Shielding and Ionization Energies in Atoms: Effective Nuclear Charge and Ionic Radius, Exams of Chemistry

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3.10 The Sizes of Atoms (in pm)

Shielding

• In a many-electron atom,

electrons are both attracted to

the nucleus and repelled by

other electrons.

• The cancelling of the nuclear

attractive force on an electron

due to the electron-electron

repulsion is called shielding.

The effective nuclear charge , Z

eff

, is the net attraction

of an electron to a nucleus after screening.

Z

eff

≈ Z – core electrons

Z

eff

increases from left to right across a row in the

periodic table.

Z

eff

increases slightly as you move down a column.

Effective Nuclear Charge

Periodic Trends in Properties of Elements

Sizes of Ions: Ionic Radius

Cations are smaller

than their parent

atoms.

Anions are larger

than their parent

atoms.

Ionic Radius

Ions increase in size as you go down a

column.

In an isoelectronic series, ionic size

decreases with an increasing nuclear

charge.

Trends in First Ionization Energies

Generally, I

1

increases

from left to right across

a period.

Within a group, I

1

decreases with

increasing atomic

number.

Trends in First Ionization Energies

There are two

discontinuities in the

first ionization

energies:

Groups 2 and 13

Groups 15 and 16

Electron Affinities

Electron affinity is the energy change

accompanying the addition of a mole of

electrons to a mole of gaseous atoms:

Cl

(g)

• e

−

 Cl

−

(g)

E = -349 kJ/mol

Why is the electron affinity for Cl negative?

Trends (?) in Electron Affinity

There are again two discontinuities. Why?