lecture 0802 trends on the periodic table. periodic trends li na k
TRANSCRIPT
Lecture 0802Trends on thePeriodic Table
PERIODIPERIODIC C
TRENDSTRENDS
PERIODIPERIODIC C
TRENDSTRENDS
Li
Na
K
Effective Nuclear Charge
Z*The 2s electron PENETRATES
the region occupied by the 1s electron.
2s electron experiences a higher positive charge than expected.
EffectiveEffective Nuclear Charge, Nuclear Charge, Z*Z*
Atom Z* e- in Valence Orbitalsest measured
Li 1 +1.28 Be 2 ------- B 3 +2.58 C 4 +3.22 N 5 +3.85 O 6 +4.49 F 7 +5.13
Increase in Increase in Z* across a Z* across a periodperiod
General Periodic Trends Atomic and ionic sizeAtomic and ionic size
Ionization energyIonization energy Electron affinityElectron affinity
Higher effective nuclear chargeElectrons held more tightly
Larger shells.Electrons held lesstightly.
Atomic Radius
Is taken as the covalent radius for non-metallic elements and as the metallic radius for metals
Atomic Radius Covalent radius is one-half
the distance between the nuclei of two identical atoms that are singly bonded to one another.
Chlorine Bond Length
Atomic Radius Covalent radii for elements
whose atoms do not bond to one another can be estimated by combining radii of those that do with the distances between unlike atoms in various molecules.
Atomic Radius
Metallic radius is one-half the closest internuclear distance in a metallic crystal.
Prediction!
0
50
100
150
200
1 3 5 7 9 11 13 15 17 19
Atomic Number
Ato
mic
Rad
ius
(pm
)
Atomic Size
Atomic Size
Size goes UP on going down a group.
Because electrons are added further from the nucleus, there is less attraction.
Size goes DOWN on going across a period.
Size goes UP on going down a group.
Because electrons are added further from the nucleus, there is less attraction.
Size goes DOWN on going across a period.
Atomic SizeAtomic SizeSize decreases across a period
owing to increase in Z*. Each added electron feels a greater and greater + charge.
LargeLarge SmallSmall
Trends in Atomic SizeTrends in Atomic Size
0
50
100
150
200
250
0 5 10 15 20 25 30 35 40
Li
Na
K
Kr
He
NeAr
2nd period
3rd period 1st transitionseries
Radius (pm)
Atomic Number
0
50
100
150
200
250
0 5 10 15 20 25 30 35 40
Li
Na
K
Kr
He
NeAr
2nd period
3rd period 1st transitionseries
Radius (pm)
Atomic Number
Sizes of Transition Elements
Sizes of Transition Elements
3d subshell is inside the 4s subshell.3d subshell is inside the 4s subshell. 4s electrons feel a more or less 4s electrons feel a more or less
constant Z*.constant Z*. Sizes stay about the same andSizes stay about the same and
chemistries are similar!chemistries are similar!
General Periodic Trends Atomic and ionic size
Ionization energy Electron affinity
Higher effective nuclear chargeElectrons held more tightly
Larger shells.Electrons held lesstightly.
Ion SizesIon SizesIon SizesIon Sizes
Li,152 pm3e and 3p
Li+, 60 pm2e and 3 p
+Does the size goDoes the size goup or down when up or down when losing an electron to losing an electron to form a cation?form a cation?
Does the size goDoes the size goup or down when up or down when losing an electron to losing an electron to form a cation?form a cation?
Ion SizesIon Sizes
CATIONS are SMALLER than the atoms from which they come.
The proton/electron attraction has gone UP and so size DECREASES.
Li,152 pm3e and 3p
Li +, 78 pm2e and 3 p
+Forming a
cation.Forming a
cation.
Ion SizesIon Sizes
F,64 pm9e and 9p
F- , 136 pm10 e and 9 p
-Does the size go up or down when gaining an electron to form an anion?
Does the size go up or down when gaining an electron to form an anion?
Ion SizesIon Sizes
ANIONS are LARGER than the atoms from which they come.
The proton/electron attraction has gone DOWN and so size INCREASES.
Forming an anion.
