1. Periodic Trends
Section 6.3

2. The Periodic Law states...

3. Periodic Law
The pattern of properties within a period repeats as you move across a period from left to right…
When elements are arranged in order of increasing atomic number, there is a periodic repetition of their physical and chemical properties

4. Periodic Trends
Trend – a predictable change
Our focus will be on the main block elements
How do electron configurations help us explain many of the trends in properties observed?

5. Trend in Atomic Radius
Measure the molecule that forms when two atoms of the same element combine
Atomic radius = half of the distance between the nuclei of the two atoms
Measured in picometers (1 pm = m)

8. Why does this trend occur?
1. Changes in n- As the principal quantum number (n) increases, the outer electrons are farther from the nucleus, so the atoms are larger.
2. Changes in Zeff – As the effective nuclear charge increases, outer electrons are pulled closer to the nucleus, so the atoms are smaller.
(Zeff = the positive charge felt by an electron)

9. 1. Down a group, n dominates
Elements have one more energy level of core electrons
These SHIELD the outer electrons
(Shielding Effect)
Atomic radius generally INCREASES in a group from top to bottom

10. 2. Across a period, Zeff dominates
Moving across a period, electrons are added to the same outer level
Shielding by inner electrons does not change
Zeff increases and outer electrons are pulled closer
Atomic radius generally DECREASES in a period from left to right

12. Try the following… Put in order of decreasing atomic size Ca, Mg, Sr
K, Ga, Ca
Br, Rb, Kr
Sr, Ca, Rb

13. Sr > Ca > Mg
K > Ca > Ga
Rb > Br > Kr
Rb > Sr > Ca

14. Ions Atom or group of atoms that has a positive or negative charge
Form when electrons are transferred between atoms
Metals tend to lose electrons, forming cations
Nonmetals gain electrons, forming anions

16. Trends in Ionization Energy
Energy needed to remove an electron from an atom
Energy needed to remove the first electron from an atom is the FIRST IONIZATION ENERGY
Produces cation with 1+ charge

17. First ionization energy tends to decrease from top to bottom in a group and increases from left to right across a period

19. Why does this trend occur?
As atomic size increases down a group, Zeff has a smaller effect on the electrons in highest level
Shielding Effect
Less energy is needed to remove electron from the energy level making first ionization energy lower

20. Across a period…
Zeff increases and shielding effect remains constant
Increase in the attraction the nucleus has for an electron
More energy is needed to remove an electron from the atom and first ionization energy is higher

21. Try these… Put the following in order of decreasing IE1 Kr, He, Ar
Sb, Te, Sn
K, Ca, Rb
I, Xe, Cs

22. He > Ar > Kr
Te > Sb > Sn
Ca > K > Rb
Xe > I > Cs