Download Atomic radii generally decrease along each period (row) of the

Atomic radii generally decrease along each period (row) of the periodic
table and increase down each group (column).
LEARNING OBJECTIVE [ edit ]
Discuss the trend of atomic size in the periodic table.
KEY POINTS [ edit ]
Atomic radii vary predictably across the periodic table. For instance, the radii generally decrease
along each period (row) of the table from left to right and increase down each group(column).
As the atomic number increases along each row of the periodic table, the additional electrons go
into the same outermost shell, causing the atomic radius to decrease due to the increasing nuclear
charge.
When moving down a group of the periodic table, the atomic radius increases because of the
presence of additional principalenergy levels, which are further away from the nucleus.
TERMS [ edit ]
noble gas
Any of the elements of group 18 of the periodic table, being monoatomic and (with very limited
exceptions) inert.
period
A horizontal row in the periodic table, which signifies the total number of electron shells in an
element's atom.
atomic radius
A measure of the size of an atom. Assuming atoms have a spherical shape, the radius of the
sphere describes the size of the atom.
Give us feedback on this content: FULL TEXT [edit ]
The atomic radius of a chemicalelement is
a measure of the size of itsatoms. It
represents the mean distance from
the nucleus to the boundary of
the surrounding cloud ofelectrons.
Atomic radii vary in a predictable manner
across the periodic table. Radii generally
decrease along each period (row) of the
table from left to right and increase down
each group (column). These trends in
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atomic radii (as well as trends in various
other chemical and physical properties of the elements) can be explained by considering the
structure of the atom.
Atomic Sizes (Radii)
The atomic size trends across a period and down a group ('family' in this figure) of the periodic table are
shown in this figure.
Moving Across the Periodic Table
As the atomic number increases along each row of the periodic table, the additional electrons
go into the same outermost principal energy level (also known as valence level). This can be
predicted to lead to
1. an increase in atomic size because of additional repulsionsbetween electrons,
2. a decrease in size because of the additional protons in the nucleus,
3. no effect at all as the two opposing tendencies of electron repulsion and nuclear
attraction balance each other out.
Experiments have shown that the first case is what happens: the increase in nuclear charge
overcomes the repulsion between the additional electrons in the valence level. Therefore, the
size of atoms decreases as one moves across a period from left to right in the periodic table.
Moving Down the Periodic Table
The principal energy levels hold electrons at increasing radii from the nucleus. In a noble gas,
the outermost level is completely filled; therefore, the additional electron that the
following alkali metal (Group I) possesses will go into the next principal energy level,
accounting for the increase in the atomic radius. Therefore, atomic size, or radius, increases
as one moves down a group in the periodic table.