1. Periodic Trends
4th block Chemistry

2. Snowman and Wedge Ionization Energy and Electronegativity
Atomic Radius

3. Trends in Atomic Size
The ATOMIC RADIUS is one half of the distance between the nuclei of TWO atoms of the same element when the atoms are joined.
We cannot measure the diameter of a single atom because an atom does not have a SHARPLY DEFINED BORDER.
All atoms of the same element are IDENTICAL.
Electrons don’t orbit the nucleus in well defined circular orbits-so an atom doesn’t have a sharply defined border.

4. Atomic Size
Measured in PICOMETERS because the distance between atoms in molecules is very small.
There are one trillion Picometers in a meter.

5. Periodic Trend in Atomic Size
Atomic size INCREASES from top to bottom within a group and DECREASES from left to right across a period.

6. Group Trends in Atomic Size
As the atomic number increases within a group, the CHARGE on the nucleus increases and the number of OCCUPIED ENERGY LEVELS increases.
Increase in positive charge draws electrons CLOSER to the nucleus.
Increase in the number of occupied orbitals shields (DOESN’T DRAW CLOSER to the nucleus) electrons in the highest occupied energy level from the attraction of the protons in the nucleus.
The shielding effect is GREATER than the effect of the increase in nuclear charge.  atomic size INCREASES

7. Periodic Trends in Atomic Size
Shielding effect is CONSTANT for all elements in a period because they are added to the SAME principal ENERGY LEVEL.
The increasing nuclear charge pulls electrons in the highest occupied energy level CLOSER to the nucleus. atomic size DECREASES

8. Questions:
1. A. Arrange these elements in order of decreasing atomic size: sulfur, chlorine, aluminum, and sodium. B. Does your arrangement demonstrate a periodic trend or a group trend?
2. Which element in each pair has atoms with a larger atomic radius?
A. sodium, lithium
B. strontium, magnesium
C. carbon, germanium
D. selenium, oxygen
Sodium, aluminum, sulfur, chlorine; periodic trend
Sodium, strontium, germanium, selenium

9. Answers: Sodium, aluminum, sulfur, chlorine; periodic trend Sodium
Strontium
germanium
selenium

10. Ions
Positive and negative IONS form when electrons are transferred between atoms.

11. Cations Electrons are LOST.
An ion with a positive (+) charge is called a CATION (ca+ion). Ex: Na1+=Na+
METALS tend to form cations.

12. Anions Electrons are GAINED.
An ion with a negative (-) charge is called an ANION. EX: Cl1- =Cl-
NONMETALS tend to form anions.

13. Questions: 1. Is Ca2+ a cation or an anion?
2. S2- a cation or an anion?
3. Do metals form cations or anions?
4. Do nonmetals form cations or anions?
Answers: 1. cation 2. anion 3. cations 4. anions
1. Cation
2. Anion
3. Cations
4. Anions

14. Ionization Energy
Sometimes there is enough energy to overcome the ATTRACTION of the protons in the nucleus.
The energy required to remove an electron from an atom is called IONIZATION ENERGY.
The energy required to remove the FIRST electron from the atom is called the FIRST ionization energy.
First ionization energy tends to DECREASE from top to bottom within a group and INCREASE from left to right across a period.
Helps predict what ions elements will form.

15. *Ionization Energies
Group 1 loses 1 electron easily = lowest first ionization energy
2nd ionization energy is the energy required to remove an electron from an ion with a 1+ charge forming an ion with a 2+ charge
Group 2 loses 2 electrons easily = lowest second ionization energy
3rd ionization energy is the energy required to remove an electron from an ion with a 2+ charge forming an ion with a 3+ charge.
Group 13 loses 3 electrons easily= lowest third ionization energy

16. Trends in Ionization Energy
Elements that gain electrons have high ionization energy.

17. Group Trends in Ionization Energy
As the size of an atom INCREASES, nuclear charge has a SMALLER effect on the electrons in the highest occupied energy level (SHIELDING). 
Less energy is required to remove an electron from the highest occupied energy level and the first ionization energy is LOWER.

18. Periodic Trends in Ionization Energy
INCREASES from left to right across a period.
Nuclear charge INCREASES and shielding remains CONSTANT. INCREASE in the nucleus attracting an electron
So more energy is needed to remove an electron from an atom.

19. Questions:
Arrange the following groups of elements in order of increasing ionization energy.
A. Be, Mg, Sr
B. Bi, Cs, Ba
C. Na, Al, S
Answers: A. Sr, Mg, Be
B. Cs, Ba, Bi
Sr, Mg, Be
Cs, Ba, Bi
Na, Al, S

20. Trends in Ionic Size
Cations are always SMALLER than the atoms from which they form. Anions are always LARGER than the atoms from which they form.
As the number of electrons increases the attraction of the nucleus for an one electron decreases.

21. Questions: Which particle has the larger radius in each atom/ion pair?
A. Na, Na+
B. S, S2-
C. I, I-
D. Al, Al3+
Answers: A. Na B. S2- C. I- D. Al Na
S2- I- Al

22. Electronegativity
Property that can predict the type of bond that will form during a reaction.
ELECTRONEGATIVITY is the ability of an atom of an element to attract electrons when the atom is in a compound.
Ionization energy is used to calculate values for electronegativity.

23. Electronegativity
In general, decreases from top to bottom within a GROUP
For representative elements (elements other than the TRANSITION metals), increases from left to right across a period.

24. Questions:
Which element in each pair has a higher electronegativity value?
A. Cl, F
B. C, N
C. Mg, Na
D. As, Ca
Answers:
A. F
B. N
C. Mg
D. As F N Mg As

25. * Reactivity How easily an element takes part in a reaction
Elements with low ionization energy (easily lose electrons) or high electronegativity (easily gain electrons) tend to be highly reactive

26. * Reactivity of Metals Increases as you go down a group
Decreases as you go across a period
Cesium is normally said to be the most reactive metal because Francium is radioactive.

27. * Reactivity of Nonmetals
Decreases as you go down a group
Increases as you go across a period
Fluorine is the most reactive nonmetal

28. Questions:
Put the following elements in order from least reactive to most reactive: K, Rb, Na, Cs, Li.
Put the following elements in order from least reactive to most reactive: N, As, Sb, P, Bi.
Answers:
Li, Na, K, Rb, Cs
Bi, Sb, As, P, N
Li, Na, K, Rb, Cs
Bi, Sb, As, P, N

29. Summary Remember Snowman and Wedge.
Nuclear Charge increases down a group and across periods.
Shielding increases down a group.

30. Practice 1. As you go down a group, what happens to radius?
2. As you go from left to right, what happens to radius?
3. Which is the most electronegative element of all?
4. If electronegativity increases, what happens to ionization energy?
5. Metals are most reactive when ionization energy is (low/high).
6. Nonmetals are most reactive when ionization energy is (low/high).
7. What is the most reactive metal?
8. What is the most reactive nonmetal?

31. Answers 1. increases 2. decreases 3. Fluorine 4. it also increases
5. low
6. high
7. Cs
8. F

32. Practice
9. What happens to the radius of a neutral atom when it forms a cation?
10. What happens to the radius of a neutral atom when it forms an anion?
11. Which is bigger: O atom or O2- atom?
12. Which is bigger: Mg atom or Mg2+ atom?
13. For which of these properties does lithium have a larger value than potassium?
A. first ionization energy
B. atomic radius
C. electronegativity
D. ionic radius
13. A and C

33. Answers: 9. becomes smaller 10. becomes larger 11. O2- 12. Mg
13. 1st ionization energy and electronegativity