1. Chapter 5
Periodic Trends

2. Periodic Trends
The properties of the elements exhibit trends and these trends can be predicted with the help of the periodic table. They can also be explained and understood by analyzing the electron configurations of the elements. This is because, elements tend to gain or lose valence electrons to achieve the stable octet formation.

3. Atomic Radius
The atomic radius is one half of the distance between the nuclei of two atoms of the same element when the atoms are joined.

4. Trends: Atomic Radius
In general, there is a decrease in atomic radii as you move from left to right across a period caused by the increasing positive charge in the nucleus and the fact that the principal energy level within a period remains the same.
Atomic radii generally increases as you move down a group because the nuclear charge increases, and electrons are added to orbitals corresponding to successively higher principal energy levels; the larger energy level the further the electrons are from the nucleus.

5. Trends in Atomic Size Size generally decreases
Size generally increases
The size of atoms tends to decrease from left to right across a period and increase from top to bottom within a group. Predicting If a halogen and an alkali metal are in the same period, which one will have the larger radius?
Increases

6. Concept Check
Arrange the following elements in order of increasing atomic radius:
Lithium, Oxygen, Carbon, Fluorine
Fluorine  Oxygen  Carbon  Lithium

7. Ions
Positive and negative ions form when electrons are transferred between atoms.

8. Ions
Some compounds are composed of particles called ions.
An ion is an atom or group of atoms that has a positive or negative charge.
A cation is an ion with a positive charge.
An anion is an ion with a negative charge.
When a sodium atom loses an electron, it becomes a positively charged ion. When a chlorine atom gains an electron, it becomes a negatively charged ion. Interpreting Diagrams What happens to the protons and neutrons during these changes?

9. Ionic Radius The gaining or losing of electrons.
The more electrons an atom loses becoming more positive, the smaller the atom.
The more electrons an atom gains becoming more negative, the larger the atom.

10. Trends in Ionic Radius
In general as you move from left to right across a period, the size of the positive ions gradually decreases.
Beginning in group 15, the size of the much-larger negative ions also gradually decreases.
As you move down a group, an ion’s outer electrons are in orbitals corresponding to higher principal energy levels, resulting in a gradual increase in ionic size.

11. Trends in Ionic Radius Relative Sizes of Some Atoms and Ions
This diagram compares the relative sizes of atoms and ions for selected alkali metals and halogens. The data are given in picometers. Comparing and Contrasting What happens to the radius when an atom forms a cation? When an atom forms an anion?

12. Trends in Ionic Radius Size generally increases
The ionic radii for cations and anions decrease from left to right across periods and increase from top to bottom within groups.

13. Concept Check Which has the larger ionic radius? Ca+2 or Sr +2
Cl -1 or S -2
S -2
Al or Al +3 Al
O or O -2
O -2

14. Trends in Ionization Energy
The energy required to remove an electron from an atom is called ionization energy.
Think of this as how strongly an atom’s nucleus holds onto its valence electrons.
Atoms with a high ionization energy are less likely to form positive ions.
Atoms with low ionization energy are more likely to lose their outer electrons.

15. Trends in Ionization Energy
First ionization energies generally increase as you move from left to right across the period because of increasing nuclear charge.
First ionization energies generally decrease as you move down a group. This occurs because there is less energy required to remove the valance electrons farther from the nucleus of an atom with a larger radius. 
Nobel gases have highest ionization energy.

16. Trends in Ionization Energy
First ionization energy tends to increase from left to right across a period and decrease from top to bottom within a group. Predicting Which element would have the larger first ionization energy—an alkali metal in period 2 or an alkali metal in period 4?
Increases

17. Concept Check
Which element in each pair has the larger ionization energy?
Li or N N
Kr or Ne Ne
Cs or Li Li

18. Trends in Electronegativity
Electronegativity is the ability of an atom of an element to attract electrons when the atom is in a compound.
In general, electronegativity values decrease from top to bottom within a group. For representative elements, the values tend to increase from left to right across a period.
Nobel gases do not have a value for electronegativity because the form very few compounds.

19. Trends in Electronegativity
Increases
Decreases
First ionization energy tends to increase from left to right across a period and decrease from top to bottom within a group. Predicting Which element would have the larger first ionization energy—an alkali metal in period 2 or an alkali metal in period 4?

20. Trends in Electronegativity

21. Concept Check
Which element in each pair has the larger electronegativity?
K or As As
N or Sb N
Sr or Be Be