Atomic Structure' Chemical Bonds'

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Lecture 18

• Atomic Structure. Chemical Bonds.

• Electronic Shells
• Chemical Bonds
• Chemical Equations

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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.
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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.
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Shells and Subshells
The subshells in a shell of given n can have any
value of l from 0 to n?1.
The n1 shell has only the single subshell
l0. The n2 shell has 2 subshells l0 and
l1. The n3 shell has 3 subshells l0, l1, and
l2.
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.
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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
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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
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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
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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).
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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
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Chemical Equations
C O2 ? CO2 H2O ? H2 O2 or, in a balanced
form, 2H20 ? 2H2 O2 C3H8 O2 ? 3CO2 4H2O
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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