1. Periodic Trends
Chemistry

2. History of the Periodic Table
1871 – Mendeleev arranged the elements according to:
1. Increasing atomic mass
2. Elements w/ similar properties were put in the same row.
1913 – Moseley arranged the elements according to:
1. Increasing atomic number
2. Elements w/ similar properties were put in the same column.

3. Periodic Groups
Elements in the same column have similar chemical and physical properties.
These similarities are observed because elements in a column have similar e- configurations (same amount of electrons in outermost shell).

4. Part I – Atomic Size

5. Atomic Radius
Measures as distance from nucleus to nucleus and divided by 2.
Unit commonly used is pm
picometer= 10-12m
Example: iodine atomic radius 140pm

6. How does atomic radius change across a period?
It is smaller to the right.
Why?
More protons in the nucleus higher electrical force pulls electrons closer to nucleus.

7. How does atomic radius change down a group?
It is larger down the group.
Why?
Valence electrons are at higher energy levels and are not bound as tightly to the nucleus because they are screened or shielded ( pushed away) by other electrons in inner levels.

8. Note
There are some exceptions. Example column 13.

9. The Periodic Table and Atomic Radius

10. Example: Which is larger: a lithium atom or a fluorine atom?

11. Example: Which is larger: an arsenic atom or a sulfur atom?

12. Part II – Ionization Energy

13. Ions What are ions? Ions are charged atoms, either + or -
Cations are positive ions
Cations form when atoms lose electrons
Anions are negative ions
Anions form when atoms gain electrons

14. Ionization energy
Ionization energy is the amount of energy needed to remove an electron from a gaseous atom.
First ionization energy –
First ionization energy is the energy required to remove a single electron from an atom.
Second ionization energy –
The second ionization energy is the energy it takes to remove another electron

15. How does ionization energy change down a group?
The first ionization energy decreases as you move down a group.
Why?
The size of the atom increases.
Electron is further from the nucleus.

16. How does ionization energy change across a period?
The first ionization energy increases as you move from left to right across a period.
Why?
Nuclear charge increases while shielding is constant.
Attraction of the electron to the nucleus increases.

18. Ionic size Metallic elements easily lose electrons.
Non-metals more readily gain electrons.
How does losing or gaining an electron effect the size of the atom (ion) ?

19. Positive ions
Positive ions are always smaller that the neutral atom. Loss of outer shell electrons.

20. Negative Ions
Negative ions are always larger than the neutral atom. Gaining electrons.

21. Ion size trends in periods.
Going from left to right there is a decrease in size of positive ions.
Starting with group 5, there is sharp increase followed by a decrease in the size of the anion as you move from left to right.

22. Ion size trends in columns.
Ion size increases as you move down a column for both positive and negative ions

23. Electronegativity: the ability of an atom in a bond to pull on the electron. (Linus Pauling)

24. Electronegativity
When electrons are shared by two atoms a covalent bond is formed.
When the atoms are the same they pull on the electrons equally. Example, H-H.
When the atoms are different, the atoms pull on the electrons unevenly. Example, HCl

25. Trends in Electronegativity
Electronegativity generally decreases as you move down a group.
Electronegativity of the representative elements (Group A elements) increases as you move across a period.

26. Electronegativities of Some Elements
Element Pauling scale F Cl O N S C H Na
Cs 0.7

27. Note Most electronegative element is F (EN 4.0)
Least electronegative stable element is Cs
(EN 0.7)

28. Summary Shielding is constant Atomic Radius decreases
Ionization energy increases
Electronegativity increases
Nuclear charge increases
Ionization energy Increases
Nuclear charge Decreases
Electronegativity Increases
Atomic radius Decreases
Shielding Decreases
Ionic size increases