PERIODIC
TRENDS
1

Period Law-
physical and chemical properties of elements are a periodic function of their atomic numbers
2

General Periodic Trends
• Atomic and ionic size
• Ionization energy
• Electron affinity, electronegativity
3

Effective Nuclear Charge
Z*
4

Effective Nuclear Charge, Z*
• Atom
• Li
• Be
• B
• C
• N
• O
• F
Z* Experienced by Electrons in
Valence Orbitals
+1.28
-------
+2.58
+3.22
+3.85
Increase in
Z* across a period
+4.49
+5.13
5

General Periodic Trends
Higher effective nuclear charge
Electrons held more tightly
6
Larger orbitals.
Electrons held less tightly.

Lithium
Periodic Trend in the
Reactivity of Metals
7
Sodium
Potassium
MOST

2. Reactivity for Metals
 As you go down a group for metals the number of energy levels increase.
 Because of this, reactivity increases because the atom is more willing to give away its electron (react).

3.Nonmetalic Trends: Gain electrons
 Nonmetals on right side, form anions
 Going right elements are more nonmetallic (better gainers of electrons)
 Going UP elements become more nonmetallic (want to gain)

8. Reactivity nonmetals: Gain e
 The reason Across = fill the energy level
 Going UP a group, nonmetals have same valence but fewer total electrons
 Flourine is the most reactive nonmetal.

Atomic Radii
11
Figure 8.9

Atomic Size
• Size increases , down a group.
• Because electrons are added into additional energy levels, there is less attraction.
• Size decreases across a period.
• Because, increased effective nuclear charge.
12

Atomic Size
Size decreases across a period owing to increase in Z*. Each added electron feels a greater and greater + charge.
13
Large Small

Trends in Atomic Size
See Figures 8.9 & 8.10
Radius (pm)
250
K
200
1st transition series
2nd period
Li
3rd period
Na
150
Kr
100
Ar
Ne
50
0
0
He
5 10 15 20
Atomic Number
25 30 35 40
14

Ion Sizes
Li,152 pm
3e and 3p
+
Li +, 78 pm
2e and 3 p
Forming a cation.
• CATIONS are SMALLER than the atoms from which they come.
• The electron/proton attraction has gone UP and so size
DECREASES .
15

Ion Sizes
-
F, 71 pm
9e and 9p
F- , 133 pm
10 e and 9 p
Forming an anion.
• ANIONS are LARGER than the atoms from which they come.
• The electron/proton attraction has gone DOWN and so size INCREASES .
• Trends in ion sizes are the same as atom sizes.
16

Trends in Ion Sizes
17
Figure 8.13

Ionization Energy
IE = energy required to remove an electron from an atom in the gas phase.
18
Mg (g) + 738 kJ ---> Mg + (g) + e-

Ionization Energy
IE = energy required to remove an electron from an atom in the gas phase.
Mg (g) + 738 kJ ---> Mg + (g) + e-
19
Mg + (g) + 1451 kJ ---> Mg 2+ (g) +
Mg + eelectrons. Therefore, IE for Mg + > Mg.

Ionization Energy
1 st IE: Mg (g) + 735 kJ ---> Mg + (g) + e-
2 nd IE: Mg + (g) + 1451 kJ ---> Mg 2+ (g) + e-
20
3 rd IE: Mg 2+ (g) + 7733 kJ ---> Mg 3+ (g) + e-
Energy cost is very high to dip into a shell of lower n (core electrons).
This is why ox. no. = Group no.

Trends in Ionization Energy
2500
1st Ionization energy (kJ/mol)
He
Ne
2000
Ar
1500 Kr
1000
21
500
0
1 3
H Li
5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35
Na K
Atomic Number

Trends in Ionization Energy
• IE decreases down a group
• Because size increases.
• IE increases across a period
• Because effective nuclear charge increases
22

A few elements GAIN electrons to form anions .
Electron affinity is the energy involved when an atom gains an electron to form an anion.
X(g) + e- ---> X (g) E.A. = ∆E
23

Trends in Electron Affinity
24

Trends in Electron Affinity
25
• Affinity for electron increases across a period (EA becomes more negative).
Atom EA
F -328 kJ
Cl -349 kJ
Br -325 kJ
I -295 kJ
• Affinity decreases down a group (EA becomes less negative).

Electron Affinity of Oxygen
O atom [He]
     
+ electron
O
- ion [He]
 
EA = - 141 kJ
∆E is
EXO thermic because O has an affinity for an e-.
26

Electron Affinity of Nitrogen
N atom [He]
 
N ion [He]
 
  

+ electron
 
∆E is zero for N due to electronelectron repulsions.
EA = 0 kJ
27

• So how is this different from electron affinity?
• Electron Affinity – is rating of how well an atom wants to gain an electron
• Electronegativity – is rating of how well an atom keeps the electron once it is bonded to another atom
28

29

Electron Configurations and the Periodic Trends
30

B
• Periodic table
31