Transcript ionization energy

If you hold a cat by the tail you learn things you cannot learn
any other way.” - Mark Twain-
PERIODIC TABLE TRENDS
TO UNDERSTAND THE PROPERTIES OF ATOMS OF AN
ELEMENT, WE NEED TO KNOW THE ELECTRON
CONFIGURATION OF THE ATOMS, ESPECIALLY HOW
MANY VALENCE (OUTER) ELECTRONS THE ATOM
HAVE.
WE ALSO NEED TO UNDERSTAND HOW STRONGLY
THE OUTER ELECTRONS ARE ATTRACTED BY THE
PROTONS IN THE NUCLEUS.
THE ELECTROSTATIC FORCE OF ATTRACTION
BETWEEN CHARGED OBJECTS IS GIVEN BY
COULOMB’S LAW AS
F = kQ1Q2/d2
WHERE F = FORCE, k = CONSTANT, Q1 and Q2 are
charges, and d = distance of separation
SO, THE FORCE OF ATTRACTION BETWEEN THE
NUCLEUS AND AN OUTER ELECTRON DEPENDS ON
THE CHARGE ON THE NUCLEUS AND THE DISTANCE
BETWEEN THE ELECTRON AND THE NUCLEUS.
THE FORCE OF ATTRACTION INCREASES AS THE
NUCLEAR CHARGE INCREASES AND DECREASES AS
THE ELECTRON MOVES FURTHER FROM THE NUCLEUS.
NOW, IT WOULD BE DIFFICULT TO CALCULATE THE
ACTUAL FORCE AN OUTER ELECTRON EXPERIENCES
IN AN ATOM WITH A NUMBER OF ELECTRONS.
BUT, WE CAN GET AN IDEA BY THINKING IN TERMS
OF EFFECTIVE NUCLEAR CHARGE.
THINK OF THE INNER ELECTRONS AS SHIELDING
THE OUTER ELECTRONS FROM THE NUCLEUS.
ANOTHER PICTURE THAT ILLUSTRATES THIS EFFECT:
SO, WE COULD THINK OF THE CHARGE AN OUTER
ELECTRON MIGHT BE SEEING AS:
Zeff = Z – S
WHERE Z = ATOMC NUMBER
S = # OF INNER ELECTRONS
LET’S TAKE A COUPLE OF EXAMPLES:
SODIUM HAS AN ATOMIC NUMBER OF 11 AND ITS
ELECTRON CONFIGURATION IS
1s22s22p63s1
Inner
electrons
SO, Zeff = 11-10 = 1
IF WE TAKE NEON, Ne, WHICH HAS Z = 10, THE
ELECTRON CONFIGURATION IS:
1s22s22p6
Inner electrons
SO, THE EFFECTIVE NUCLEAR CHARGE AN
OUTER ELECTRON WOULD EXPERIENCE
IS:
Zeff = 10 – 2 = 8
THIS SHIELDING EFFECT BY INNER ELECTRONS
WOULD HELP EXPLAIN HOW ATOMIC SIZE VARIES
IN THE PERIODIC TABLE.
THERE ARE TWO TRENDS IN ATOMIC SIZE:
1. WITHIN A GROUP (FAMILY) THE SIZE TENDS TO
INCREASE AS WE MOVE FROM TOP TO BOTTOM. AS
WE MOVE DOWN IN A GROUP, THE OUTER
ELECTRONS ARE IN SHELLS WITH HIGHER PRINCIPAL
QUANTUM NUMBERS, n. THE OUTER ELECTRONS
SPEND MORE TIME FURTHER FROM THE NUCLEUS.
2. WITHIN A GIVEN ROW, THE ATOMIC SIZE TENDS
TO DECREASE AS WE MOVE FROM LEFT TO RIGHT.
THE EFFECTIVE NUCLEAR CHARGE, Zeff , IS
INCREASING, AND THE ATTRACTION FOR THE
ELECTRONS BY THE NUCLEUS IS INCREASING.
COMMENTS ON ION SIZE:
1.CATIONS (POSITIVE IONS) ARE SMALLER THAN
THEIR PARENT ATOMS. OUTER ELECTRONS HAVE
BEEN LOST, AND THE NUCLEAR CHARGE REMAINS
THE SAME. THERE IS ALSO LESS ELECTRONELECTRON REPULSION.
2.ANIONS ARE LARGER THAN THEIR PARENT ATOMS.
THE NUCLEAR CHARGE REMAINS THE SAME, AND
THERE IS MORE ELECTRON-ELECTRON REPULSION.
3.AS YOU MOVE DOWN IN A GROUP OR FAMILY, ION
SIZE INCREASES.
IONIZATION ENERGY
THE IONIZATION ENERGY IS THE ENERGY REQUIRED
TO REMOVE AN ELECTRON FROM A GASEOUS ATOM.
Na (g)  Na+ (g) + eREMOVAL OF SUCCESSIVE ELECTRONS WOULD
REQUIRE MUCH GREATER ENERGIES.
LET’S TAKE A LOOK AT HOW FIRST IONIZATION
ENERGIES (ENERGY TO REMOVE THE FIRST
ELECTRON) VARY IN THE PERIODIC TABLE.