Forming an anion.F, 71 pm
9e and 9pF-, 133 pm10 e and 9 p
-
Trends in Ion Sizes
Trends in Ion Sizes
Trends in ion sizes are the same as atom sizes.
Redox Reactions
Redox Reactions
Why do metals lose electrons in their reactions?
Why does Mg form Mg2+ ions and not Mg3+?
Why do nonmetals take on electrons?
Why do metals lose electrons in their reactions?
Why does Mg form Mg2+ ions and not Mg3+?
Why do nonmetals take on electrons?
Ionization Energy (General)
Is the energy required to remove the outermost electron from an atom or a positive ion in the ground state.
First Ionization Energy
Energy required to remove the first electron from a neutral atom in the gaseous state.
Ionization EnergyIonization Energy
Mg (g) + 738 kJ Mg+ (g) + e-
Prediction!
0
500
1000
1500
2000
2500
1 3 5 7 9 11 13 15 17 19
Atomic Number
1st
Ion
izat
ion
En
erg
y (
kJ/m
ol)
Trends in Ionization EnergyTrends in Ionization EnergyTrends in Ionization EnergyTrends in Ionization Energy
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 350
500
1000
1500
2000
2500
1st Ionization energy (kJ/mol)
Atomic NumberH Li Na K
HeNe
ArKr
Atomic RadiiAtomic Radii
Trends in Ionization Energy
Trends in Ionization Energy
IE increases across a period because Z* increases.
Metals lose electrons more easily than nonmetals.
Metals are good reducing agents.
Nonmetals lose electrons with difficulty.
Trends in Ionization Energy
Trends in Ionization Energy
IE decreases down a group
Because size increases. Reducing ability
generally increases down the periodic table.
Remember Li, Na, K
Second Ionization Energy
Energy needed to remove the outermost electron from a +1 ion.
Energy needed to remove the second electron from a neutral atom.
Ionization EnergyIonization EnergyMg (g) + 738 kJ Mg+ (g) + e-
MgMg+ + (g) + 1451 kJ (g) + 1451 kJ Mg Mg2+2+ (g) + e (g) + e-
Mg+ has 12 protons and only 11 electrons. Therefore, IE for Mg+ > Mg.
0
1000
2000
3000
4000
5000
6000
7000
8000
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Atomic Number
2nd
Io
niz
atio
n E
ner
gy
(kJ/
mo
l)
Ionization EnergyIonization EnergyMg (g) + 735 kJ Mg+ (g) + e-
Mg+ (g) + 1451 kJ Mg2+ (g) + e-
MgMg2+2+ (g) + 7733 kJ (g) + 7733 kJ Mg Mg3+3+ (g) + e (g) + e--
Energy cost is very high to dip into a shell of lower n.
This is why ox. no. = Group no.
General Periodic Trends Atomic and ionic size
Ionization energy Electron affinity
Higher effective nuclear chargeElectrons held more tightly
Larger shells.Electrons held lesstightly.
Electron Affinity A few elements GAIN
electrons to form anions. E.A. is the energy released or
absorbed when an electron is added to the valence level of a gas-phase atom.
A(g) + e- A-(g) E.A. = ∆E
Prediction!
Trends in Electron AffinityTrends in Electron Affinity
Electron Affinity of Oxygen
∆E is EXOthermic because O has an
affinity for an e-.
[He] O atom
EA = - 141 kJ
+ electron
O [He] - ion
Electron Affinity of Nitrogen
∆E is zero for N- due to electron-electron repulsions.
EA = 0 kJ
[He] N atom
[He] N- ion
+ electron
See Figure 8.12 and Appendix F
Affinity for electron increases across a period (EA becomes more negative).
Affinity decreases down a group (EA becomes less negative).
Atom EAAtom EAFF -328 kJ-328 kJClCl -349 kJ-349 kJBrBr -325 kJ-325 kJII -295 kJ-295 kJ
Atom EAAtom EAFF -328 kJ-328 kJClCl -349 kJ-349 kJBrBr -325 kJ-325 kJII -295 kJ-295 kJ
Trends in Electron Affinity
General Periodic Trends Atomic and ionic size
Ionization energy Electron affinity
Higher effective nuclear chargeElectrons held more tightly
Larger shells.Electrons held lesstightly.