Transcript Periodic Trends

IN THE NAME OF ALLAH, THE
BENEFICENT THE MERCIFUL
Chapter 6
Ionic Bonds and some Main
Group Chemistry
By
Dr AYESHA MOHY-UD-DIN
Periodic Law
• States: when elements are arranged by
increasing atomic number, there is a
periodic repetition of their physical and
chemical properties
Chemical Bond! Why it is formed
• Forces that hold the atoms together in a
compound or molecule are called chemical
bonds.
• Atoms form bond whenever there is a net
decrease in their energy and an increased
attraction.
Electronic Configuration
(Arrangement of electrons)
Alkali Metals
Lithium (Li)
1s22s1
Sodium (Na)
1s22s22p63s1
Potassium (K)
1s22s22p63s23p64s1
Alkaline Earth Metals
Beryllium (Be)
Magnesium (Mg)
Calcium (Ca)
1s22s2
1s22s22p63s2
1s22s22p63s23p64s2
Halogens
Fluorine
(F)
1s22s22p5
Chlorine
(Cl)
1s22s22p63s23p5
Bromine
(Br)
1s22s22p63s23p64s23d104p5
Noble Gases
Neon
(N)
1s22s22p6
Argon
(Ar)
1s22s22p63s23p6
Krypton 1s22s22p63s23p64s23d104p6
(Kr)
6.3
Trends in Atomic Size
•The atomic radius is one half of the distance
between the nuclei of two atoms of the same
element when the atoms are joined.
Trends in Atomic Size
• Atomic radius is ½ the
distance between the
nuclei of 2 like atoms
• As you move DOWN
a group, atomic size
increases
• As you move LEFT to
RIGHT across a
period size decreases
WHY?
As you move across the
period
effective
nuclear
charge (Zeff) increases. Atom
gain electrons and protons.
More protons hold electrons
tighter. Size Decreases
As you move down the group
successive
shells/orbitals
are
occupied. atom gain more electrons
which are in far orbitals away from the
nucleus.
The nucleus has less
influence and the inner shell electrons
shield outer electrons from the
attraction of nucleus and the net Zeff
decreases. Size Increases
6.3
Ions & Ionic Radii (IR)
•Metals elements lose valence electrons to
form cations. Cation radii (IR) are always
smaller than atomic radii (AR) [i.e. IR < AR]
•The electron/proton attraction has gone UP
and so size DECREASES
6.3
Ions & Ionic Radii (IR)
•Non-metal elements gain valence electrons to
form anion ions. Anion radii (IR) are always
larger than atomic radii (AR) [i.e. IR > AR]
•The electron/proton attraction has gone
DOWN and so size INCREASES.
6.3
Trends in Ionic Size
• Relative Sizes of Some Atoms and Ions
6.3
Trends in Ionic Size
Size generally increases
• Trends in Ionic Size
Ionization Energy (Ei) & its Trends
• Ionization Energy is the energy required
to remove outermost electron from an
isolated gaseous atom.
• Energy is usually absorbeded (+ve sign)
• As you move DOWN a group ionization
energy DECREASES
• As you move LEFT to RIGHT across a
period ionization energy INCREASES
6.3
Trends in Ionization Energy
Ionization energy vs. atomic number
He
Ionization energy (kJ/mol)
2500
Ne
2000
Ar
F
1500
N
H
Cl
C
Be
1000
O
P S
B
500
Mg Si
Al
Li
Ca
Na
K
0
0
2
4
6
8
10
12
Element
14
16
18
20
Why?
11p+
12n°
11p+
12n°
10p+
10n°
Na has 11
electrons
Na+ has 10
electrons
Ne has 10
electrons
electron configuration of Na+ resembles Ne
Alkali metals become like noble gases.
OCTET is completed.
Higher Ionization Energies
• Ionization is not limited to the removal of a
single electron from an atom. Two, three, or
even more electrons can be removed
sequentially from an atom.
Mg (g) + 735 kJ ---> Mg+ (g) + eMg+ (g) + 1451 kJ ---> Mg2+ (g) + e• successively larger amounts of energy are
required for each successive ionization step
because it is much harder to remove a
egatively charged electron from a positively
charged ion than from a neutral atom.
Mg2+ (g) + 7733 kJ ---> Mg3+ (g) + eEnergy cost is very high to dip into a lower shell
Electron Affinity (Eea) & its Trends
An element’s electron affinity is the energy
change that occurs when an electron is added
to an isolated atom in the gaseous state.
Energy is usually evolved (-ve sign)
The more negative the Eea, the greater the
tendency of the atom to accept an electron and
the more stable the anion that results.
Cl(g) + e- ---> Cl-(g) ∆E= -384.6 kJ/mol
Trends in Electron Affinity (Eea)
• As you move DOWN a group electron
affinity DECREASES
• As you move LEFT TO RIGHT across a
period electron affinity INCREASES
• An atom that forms an unstable anion by
addition of an electron has, in principle,
a positive value of but no experimental
measurement can be made in such
circumstances. All we can say is that the
for such an atom is greater than zero.
Summary of Periodic Trends
Lattice Energy
For an ionic compound the
+
+
lattice enthalpy is the enthalpy
+
+
+
change when one mole of solid
+
+ in its standard state is formed
from its ions in the gaseous
state.
The lattice enthalpy cannot be
measured directly and so we make use
of other known enthalpies and link
them together with an enthalpy cycle.
This enthalpy cycle is the Born-Haber cycle.
kJmol-1
Born-Haber Cycle for Sodium
Na (g) + Cl(g)
Chloride
+
-348.6 kJ/mol
495.8 kJ/mol
Na+(g) + Cl-(g)
Na(g) + Cl(g)
122 kJ/mol
Lattice Energy for
Sodium Chloride
107.3 kJ/mol
Na(g) + 1/2 Cl2(g)
107.3 kJ/mol
0
Na(s) + 1/2 Cl2(g)
H = -786 kJ/mol
L
NaCl(s)
-
+
-
+
-
+
-
+
-