Effective Nuclear Charge Effective Nuclear Charge (ENC)(ENC)

• Effective Nuclear Charge is the pull felt Effective Nuclear Charge is the pull felt on the valence electrons by the protons on the valence electrons by the protons in the nucleusin the nucleus

• How tightly the valence electrons are How tightly the valence electrons are pulled by the nucleus determines how pulled by the nucleus determines how the element will react in forming ionsthe element will react in forming ions

Inner Shell ShieldingInner Shell Shielding

• Inner shells (energy levels) of electrons Inner shells (energy levels) of electrons “shield” or hide the valence electrons “shield” or hide the valence electrons from the protons in the nucleusfrom the protons in the nucleus

• Magnesium has 12 protonsMagnesium has 12 protons• Magnesium has 10 inner shell electronsMagnesium has 10 inner shell electrons• The valence electrons only feel the pull The valence electrons only feel the pull

of 2 protonsof 2 protons

Determining ENCDetermining ENC

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

Effects of Effective Nuclear Effects of Effective Nuclear ChargeCharge

On –On –

SizeSize

Ionization EnergyIonization Energy

ElectronegativityElectronegativity

Electron AffinityElectron Affinity

SizeSize

• As ENC increases, the size As ENC increases, the size decreases.decreases.

• Therefore, the size decreases as Therefore, the size decreases as you go across the periodic tableyou go across the periodic table

• The size The size increasesincreases as you go down as you go down a familya family

Ionization EnergyIonization Energy

• This is the amount of energy needed to This is the amount of energy needed to remove an electron – making a positive remove an electron – making a positive ion.ion.

• Written asWritten as– A + energy -> AA + energy -> A++ + e + e--

• As ENC increases, energy needed to As ENC increases, energy needed to remove an electron increases.remove an electron increases.

• Energy to remove subsequent electrons Energy to remove subsequent electrons also increasesalso increases

Electron AffinityElectron Affinity

• This is the change in energy as an This is the change in energy as an atom gains an electron – making a atom gains an electron – making a negative ion.negative ion.

• Written asWritten as– A + eA + e- - -> A-> A-- + energy + energy

• As ENC increases, Electron Affinity As ENC increases, Electron Affinity increases.increases.

ElectronegativityElectronegativity

• The ability of an atom to attract The ability of an atom to attract electrons.electrons.

• As ENC increases, Electronegativity As ENC increases, Electronegativity increases.increases.

Ion sizeIon size

• Positive (cat) ions getPositive (cat) ions get– SmallerSmaller

• Negative (an) ions getNegative (an) ions get– LargerLarger