Download Chapter 8 - Chemistry

Chapter 8
(Essentials of General Chemistry, 2nd Edition)
(Ebbing and Gammon)
Electron Configuration
and Periodicity
Electron Spin
spin quantum number (ms)
-describes the spin orientation of an electron
electron spin
- beam of hydrogn atoms is split into two since each
electron in each atom acts as tiny magnet with only
two possible orientations
- electrons acts as a ball of spinning charge and like a
circulating electric charge, would create a magnetic
field
- resulting directions of spin magnetism correspond
to ms = +1/2 and ms = –1/2
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Electron Spin
Electron spin magnetism
Spin quantum numbers
ms +1/2 and – 1/2
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Electron Configurations: Orbital Diagrams
electron configuration
- a particular distribution of electrons among the
available subshells
- notation for configuration lists the subshell
symbols, one after the other, with a subscript giving
the number of electrons in that subshell
eg. boron 1s2 2s2 2p1
orbital diagram
- a diagram to show how the orbitals of a subshell are
occupied by electrons
1s
2s
2p
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Pauli Exclusion Principle
- no two electrons in an atom can have the same four
quantum numbers
restated: an orbital can hold at most two electrons, and then
only if the electrons have opposite spins
Note: each subshell holds a maximum of twice as many
electrons as the number of orbitals in the subshell
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The Pauli Exclusion Principle
The maximum number of electrons and their orbital
diagrams are:
Sub shell
Number of
Orbitals
Maximum
Number of
Electrons
s (l = 0)
1
2
p (l = 1)
3
6
d (l =2)
5
10
f (l =3)
7
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Building-Up Principle (Aufbau Principle)
ground state
- the configuration with the lowest energy level of the
atom
- chemical properties of an atom are related primarily to
the electron configuration of its ground state
excited state
- associated with energy levels other than the lowest
Aufbau Principle
- a scheme used to reproduce the electron
configurations of atoms by successfully filling
subshells with electrons
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- following this principle, electron configuration of an atom
may be obtained by successively filling subshells in the
following order:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f
- this order can be easily obtained by using the periodic
table as a template
- filling orbitals of lowest energy first, usually gives lowest
total energy (ground state) of the atom
- recall: energy of orbital depends only on quantum
numbers n and l
- orbitals with same n and l but different ml (different
orbitals of same subshell) have same energy
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- energy depends primarily on n, increasing with its value
- energies of orbitals with same n increase with the l
quantum number
- that is, 3p orbital has slightly greater energy than 3s orbital
- exception: when subshells have nearly same energy,
building- up order is not strictly determined by order of
energies
- ground-state configurations are determined by total
energies of atoms which depend not only on energies of
subshells but also on energies of interaction among
different subshells
- note that for elements with Z=21 or greater, the energy of
the 3d subshell is lower than the energy of the 4s subshell
(opposite to the building-up order)
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Order for Filling Atomic Subshells
1s
2s
3s
4s
5s
6s
2p
3p
4p
5p
6p
3d
4d 4f
5d 5f
6d 6f
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Orbital Energy Levels in Multi-electron Systems
3d
Energy
4s
3p
3s
2p
2s
1s
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Electron Configuration and the Periodic Table
noble gases
- relatively unreactive
- configurations in which p subshell has just filled
(neon, argon, krypton; also He with filled 1s subshell)
alkaline earth metals (Group IIA)
- noble-gas core plus two out electrons with an ns2
configuration
Beryllium
Magnesium
Calcium
1s22s2
1s22s22p63s2
1s22s22p63s23p64s2
or
or
or
[He]2s2
[He]3s2
[He]4s2
noble-gas core
- an inner-shell configuration corresponding to one of
the noble gases
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Group IIA
Boron
Aluminum
Gallium
1s22s22p1
1s22s22p63s23p1
1s22s22p63s23p63d104s24p1
or [He]2s22p1
or [He]3s23p1
or [He]3d104s24p1
- boron and aluminum
– noble-gas core plus three electrons with
configuration ns2np1
- gallium
– pseudo-noble-gas core plus ns2np1
pseudo-noble-gas core
- noble-gas core together with (n-1)d10 electrons
- these electrons are usually not involved in
chemical reactions
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valence electron
- an electron in an atom outside the noble-gas or
pseudo-noble gas core
- primarily involved in chemical reactions
- similarities among configurations of valence
electrons (valence-shell configurations) account for
similarities of the chemical properties among groups
of elements
- figure 8.5 (p.231)
– note similarity in electron configuration
within any group (column)
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Valence-shell Configurations
Figure 8.5: Periodic Table
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main-group (or representative) elements
- have valence-shell configurations nsansb as the
outer s or p subshell is being filled
d-block transition elements (transition elements)
- a d subshell is being filled
f-block transition elements (inner-transition elements)
- an f subshell is being filled
Exceptions to the Building-up Principle
Element
Predicted
Experimental
Cr
[Ar]3d44s2
[Ar]3d54s1
Cu
[Ar]3d94s2
[Ar]3d104s1
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Electron Configuration Using the Periodic Table
Configuration of the outer (valence-shell) electrons
- for main-group elements is nsanpb
- n = principal quantum number of outer shell
and period for the element
- total valence electrons (a+b) comes from
group number
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Orbital Diagrams and Hund’s Rule
