Transcript Periodic Trends

Chapter 3 Atoms and Elements
3.8
Periodic Trends
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
1
Valence Electrons
The valence electrons
• determine the chemical properties of the elements.
• are the electrons in the highest energy level.
• are related to the group number of the element.
Example: Phosphorus has 5 valence electrons.
5 valence electrons
P Group 5A(15)
2, 8, 5
2
Groups and Valence Electrons
All the elements in a group have the same number of
valence electrons.
Example: Elements in group 2A(2) have two (2)
valence electrons.
Be
2, 2
Mg
2, 8, 2
Ca
2, 8, 8, 2
Sr
2, 8, 18, 8, 2
3
Periodic Table and Valence
Electrons
Representative Elements Group Numbers
1
2
3
4
5
6
7
H
1
Li
2,1
Be
2,2
Li
Mg
2,8,1 2,8,2
Al
2,3
C
2,4
N
2,5
Ge
Si
2,8,3 2,8,4
P
2,8,5
O
2,6
F
2,7
S
Cl
2,8,6 2,8,7
8
He
2
Ne
2,8
Ar
2,8,8
4
Learning Check
State the number of valence electrons for each.
A. O
1) 4
2) 6
3) 8
B. Al
1) 13
2) 3
3) 1
2) 5
3) 7
C. Cl
1) 2
5
Solution
State the number of valence electrons for each.
A. O
2) 6
B. Al
2) 3
C. Cl
3) 7
6
Learning Check
State the number of valence electrons for each.
A. calcium
1) 1
2) 2
3) 3
B. group 6A (16)
1) 2
2) 4
3) 6
C. tin
1) 2
2) 4
3) 14
7
Solution
State the number of valence electrons for each.
A. calcium
2) 2
B. group 6A (16)
3) 6
C. tin
2) 4
8
Learning Check
State the number of valence electrons for each.
A. 2, 8, 5
B. 2, 8, 8, 2
C. 2, 7
9
Solution
State the number of valence electrons for each.
A. 2, 8, 5
5
B. 2, 8, 8, 2
2
C. 2, 7
7
10
Electron-Dot Symbols
An electron-dot symbol
• shows the valence electrons
around the symbol of the
element.
• for Mg has two valence
electrons as single dots on the
sides of the symbol Mg.
.
.
·Mg · or Mg · or ·Mg or ·Mg
·
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
11
Writing Electron-Dot Symbols
Electron-dot symbols for
• groups 1A(1) to 4A(14) use single dots.
·
·
Na ·
· Mg ·
· Al ·
·C·
·
• groups 5A(15) to 7A(17) use pairs and single dots.
··
··
··
·P·
·
:: O ·
: Cl :
·
·
12
Groups and Electron-Dot Symbols
In a group, all the electron-dot symbols have the
same number of valence electrons (dots).
Example: Atoms of elements in Group 2A(2) each
have 2 valence electrons.
· Be ·
· Mg ·
· Ca ·
· Sr ·
· Ba ·
13
Learning Check

A.
X is the electron-dot symbol for
1) Na
2) K
3) Al

B.

X

1) B
is the electron-dot symbol of
2) N
3) P
14
Solution

A.
X is the electron-dot symbol for
1) Na
2) K

B.

X


is the electron-dot symbol of
2) N
3) P
15
Atomic Size
Atomic size is described using the atomic radius; the
distance from the nucleus to the valence electrons.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
16
Atomic Radius Within A Group
Atomic radius increases
going down each group
of representative
elements.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
17
Atomic Radius Across a Period
Going across a period left to right,
• an increase in number of protons increases attraction
for valence electrons.
• atomic radius decreases.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
18
Learning Check
Select the element in each pair with the larger atomic
radius.
A.
B.
C.
Li or K
K or Br
P or Cl
19
Solution
Select the element in each pair with the larger atomic
radius.
A. K
B. K
C. P
20
Ionization Energy
Ionization energy is the energy it takes to remove a
valence electron.
Na(g) + Energy (ionization)  Na+(g) + e-
21
Ionization Energy In a Group
Going up a group of
representative elements,
• the distance decreases
between nucleus and
valence electrons.
• the ionization energy
increases.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
22
Ionization Energy
• Metals have
•
lower
ionization
energies.
Nonmetals
have higher
ionization
energies.
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
23
Learning Check
Select the element in each pair with the higher ionization
energy.
A.
B.
C.
Li or K
K or Br
P or Cl
24
Solution
Select the element in each pair with the higher ionization
energy.
A.
B.
C.
Li
Br
Cl
25