1. Periodic Trends

2. Periodic trends
Periodic trends – properties of elements that change in a predictable way as you move through the periodic table

3. Atomic Radius
What happens to atomic radii as you move across a period or down a group?

4. Atomic radius increases down a group
Atomic radius decrease across a period

5. As you move down a group you have an increasing number of electrons in orbitals further away from the nucleus

6. As you move across a period there are an ever increasing number of protons in the nucleus that exert an attractive force on electrons in orbitals with the same principle quantum number. This stronger force pulls them closer.

7. Ionic Radii
Ionic radii are a bit more complex

8. Cations When an atom loses electrons it shrinks
Outermost orbital becomes empty
Reduced repulsive forces between electrons allow them to be drawn closer to the nucleus

9. Anions When atoms gain electrons they grow
electrons repel each other requiring more space forcing them further from the nucleus

10. Ionic Radius Trends Ionic radii increase as you move down a group
Ionic radii of cations decreases from left to right across a period
Ionic radii of anions decreases from left to right across a period

11. Ionization Energy
Ionization energy – the energy needed to remove electrons in the gaseous state
measure of how strongly held electrons are
higher IE indicates atoms hold valence electrons more strongly
low IE indicates weak hold on valence electrons
atoms with high IE unlikely to form cations

12. Ionization Energy

13. Ionization Energy Trends
IE decrease down a group
IE increase moving left to right across a period

14. Ionization Energy Trends
Each additional electron removed costs more energy
Once all valence electrons are removed the energy cost of removing electrons will be dramatically higher
atoms hold inner core electrons much more strongly than valence electrons
Theoretically valence electrons from the right side of the periodic table can be removed but this is unlikely

15. Electronegativity
electronegativity – ability of an atom to attract electrons in a chemical bond

16. Trends in Electronegativity
Increases across a period (left to right)
Decreases down a group
Fluorine is the most electronegative element
Least electronegative, Cs & Fr
Electronegativities are not generally considered meaningful for noble gases since they do not react

17. Electronegativity and Chemical Bonds
Covalent bonds – when atoms share electrons they will be more attracted to the atom with a higher electronegativity
when this difference is high the bond is said to be polar
The atom with higher electronegativity will have a slightly negative charge – δ-
The atom with less electronegativity will have a slightly positive charge – δ+

18. Water: example of a polar covalent bond

19. Nonpolar covalent bond
When the electrons are shared equally the bond will be a nonpolar covalent bond
best examples are bonds between atoms of the same element (eg H2)
Ionic bonds – when a cation and anion are held together by electrostatic attraction instead of electron sharing

20. Bonding spectrum
Bonding spectrum can be defined based on differences in electronegativity