1. Periodic Trends!

3. Periodic Law  When elements are arranged in order of increasing atomic number, there is a periodic pattern in their physical and chemical properties.

4.  There are many trends in the periodic table, but you must know the reasoning behind 3 of them

5. Atomic Radius:  Gives us an idea of the size of an atom  The trend:  A.R. increases as you move down a group  A.R. decreases as you move across a period from left to right

7. Why?  As atomic # increases, the number of protons and electrons increases  To accommodate the electrons, there must be more energy levels.  This is why A.R. increases as you move down a group.  As you go across a period, an atom is not gaining energy levels  The atom is still gaining protons and electrons, though, so the atom becomes more and more dense  This causes the electrons to be drawn closer to the positive nucleus.  This is why A.R. decreases as you move across a period.

8. Ionization Energy  The energy required to remove an electron from the valence shell of an atom  The trend:  I.E. decreases as you go down a group  I.E. increases as you go across a period from left to right

9. Draw the arrows for the trend on the periodic table sheet!

10. Why? As you go down a group, the atomic radius increases  This means that the valence electrons in a large atom are further away from the nucleus, than the valence electrons in a small atom  If the valence electrons are far away from the positive nucleus, it will take less energy to remove them. Large atoms tend to be lower down in a group.  This is why I.E. decreases as you go down a group.  As you go across a period, atomic radius decreases  This means the electrons will be held closer and closer to the positive nucleus as you go across the period  This means that atoms of elements to the right of the periodic table will have valence electrons that are harder to remove.  This is why I.E. increases as you go across a period.

11. Electronegativity  the tendency of an atom to attract electrons to itself when it is chemically combined with other elements  The trend:  E.N. decreases as you go down a group  E.N. increases as you go across a period left to right

12. Draw the arrows for the trend on the periodic table sheet!

13. Why?  As you go down a group, the atomic radius increases  This means that in elements lower down in the group, the additional electrons will not feel a strong attraction to the nucleus, which is already surrounded by electrons.  This is why E.N. decreases as you go down a group.  As you go across a period, the atomic radius decreases  This means that in elements further to the right, additional electrons will feel a stronger attraction to the nucleus than in elements further to the left.  This is why E.N. increases as you go across a period.