IONIC RADIUS
By:
Rachel Smith, Lauren Pyfer, Peter Xu
IONIC RADIUS
•
The ionic radius is the measure of an atom's ion in a crystal
lattice
•
Values for ionic radius are difficult to obtain and tend to
depend on the method used to measure the size of the ion
•
A typical value for an ionic radius would be from 30 pm (0.3
Å) to 200 pm (2 Å)
•
Ionic radius may be measured using x-ray crystallography or
similar techniques.
IONIC RADIUS TREND
• Ionic radii are difficult to measure with certainty
• In order to make reliable comparisons using ionic radii,
they must must be from the same source.
• Details are difficult to determine due to many
uncertainties in the use of atomic radii.
TREND DOWN A GROUP
• Adding extra layers of electrons when moving down a
group, the ions get larger.
TREND ACROSS A PERIOD
• Cations and anions must be looked at separately.
• Within the group of positive and negative ions, the ionic
radii falls as you go across a period.
• Since an additional layer of electrons is added in the
anions, there is a large jump in radii immediately
starting in the group of anions.
IONIC RADIUS-REASONS FOR TREND
ANIONS
• Anions are larger than their non-charged versions
because:
• Electrons repel each other, forcing them to be farther
apart.
• There are more electrons than protons. Thus, the
strength of the protons pulling on each electron is no as
great as if the numbers of protons and electrons were
the same.
IONIC RADIUS-REASONS FOR TREND: CATION
• Cations are smaller than their non-charged versions because:
• There is less repulsion between electrons.
• There are more protons than electrons. Thus, the strength of the
protons pulling on each electron is greater than if the numbers
of protons and electrons were the same.
• If there is at least one electron in a shell, the shell exists. This
means that the only time a shell does not exist is when there are
absolutely no electrons on the shell anywhere.
IONIC RADIUS-EXCEPTIONS TO TREND
• The only ions follow the trends of ionic radii.
• An abnormal ionic radius often means that the ionic bond is
significantly covalent in part.
• The reason is that, actually, no bond is completely ionic.
• Transition metals in particular are partially covalent. Sometimes,
they do not completely lose their electrons.
IONIC RADIUS-EXCEPTIONS TO TREND