1. Electron Configurations and the Periodic Table.

2. Configurations Within a Group
Look at the electron configurations for the Halogens F
1s2 2s2 2p5 Cl
1s2 2s2 2p6 3s2 3p5 Br
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p5 I
1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p5
All of the elements in Group 7 end with 5 electrons in a p subshell.
In fact, every Group ends with the same number of electrons in the highest energy subshell

3. Configurations and the Periodic Table
s-block
p-block
d-block s1 s2 p1 p2 p3 p4 p5 p6 d1 d2 d3 d4 d5 d6 d7 d8 d9
d10
f-block f1 f2 f3 f4 f5 f6 f7 f8 f9
f10
f11
f12
f13
f14

4. How to remember the filling order?1s 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d
s subshells begin in level 1, so begin the s-block with “1s”
p subshells begin in level 2, so begin the p-block with “2p”
d subshells begin in level 3, so begin the d-block with “3d”
f subshells begin in level 4, so begin the f-block with “4f”

5. How to remember the filling order?1s 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d
To see the filling order of subshells, read from left to right, top to bottom!
Note that this tool shows that the 3d energy level is filled after the 4s energy level!

6. Where Does the Noble Gas Leave Off?
How do you know where to start off after using a noble gas?
Use the periodic table! 1s He 2s 2p Ne 3s 3p Ar 4s 3d 4p Kr 5s 4d 5p Xe 6s 4f 5d 6p Rn 7s 5f 6d
The noble gas fills the subshell that it’s at the end of.
Begin filling with the “s” subshell in the next row to show valence electrons.

7. Isotopes

8. What are isotopes?
Isotopes - n. Atoms of the same element with a different number of neutrons
Some isotopes are radioactive—but not all…many are quite stable!

9. Isotopes Example
Hydrogen-2
Hydrogen-1
Mass # = 1 amu
Mass # = 2 amu
If they have different number of neutrons, and neutrons have a mass of 1 amu…
Then isotopes of the same element will have different masses!
But because their protons are the same, they are the same element!

10. C C Carbon-12 Carbon-13 Identifying Isotopes
Isotopes can be differentiated by their different mass numbers in the element symbol 12 C 13 C
Carbon-12
Carbon-13
Or by the mass number following their name.

11. Mass Number versus Atomic Mass
Average Atomic Mass
# of protons + # of neutrons
Average of actual masses
Always a whole number
Not a whole number
For one specific isotope only
Weighted average of all isotopes
Is not found on the periodic table
Is found on the periodic table

12. Calculating Average Atomic Mass
Average atomic mass is a weighted average (it takes into account how often each isotope occurs).
Actual mass (not mass number)
“Sum of”
Average
atomic
mass  ( )  =
Abundance of isotope
Mass of isotope
What fraction of the time is that isotope present?

13. Example of Finding Avg Atomic Mass
Find the atomic mass of chlorine if
Chlorine-35 has a mass of amu
and Chlorine-37 has a mass of amu and is present 24.22% of the time.
Remember that percents add up to 100.
So they said the second isotope is present 24.22% of the time.
This means that the first isotope is present = 75.78% of the time
Isotope
Mass
Percent
Decimal 1
amu
75.78
0.7578 2
amu
24.22
0.2422
This chart summarizes the information in the problem:
= amu
(this is what’s on the periodic table for Cl!)