1. Effective Nuclear Charge (ENC)
Effective Nuclear Charge is the pull felt on the valence electrons by the protons in the nucleus
How tightly the valence electrons are pulled by the nucleus determines how the element will react in forming ions

2. Inner Shell Shielding
Inner shells (energy levels) of electrons “shield” or hide the valence electrons from the protons in the nucleus
Magnesium has 12 protons
Magnesium has 10 inner shell electrons
The valence electrons only feel the pull of 2 protons

3. Determining ENC
ENC, in main group elements, is approximately equal to the number of valence electrons

4. Effects of Effective Nuclear Charge
On –
Size
Ionization Energy
Electronegativity
Electron Affinity

5. Size As ENC increases, the size decreases.
Therefore, the size decreases as you go across the periodic table
The size increases as you go down a family

6. Ionization Energy
This is the amount of energy needed to remove an electron – making a positive ion.
Written as
A + energy -> A+ + e-
As ENC increases, energy needed to remove an electron increases.
Energy to remove subsequent electrons also increases

7. Electron Affinity
This is the change in energy as an atom gains an electron – making a negative ion.
Written as
A + e- -> A- + energy
As ENC increases, Electron Affinity increases.

8. Electronegativity The ability of an atom to attract electrons.
As ENC increases, Electronegativity increases.

9. Ion size
Positive (cat) ions get
Smaller
Negative (an) ions get
Larger