CH225.4
Today:
• The Periodic Table
• Atomic radius
(p. 18 – 25 of the textbook)
Read Lecture 5 for next class
Orbital energies
The Periodic Table lists all elements in the order of increasing number of electrons in
the atoms, grouped by shells (periods). This is the order of increasing orbital
energies when they are first filled (bottom up). Note that 4s orbital fills before 3d in
Period 4. ns and (n-1)d orbitals have similar energies in transition metals.
Periodic Table
Shell
(n = 7)
Period 7
(n = 6)
Period 6
(n = 5)
Period 5
(n = 4)
Period 4
(n = 3)
Period 3
(n = 2)
Period 2
(n = 1)
Period 1
2
The Periodic Table
3
Hund’s rule
Electrons singly “occupy” all degenerate (and nearly degenerate) orbitals until there
is no room left. All the unpaired electrons have the same spin.
For example, carbon (Z = 6) has the following electronic configuration:
2p2
2s2
1s2
2p2
valence electrons
and not
1s2
core electrons
valence electrons
Cr, Z = 24
4s
2s2
3d
3d and 4s orbitals are
accidentally nearly
degenerate in chromium.
3p
3s
core electrons
2p
2s
1s
4
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Instructor: Nissa Garcia
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college level chemistry.
Ground State Electron Configuration: Definition & Example
Chapter 5 / Lesson 6
5
Examples of main-group elements
Oxygen, Z = 8 1s22s22p2
Silicon, Z = 14 1s22s22p63s23p2
4 electrons, 4 orbitals
In the valence shell
6 electrons, 4 orbitals
In the valence shell
10 core electrons
2 core electrons
All elements of the main groups (s & p block) have 4 orbitals in the valence shell,
6
Example of a d-element
Iron, Fe (Z = 26)
1s22s22p63s23p23d64s2
8 electrons and 9 orbitals
in the valence shell
4p
18 core electrons
All d-block elements have 9 orbitals in the valence shell: ns, np, and (n-1)d
7
Atomic radius
One of the useful properties of an element is the size of its atom.
By definition, the atomic radius of an element is half the distance between the atoms
in its homonuclear compound.
Example: Hydrogen (in H2)
Example: Carbon (in diamond)
0.74 Å
H
H
C
C
0.77 Å
0.37 Å
Example: Chlorine (in Cl2)
Cl
Cl
0.99Å
8
Atomic radius
An estimate of an A-B single bond length can be obtained by combining the atomic
radii of the atoms A and B.
Example: C-H bond length is predicted to be 0.77 + 0.37 = 1.14 Å. The experimental
values are between 1.08 and 1.10 Å.
9
Atomic radius
Atomic radii increase down a group, and within the s and p-blocks they decrease
from left to right across a period. There is the so-called lanthanide contraction
observed as a decrease in radius for elements following the f-block in Period 6.
Group 1
f-block (Lanthanides)
3d
5d
4d
I
Group 7
5d-metals
10