Effective Nuclear Charge

(Bohr model of Li vs C)

Slater's Rules

effective nuclear charge = Z_eff = Z - s

where s is the shielding parameter and is calculated by empirical rules.

1. Write out the electron configuration as follows:

(1s)(2s,2p)(3s,3p)(3d)(4s,4p)(4d)(5s,5p)...

2. Electrons to the right of the group of electrons in question contribute nothing to the shielding of that group of electrons.

3. All other electrons in the same group shield the electron in question to an extent of 0.35 each.

4. If the electron in question is an s or p electron:

a) all electons in the n - 1 shell shield to an extent of 0.85 each

b) all electrons in shells n - 2 or less shield perfectly, i.e., their contribution is 1.00 each.

5. If the electron in question is a d or f electron: all electrons lying to the left of the nd or nf group contribute 1.00. (why?)

(example with N)

Z_eff is a major factor in determining periodic variation in atomic parameters:

Element.....Z_eff

Li...............1.3

Be..............1.95

B................2.60

C................3.25

N................3.90

O................4.55

F.................5.20

Ne...............5.85

steady increase in Z_eff

Element.....Z_eff

H.................1.0

Li................1.3

Na...............2.2

K.................2.2

Rb...............2.2

Cs...............2.2

slow increase in Z_eff

Atomic Radii, r_A

The the metallic radii and covalent radii are together referred to as the atomic radii:

Ionic Radii, r_I

Vertical trends:

1 Main group elements: r_I __________ while descending a group since n increases:

Li⁺ < Na⁺ < K⁺ < Rb⁺ < Cs⁺

2. d block elements:

3d < 4d, 5d

Cr < Mo, W

Lanthanide Contraction: _________________________________________________________________

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Horizontal trends: isoelectronic series:

O^2- > F^- > Na⁺ > Mg²⁺ > Al³⁺ (each has [Ne] configuration)

Z ___________ so orbitals contract.

constant charge:

Ca²⁺ > Ti²⁺ > V²⁺

(anomalies not uncommon in d block)

Other trends:

anions > cations (why?)

(periodic table of ions)

For a given element, the greater the charge, the smaller the ion.

Cr²⁺ > Cr³⁺ > Cr⁶⁺

Electronegativity, c (Pauling)