THE MULTI-PROTON NUCLEUS, ORBITAL CONTRACTION, EFFECTIVE NUCLEAR CHARGE & SLATER’S RULES

Multi-proton nucleus

A nucleus with more than one proton; any atom except hydrogen

Orbital contraction

The shrinking of atomic orbitals due to increased effective nuclear charge

Trend of orbital contraction

Orbital size decreases across a period; increases down a group

Effective nuclear charge (Zeff)

The net positive charge felt by an electron after screening is considered

Zeff formula

Zeff = Z - S (nuclear charge minus screening constant)

Screening effect

Inner electrons shield outer electrons from full nuclear charge

Effect of screening on Zeff

Higher screening reduces Zeff; lower screening increases Zeff

Screening constant (S)

A measure of how much shielding is provided by other electrons

Slater's Rules

A set of rules to estimate the screening constant S for an electron

Slater's grouping order

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

Slater's Rule for s or p electrons

0.35 for same group, 0.85 for (n-1) shell, 1.00 for lower shells

Slater's Rule for d or f electrons

0.35 for same group, 1.00 for all lower groups

No contribution rule

Electrons in higher orbitals do not contribute to screening

Zeff of 4s in K (Z = 19)

4s feels Zeff = 19 - (8×0.85 + 10×1.00) = 2.20

Zeff of 3d in K (Z = 19)

3d feels Zeff = 19 - (18×1.00) = 1.00

Why K prefers 4s over 3d

4s electron experiences higher Zeff than 3d, so it's lower in energy

Ionization energy

Energy required to remove the outermost electron from an atom

Ionization energy trend

IE increases across a period; decreases down a group

Reason for IE trend

Stronger Zeff and smaller radius make electrons harder to remove

Electron affinity

Energy change when an atom gains an electron

Electron affinity trend

Generally becomes more negative across a period; less clear down groups

Zeff trend across period

Increases from left to right due to rising Z and low added shielding

Zeff trend down group

Decreases slightly due to increased distance and more shielding

Screening trend across period

Small increase, but not enough to cancel increase in Z

Screening trend down group

Increases due to more inner shells added