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Transcript
Lecture 18
Atomic Structure.
Chemical Bonds.
• Electronic Shells
• Chemical Bonds
• Chemical Equations
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.
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:
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 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
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
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).
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
Chemical Equations
C + O2  CO2
H2O  H2 + O2 or, in a balanced form, 2H20  2H2 + O2
C3H8 + O2  3CO2 + 4H2O
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