1. PERIODICITY
Have you ever noticed that people in the same family have similar characteristics? For example, if all of your family has blue eyes, it is likely that you will have blue eyes also. These biological characteristics are caused by common genetic structures. In this unit, you will learn about chemical families that also exist.

2. CHEMICAL FAMILIES
Chemical similarities are caused by elements having the same number of Valence Electrons.
Valence electrons are the electrons in the outer energy level of an atom.

3. Family
# valence electrons
Alkali Metals (IA) 1
Alkaline Earths (IIA) 2
Halogens (VIIA) 7
Noble Gases (VIIIA) 8

4. In this section you have learned:
Chemical properties are determined by the number of valence electrons.
The number of valence electrons is a periodic property.
Elements with similar chemical properties are arranged in columns (called families) on the periodic table.

5. Periodic Nature of Atomic Radius
The size of the atom is measured by measuring the distance between the nuclei of two neighboring atoms in a solid.
Half of this distance is called the atomic radius.

6. There are 2 properties of an atom that affect the radius:
The number of energy levels.
The attraction of the outer electron for the positive nucleus.
To help understand how these two properties affect the radius, let’s look at the periodic table.

7. What pattern do you see?
Element
# Energy Levels
Radius H 1
.32 Li 2
1.23 Na 3
1.54 K 4
2.03 Rb 5
2.16 Cs 6
2.35

8. What pattern did you see?
As you go down a column on the periodic table, each row is another energy level. More energy levels means the valence electrons are further from the nucleus and the atom has a larger radius.
This pattern holds true for all columns on the periodic table.

9. What pattern do you see??? I II
III IV V VI
VII
VIII Li Be B C N O F Ne
# protons 3 4 5 6 7 8 9 10
Radius
152
112 85 77 75 73 72 71
These are the elements in row 2 of the periodic table, each has 2 energy levels.
As the positive pull on the nucleus gets stronger, the radius gets smaller.

10. Let’s try a problem!
Which is larger, an atom of silver or an atom of gold?
There are 2 properties of an atom that affect the radius:
#1 - The number of energy levels.
#2 - The attraction of the outer electron for the positive nucleus.
Check the number of energy levels first.
Silver is in row 5, so it has 5 energy levels.
Gold is in row 6, so it has 6 energy levels.
An atom of gold has a larger radius than an atom of silver.

11. How about another problem?
Which is larger, an atom of silver or an atom of tin?
There are 2 properties of an atom that affect the radius:
#1 - The number of energy levels.
#2 - The attraction of the outer electron for the positive nucleus.
Check the number of energy levels first.
Silver is in row 5, so it has 5 energy levels.
Tin is in row 5, so it has 5 energy levels.

12. Are they the same size?
NO, let’s check the The attraction of the outer electron for the positive nucleus.
Silver has 47 protons and tin has 50 protons.
Therefore, the nucleus of tin attracts the electrons more, pulls them in closer and is therefore a smaller atom.

13. Ionization Energy Apply the same ideas to the ionization energy.
There are 2 properties of an atom that affect the ionization energy:
#1 - The number of energy levels.
#2 - The attraction of the outer electron for the positive nucleus.
Which has a higher ionization energy? Ca or Ba?
Hint: Remember that higher ionization energy means it is harder to remove an electron.

14. Calcium’s IE is higher 1st, let’s look at the number of energy levels.
Ca has 4 energy levels.
Ba has 6 energy levels.
Since the electrons in Ca are much closer to the nucleus, it is harder to remove the electrons.
Therefore, Ca has a higher ionization energy than Ba.

15. Lower energy means easier
Which element has the lowest ionization energy, Aluminum or Chlorine?
There are 2 properties of an atom that affect the ionization energy:
#1 - The number of energy levels.
#2 - The attraction of the outer electron for the positive nucleus.
Aluminum and Chlorine both have 3 energy levels, they are both in row 3.

16. So check the number of protons!
Al has 13 protons and Cl has 17 protons.
With more protons, the Cl will pull in the electrons more and make it harder to remove the electrons.
Therefore, Al has the lower ionization energy.

17. Ions Ions are charged particles. They can be made by losing electrons.
Ag  Ag+ + 1e-1
They can be made by gaining electrons.
Br + 1e-1  Br-1

18. Other Periodic Tables