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

Quantum Numbers ThreeThree 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 m l  the magnetic quantum number The fourth quantum number is m s  spin magnetic quantum number.fourth In an unexcited hydrogen atom, the electron is in its quantum state of the lowest energy. Only 1 electron can exist in each quantum state of an atom.

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.

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.

Number of Electrons in Closed Shells The larger the l, the more electrons can occupy the corresponding subshell. 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:totalclosed n = 1 may have 2 electrons with l = 0 n = 2 may have 2 l =0 electrons and 6 l = 1 electrons

Chemical Properties of Elements Inert (Noble) Gases: Outer shells are closed  the electrons are tightly bound to the atom  chemically passiveclosed 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

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.lack 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

Chemical Bonds Atoms are held together by electric forces Three types of chemical bonds:types Covalent bond  one or more pairs of electrons are shared by atoms (H 2 ). 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).

Atom Groups Some groups of atoms remain together during chemical reactions. Examples: SO 4 (charge  2)  Na 2 SO 4, CuSO 4 NO 3 (charge  1)  Ca (NO 3 ) 2 Conventions about naming compounds: NaCl – sodium chloride (only 2 elements) Ba(OH) 2 – barium hydroxide (contains the OH  ion) Na 2 SO 4 – sodium sulfate (oxygen and 2 more elements) CO – carbon monoxide, CO 2  dioxide

Chemical Equations C + O 2  CO 2 H 2 O  H 2 + O 2 or, in a balanced form, 2H 2 0  2H 2 + O 22H 2 0 C 3 H 8 + O 2  3CO 2 + 4H 2 O

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