Atomic Radius
By: Shourik, Holly, Zach, and Elizabeth
The Property of Atomic Radius
● Defined as the measure of the size of an atom; the
mean distance from the nucleus to the boundary of the
surrounding electron cloud
● The radius is dependent on the type of bond present, if
there is one, whether that be covalent, ionic, metallic, or
Van der Waals
○ these bonds affect how far our the valence electron
shell extends
● Atomic radius is variable due to the fact that effective
nuclear charge changes among individual elements
○ this affects the attraction of the electrons in the
valence shell. Thus the effective nuclear charge
determines how "tightly packed" the valence electron
cloud is.
What is the trend as you go down a
column?
The atomic radius increases as the atomic
number increases, moving down a column.
Group 13
Group 14
Group 15
Smaller
Small
Big
Bigger
Why do atomic radii increase as you
go down a group of elements?
Moving down the group of elements, the atoms increase in
mass. They have more protons and also more electrons.
The radius increases not only because there is more mass
to the atom, but also because there are more electrons that
will be undergoing electron-electron repulsion. The radius
is determined by the valence electrons and the valence
electrons will be on higher shell levels as we go down a
group, meaning that a greater amount of electrons will
consequently be below the valence electrons, shielding
them from the nucleus and at the same time causing the
valence electrons to be pushed out to further reaches of the
atom, thus extending the radius.
What is the trend as you go across a
row?
Not including the noble gases, atomic radii get
smaller as you move across a period.
Why do atomic radii decrease as you
go across a period?
As you go across a period, the number of
protons in the nucleus increases. This causes
the electrons to experience a greater effective
nuclear charge, which pulls the electrons in
more tightly, thus causing the radius to
decrease. While electrons are being added as
you move across a period, they are all in the
same shell at about the same distance from the
nucleus, so the shielding effect does not affect
the radius size as you go across a period.
What are the units used to measure
atomic radius?
Angstroms (Å): This the most common unit
used. Equivalent to 1.0 x 10^-10 meters.
Nanometer (nm): Equivalent to 1.0 x 10^-9
meters.
Picometer (pm): Equivalent to 1.0 x 10^-12
meters.
How is atomic radius measured?
The radius of an atom is dependent on the
distance from the nucleus to the outermost
boundary of its probability cloud. Therefore,
the atomic radius of an atom is not a fixed
amount. The only way to determine the radius
is to find the distance between the nuclei of two
bonded similar atoms and dividing the distance
by 2.
Exceptions to This Trend
● Transition metals
● Across a period: decreases initially from left to right, changes
●
●
very little in the middle, and ends with a small increase ub
suze at end of row
Down a column: maintains pattern of increasing uniformly
Rationale for irregularity
○
○
○
electron-electron repulsion cancels this decrease out halfway through
the period
the 'additional' electron for each successive element enters the the
inner 3d sub shell where it provides a more effective shield between
the nucleus and the outer 4s sub shell.
although each successive nucleus has one more proton, the extra
positive charge is partly shielded by the extra electron in an underlying
3d sub shell; this is why halfway through, we see an increase in radii
Questions???