C + O2 CO2 H2O  H2 + O2 or, in a balanced form, 2H20  2H2 + O2 C3H8 + O2  3CO2 + 4H2O

  11. Summary Quantum theory explains chemical properties of elements The exclusion principle governs the distribution of electrons in shells and subshells The number of electrons in the outermost subshells defines chemical properties of elements

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