THE FACTORS THAT INFLUENCED ATOMIC SIZE ALSO
INFLUENCE IONIZATION ENERGIES. THE ENERGY
NEEDED TO REMOVE AN ELECTRON FROM THE OUTER
SHELL DEPENDS BOTH ON THE EFFECTIVE NUCLEAR
CHARGE AND THE DISTANCE OF THE ELECTRON FROM
THE NUCLEUS.
As proton # increases. More protons means
greater attraction between nucleus and outer
electron thus higher ionization energy.
The greater attraction also means that outer
electrons are brought closer to the nucleus,
thus smaller atomic radius results.
Li (enc = 1)
++
+
Be (enc = 2)
++
+
+
B (enc = 3)
+++
++
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
Atomic radius vs. atomic number
Atomic Radius (pm)
250
K
200
Na
Li
150
Mg
Al Si
Be
100
Ca
P S Cl
B C N
O F
Ar
Ne
50
H
0
0
He
2
4
6
8
10
12
Element
14
16
18
20
ATOMS WITH LOW IONIZATION ENERGIES WILL TEND
TO FORM POSITIVE IONS. THOSE WITH HIGH
IONIZATION ENERGIES WILL NOT.
THE ELECTRON AFFINITY IS THE ENERGY GAIN WHEN
A NEUTRAL GASEOUS ATOM GAINS AN ELECTRON.
Cl (g) + e-  Cl- (g)
DE = -349 kJ/mole
Note: the negative sign indicates that energy is released.
The greater the attraction between an atom and an
electron, the more negative the electron affinity.
THE ELECTRON AFFINITIES FOR THE NOBLE GASES ARE
GREATER THAN ZERO, AS IT WOULD REQUIRE PUTTING
AN ELECTRON IN A HIGHER SHELL.
THE ELECTRON AFFINITIES FOR GROUP 5A ELEMENTS
ARE UNUSUAL. THESE ATOMS HAVE HALF-FILLED P
SUBSHELLS, SO AN ADDITIONAL ELECTRON WOULD
HAVE TO GO INTO A ORBITAL THAT IS ALREADY
OCCUPIED WITH ONE ELECTRON.
NOW, WHAT DO IONIZATION POTENTIALS AND
ELECTRON AFFINITIES TELL US ABOUT THE ELEMENTS.
ELEMENTS WHOSE ATOMS HAVE LOW IONIZATION
POTENTIALS AND LOW ELECTRON AFFINITIES WILL
TEND TO LOSE ELECTRONS AND FORM POSITIVE IONS.
THESE ARE METALS.
ELEMENTS WHOSE ATOMS HAVE HIGH IONIZATION
POTENTIALS AND HIGH ELECTRON AFFINITIES WILL
TEND TO GAIN ELECTRONS AND FORM NEGATIVE
IONS.
THESE ARE NONMETALS.
METALS
1.METALS TEND TO HAVE LOW IONIZATION ENERGIES
AND TEND TO FORM POSITIVE IONS
2.HAVE A SHINY LUSTER AND MOST ARE SILVERY
3.SOLIDS ARE MALLEABLE AND DUCTILE
4.GOOD CONDUCTORS OF HEAT AND ELECTRICITY
5.TEND TO FORM CATIONS IN AQUEOUS SOLUTION
6.MOST METAL OXIDES ARE IONIC SOLIDS THAT ARE
BASIC
NONMETALS
1.NONMETALS HAVE HIGH ELECTRON AFFINITIES AND
TEND TO GAIN ELECTRONS TO FORM NEGATIVE IONS.
2.DO NOT HAVE A LUSTER.
3.SOLIDS ARE USUALLY BRITTLE.
4.POOR CONDUCTORS OF HEAT AND ELECTRICITY.
5.MOST NONMETAL OXIDES ARE MOLECULAR
SUBSTANCES THAT FORM ACIDIC SOLUTIONS.
METALLOIDS HAVE PROPERTIES BETWEEN THOSE OF
METALS AND NONMETALS. A GOOD EXAMPLE IS
SILICON. IT LOOKS LIKE A METAL, BUT IS VERY
BRITTLE AND IS A NONCONDUCTOR OF ELECTRICITY.
Definition
ATOMIC
RADIUS
RADIUS OF ATOM
IONIZATION THE ENERGY REQUIRED TO
ENERGY
REMOVE THE OUTERMOST
ELECTRON FROM AN ATOM
ELECTRON
AFFINITY
IONIC
RADIUS
Trend
INCREASES AS YOU
MOVE DOWN IN A
GROUP AND FROM
RIGHT TO LEFT ON
PERIODIC TABLE
INCREASES AS YOU
MOVE UP AND TO THE
RIGHT
ENERGY RELEASED WHEN AN
ELECTRON IS ADDED TO THE
OUTERMOST SHELL OF AN
ATOM
INCREASES IN
MAGNITUDE AS YOU
MOVE UP AND TO THE
RIGHT
RADIUS OF ION
•INCREASES DOWN IN
A GROUP
•CATIONS DECREASE
LEFT TO RIGHT
•ANIONS DECREASE
LEFT TO RIGHT