1. Periodic Table Trends Effective Nuclear Charge
Elena Man

2. Periodic Trends
In this workshop, we will rationalize observed trends in
Sizes of atoms and ions.
Ionization energy.
Electron affinity.

3. Effective Nuclear Charge

4. In a many-electron atom, electrons are both attracted to the nucleus and repelled by other electrons. The nuclear charge that an electron experiences depends on both factors.
The effective nuclear charge, Zeff is the amount of positive nuclear charge “felt” by a given electron. It is found this way:
Zeff= Z−S
Where Z is the atomic number and S is a screening constant - number of inner shell electrons

5. Atomic Radius Atomic Radius – size of an atom
distance from nucleus to outermost e-

6. Sizes of Atoms The bonding atomic radius tends to
-Decrease from left to right across a row (due to increasing Zeff)
-Increase from top to bottom of a column (due to increasing value of n).

7. Sizes of Ions Cations are smaller than their parent atoms:
The outermost electron is removed and repulsions between electrons are reduced.
Anions are larger than their parent atoms
Electrons are added and repulsions between electrons are increased.

8. Ionic Radius Trend
Group Trend – As you go down a column, atomic radius increases.
Periodic Trend – As you go across a period (L to R), atomic radius decreases.

9. Sizes of Ions Ionic size decreases with an increasing nuclear charge
Ions increase in size as you go down a column:
This increase in size is due to the increasing value of n.
In an isoelectronic series, ions have the same number of electrons:
Ionic size decreases with an increasing nuclear charge

10. Ionization Energy
The ionization energy is the amount of energy required to remove an electron from the ground state of a gaseous atom or ion.
The first ionization energy is the energy required to remove the first electron.
The second ionization energy is the energy required to remove the second electron, etc.

11. Ionization Energy
Ionization Energy – energy needed to remove outermost e-

12. Ionization Energy
Group Trend – As you go down a column, ionization energy decreases.
As you go down, atomic size is increasing (less attraction), so it is easier to remove an e-.
Periodic Trend – As you go across a period (L to R), ionization energy increases.
As you go L to R, atomic size is decreasing (more attraction as Zeff increases), so it is more difficult to remove an e-
(Remember that metals want to lose e-, but nonmetals do not).

13. Energy to Take Off Several Electrons
How do electron configurations explain why there is a big change in energy at the red line? It requires more energy to remove each successive electron. When all valence electrons have been removed, the ionization energy takes a quantum leap.

14. However, there are two apparent discontinuities in this trend:
The first occurs between Groups IIA and IIIA.
In this case the electron is removed from a p orbital rather than an s orbital. The electron removed is farther from the nucleus. There is also a small amount of repulsion by the s electrons.
The second discontinuity occurs between Groups VA and VIA.
The electron removed comes from a doubly occupied orbital. Repulsion from the other electron in the orbital aids in its removal.

15. Electron Affinity
Electron affinity is the energy change accompanying the addition of an electron to a gaseous atom:
Cl (g) + e−  Cl− (g)
In general, electron affinity becomes more exothermic as you go from left to right across a row.

16. Electronegativity Trend (really electron affinity)
Group Trend – As you go down a column, electron affinity decreases. As you go down, atomic size is increasing, so less attraction of electrons to the nucleus. Periodic Trend – As you go across a period (L to R), electron affinity increases. As you go L to R, atomic size is decreasing, so the electrons are more attracted to the nucleus.

17. There are again, however, two discontinuities in this trend.
The first occurs between Groups IA and IIA. The added electron must go in a p orbital, not an s orbital. The electron is farther from the nucleus and feels repulsion from the s electrons.
The second discontinuity occurs between Groups IVA and VA.
Group VA has no empty orbitals. The extra electron must go into an already occupied orbital, creating repulsion.

18. Electron Affinity

19. Properties of Metal, Nonmetals

20. Electronegativity
Electronegativity- tendency of an atom to attract e-.

21. Reactivity Reactivity – tendency of an atom to react.
Metals – lose e- when they react, so metals’ reactivity is based on lowest Ionization Energy (bottom/left corner)
Low I.E = High Reactivity.
Nonmetals – gain e- when they react, so nonmetals’ reactivity is based on high electronegativity (upper/right corner).
High electronegativity = High reactivity