Download effective nuclear charge

7.1 Development of the Periodic
Table
Abundance of Elements
 Presently 118 elements
known
 92 occur naturally rest
are made in labs
 Most abundant element in
Earth’s crust, oceans, and
atmosphere:
Oxygen 49.2%
Silicon 25.7%
Aluminum 7.5%
Discovery of elements is ongoing
process since ancient times.
 Gold appears in nature in
elemental form= discovered
thousands of years ago
 Technetium radioactive and
unstable= wasn’t discovered
until 20th century bc of
development of technology
 Majority of elements readily
form compounds not found
in nature in elemental form=
scientist where unaware of
existence
 19th century= advances in
chemistry made it easier to
isolate elements 31 in 1800
to 63 by 1865
Organizing the Elements
As # of elements increased, scientists
investigated ways to classify them:
 1869 Dmitri Mendeleev (Russian
chemist) organized the known
elements in order of increasing
atomic weights - noted similar
chemical and physical properties
 Left spaces in pt to keep elements
properly lined up according to
chemical properties = able to
predict the properties and the mass
#s of new elements that had not
been discovered yet.
 http://www.excellup.com/classten/scienceten/periodi
ctenImage/10_sc_periodicTable4.PNG
Improving the
Periodic Table
1913 2 yrs after Rutherford
proposed nuclear model of atom…
 Henry G.J. Moseley- developed
concept of atomic numbers
How?
 observed spectra of 38 different
elements that were bombarded
with high energy e-  emitted
different frequencies of x-rays (each
element was unique)
 observed frequencies increased as
atomic mass increased
 arranged x-ray freq in order of
increasing nuclear charge
(or number of protons in nucleus)
Result
 Led to modern def of atomic
number and organization of pt by
atomic number
 Clarified some problems in pt which
was based on atomic weights
 ex: Argon mass 39.948 (atomic
number 18) is greater than
Potassium mass 39.0983 (atomic
number 19) yet Ar has chemical
and physical properties more like
Ne and Kr than Na and Rb
 Mosley's studies made it possible to
identify holes in periodic table
lead to discovery on previously
unknown elements
7.2 Effective Nuclear Charge
 e- are attracted to nucleus,
but repelled by other e Thus e- “shield” other efrom nucleus
 This shielding reduces the
full nuclear charge to an
effective nuclear charge
(Zeff) the nuclear charge
an e- actually experiences
 Z= # protons, S=screening
constant (# of core e-)
 Coulomb’s law strength of 2
electrical charges depends on
magnitude(size) of charge and
distance between them
 force of attraction increases as charge
increases and distance from nucleus
decreases
Periodic Trends
 Zeff increases as move across any row(period) on pt
 Zeff increases slightly as go down family bc larger e- cores are less
able to screen the outer e- from nuclear charge
Groups- columns of
elements that have
similar chemical
properties
ex: Group 1 Alkali Metals
Protons – positively
charged particle found in
nucleus= atomic number
ex: Li 3
Neutron- neutral
charged particle found in
nucleus ex: Li 4
Electron- negative
charge particle found
around in electron cloud
Ex: Li 3
Rows- horizontal
periods of elements
that increase in
atomic number
Metalloids: elements that
share properties metals and
some nonmetals ex: Boron
Metals- physical
properties include
hard, shiny, solid,
good conductor
Nonmetals- physical
properties include gas or
brittle solid, not malleable
or ductile, poor
conductors ex: Oxygen
Valence
Electronselectrons in
the outer
energy
level; each
group has
the same
number ex:
Noble gases
have 8
Homework
 pg 289 7.7 - 7.12
7.3 Sizes of Atoms
Bonding Atomic
Radius
the radius of an atom;
defined as distance
separating it from
other atoms which it is
chemically bonded
Knowing atomic radii allows us to estimate the bond
lengths between different elements in molecules…
Bond Lengths in a Molecule
Using radii for C, S, and H from Fig 7.7 predict the lengths of
the C-S,C-H, and S-H bonds in the molecule methyl
mercaptan CH3SH:
C-S = Radius of C + Radius of S
= 0.77 A + 1.02 A = 1.79 A (E=1.82)
C-H= 0.77 A + 0.37 A = 1.14 A (E=1.10)
S-H= 1.02 A + 0.37 A = 1.39 A (E=1.33)
Practice Exercise
 Predict which will be greater,
the P-Br bond in PBr3 or the
As-Cl bond in AsCl3.
As-Cl 1.19 + 0.99 = 2.18 A
P-Br 1.06 + 1.14= 2.2A
What about
Nobel Gases?
Imagine Argon gas atoms colliding
with each other in the course of
motions ricochet apart like billiard
balls
Why? When e- clouds collide they
cannot penetrate each other to any
significant extent
Nonbonding atomic radius
the closest distances separating the
nuclei during collisions determine the
apparent radii of the Ar atoms
Periodic Trends in Atomic Radii
Arrange the following in order of
increasing atomic radius: Na, Be, Mg
Be<Mg<Na
Periodic Trends in Ionic Radii
 Cations are
smaller than
parent ions
 Anions are
larger than
their parent
atoms
Isoelectronic Series
 def. groups of ions all containing the same number of electrons
 List them in order of increasing atomic number= increase in nuclear
charge= radius decreases as e- are more strongly attracted to
nucleus
Arrange the ions K+,Cl-,Ca2+, S2- in
decreasing order:
Atomic Numbers:
S(16), Cl(17), K(19), Ca(20)
S-2>Cl->K+>Ca2+
7.4 Ionization Energy
 def. minimum energy required to remove an electron from the
ground state of the isolated gaseous atom or ion
 energy must be added to remove an e-
 Every element exhibits a large increase in IE when e- are
removed from noble-gas core
 Supports idea that sharing and transfer of e- give rise to
chemical bonding and reactions- inner e- too tightly bounds to
nucleus
Which will have the lowest/highest
1st IE: B, Al, C, Si ?
 lowest: Al
 highest: C
Electron Configurations of Ions
 When e- removed from atom to form cation eremoved 1st from occupied orbitals having the largest
principal quantum number
 If there is more than one occupied subshell for a given value of
n, then e- are first removed from the orbital with the highest l
value. Ex: tin atom loses its 5p electrons before losing its 5s
electrons
 Anion- e- are added to empty or partial filled orbital
having lowest n value
Write EC for Ca2+ , S2- , Cr3+
[Ar]
[Ar]
[Ar] 4s2 3d1
7.5 Electron Affinities
 def. the energy change that occurs when an e- is added to a gaseous
atom or ion
 measure of attraction of atom for the added e energy is released when e- added the more negative the EA, the greater
the attraction of the atom for an e ex: Cl(g) + e-  Cl-(g) ΔE= -349kJ/mol
Electron Affinity > 0
 For some elements (ex Noble gases) EA is + = the anion is
higher energy than are the separated atom and eAr(g) + e-  Ar-(g) ΔE>0
 +EA means at e- will not attach itself to an Ar atom
 Ar- unstable and does NOT form
Homework
 pg 289
 7.17 – 7.55 odd