Consider possible arrangements of electrons in the
ground-state configuration of C (1s22s22p2):
1s
2s
2p
Diagram 1:
Diagram 2:
Diagram 3:
- these diagrams represent different states of the carbon
atom
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Hund’s Rule
Friedrich Hund (1927)
- the lowest energy arrangement of electrons in a subshell is
obtained by putting electrons into separate orbitals of the
subshell with the same spin before pairing electrons
Magnetic Properties of Atoms
- the magnetic properties of an atom are best observed
by subjecting it to the field of a strong magnet
paramagnetic substance
- a substance that is weakly attracted by a magnetic field,
and this attraction is generally the result of unpaired
electrons
diamagnetic substance
- a substance that is not attracted by a magnetic field or is
very slightly repelled by such a field
- means that the substance has only paired electrons
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Some Periodic Properties
periodic law
- states that when the elements are arranged by atomic
number, their physical and chemical properties vary
periodically
Physical properties:
a.) atomic radius
b.) ionization energy
c.) electron affinity
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Atomic Radii
Atomic radius (or size of an atom)
- measured as covalent radii
– obtained from measurements of distances
between the nuclei of atoms in the chemical
bonds of molecular substances
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Observed variations in Atomic Size
1.) decrease in atomic radii from left to right across
the periodic table
- b/c as atomic number increases, the effective
nuclear charge also increases
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effective nuclear charge
- the positive charge that an electron experiences
from the nucleus, equal to the nuclear charge but
reduced by any shielding or screening from any
intervening electron distribution
Consider Li:
- nuclear charge is +3 but effective nuclear charge is
+1
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2.) atomic radii increase going down a group of the
periodic table
Atomic Radii of the Elements (pm)
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Ionization Energy
first ionization energy (first ionization potential)
- the minimum energy needed to remove the highestenergy (that is, the outermost) electron from the
neutral atom in the gaseous state
Consider Li:
Li (1s22s1)
Li+(1s2) + eI.E. = 520 kJ/mol
General trends:
1.) increasing ionization energy with atomic number in a
given period
2.) decreasing ionization energy down any column of
main-group elements
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- electrons of an atom can removed successively with
ionization energies increasing as more electrons are
removed
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Electron Affinity
electron affinity
- the energy change for the process of adding an
electron to a neutral atom in the gaseous state to
form a negative ion
Cl ([Ne]3s23p5) + e-
Cl- ([Ne]3s23p6)
E.A.= -349kJ/mol
- if the negative ion is stable, the energy change for
its formation is a negative number (energy is
released)
- large negative numbers indicate a very stable
negative ion is formed (as is the case with Cl-)
- small negative number indicate that a less stable ion
is formed
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General trend:
- broadly speaking, more negative electron affinities
from left to right in any period
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Periodicity in Main-Group Elements
- chemical and physical properties of main-group elements
clearly display periodic character
- move left to right in any row, metallic character of
the elements decreases
- proceed down a column, elements tend to increase
in metallic character
- low ionization energy – tend to be metallic
- high ionization energy – tend to be nonmetallic
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- basic-acidic behavior of oxides of elements is good
indicator of metallic-nonmetallic character
basic oxide
- an oxide that reacts with acids
- metal oxides are basic
acidic oxide
- an oxide that reacts with bases
- nonmetal oxides are acidic
amphoteric oxides
- an oxide that has both basic and acidic properties
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Hydrogen (1s1)
- electron configuration places is in Group IA
- properties different and better to consider it belonging
to its own group
Group IA Elements; Alkali Metals (ns1)
- reactivities increase as you move down a group
- form basic oxides of the formula R2O
Group IIA Elements; Alkaline Earth Metals (ns2)
- reactivities increase as you move down a group; but
in general are less reactive than the alkali metals
- form basic oxides of the formula RO
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Group IIIA Elements (ns2np1)
- shows significant increase in metallic character
down the group
- oxides have general formula R2O3 (include acidic
oxide and amphoteric oxides)
Note: indicative of increase in metallic character
Group IVA Elements (ns2np2)
- distinct change in metallic character down the
group
- oxides have general formula RO2 (includes acid and
amphoteric oxides)
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Group VA Elements (ns2np3)
- distinct change in metallic character down the
group
- oxides have general empirical formulas R2O3 and
R2O5 with molecular formulas of R4O6 and R4O10
(includes acid, amphoteric and basic oxides)
Group VIA Elements; the Chalcagens (ns2np4)
- distinct change in metallic character down the
group
- oxides have general formula RO2 and RO3 (includes
acid and amphoteric oxides)
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Group VIIA Elements; the Halogens (ns2np5)
- reactive nonmetals with general formula, X2
- each halogen forms several compounds with
oxygen; generally unstable, acidic oxides
Group VIIIA Elements; the Noble Gases (ns2np6)
- exist as gases consisting of uncombined atoms
- thought to be inert until 1960s
- now, compounds of xenon, krypton and radon are
known (although still relatively unreactive)
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