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

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

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

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

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

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

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.

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

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