8.3 Periodic Trends in Atomic Properties
– Periodicity is based on the electron configuration
which depends on the # of electrons which in turn
depends on the number of protons (atomic #)
Trends in Atomic Size
•Atomic radius – half of the distance between
the centers of two adjacent identical atoms
–Metallic radius – for metals in the solid phase
–Covalent radius – for nonmetals in molecules
• Atomic radii increase down a group and
decrease from left to right across a period
(for main group elements)
•Down a group – the valence shell principal
quantum number (n) increases ⇒orbitals
and electron clouds become larger
•Across a period – the nuclear charge
increases while the new electrons enter the
same principal shell (do not shield each other
effectively) ⇒the effective nuclear charge
(Zeff) increases and draws the electrons
closer to the nucleus
Example: Compare the sizes of Ge, Sn and Se.
Sn is below Ge ⇒Sn>Ge
Ge is to the left of Se ⇒Ge>Se
• For the transition elements, the size trend
across a period is not as pronounced because
electrons are added to inner shells which
provides better shielding of the outer
electrons, so Zeff does not increase as much
