Atom and Ion Sizes  
Atomic and ionic size is determined by a balance between the attractions the electrons feel for the nucleus and the repulsions they feel for each other.
It is hard to imagine how small atoms really are. Atoms range in size from 1.4 X10-10 to 5.7 X 10-10 m in diameter.  Their radii, which is the usual way that size is specified, range from 7.0 X 10-11 to 2.9 X 10-10 m.  These numbers are also difficult to imagine.  A million carbon atoms lined up side by side would be about 0.2 mm, or about the diameter of the period at the end of this sentence.  The sizes of atoms are rarely expressed in meters because the numbers are so cumbersome.  Scientists have traditionally used the angstrom (A), which is defined as

                                                1 A = 1 X 10-10 m

However, since the advent of SI Metric the angstrom has been replaced by the nanometer (10-9 m), and the picometer (10-12 m).  We will use picometers, but because much of the earlier scientific literature has been written in angstroms, you may someday find it useful to remember the conversions:

                                                1 A = 100 pm
                                                1 A = 0.1 nm

Atomic Size
Atomic sizes vary in a more or less systematic way in the periodic table.  Atoms become larger from top to bottom in a group; they become smaller from left to right in a period.   There are two things that control atom sizes. One is the number of shells, and the other is the strengths of the attractions felt by the valence electrons toward the center of the atom.

     Going from top to bottom in a group we add shells. Therefore, an orbital containing the valence electrons becomes larger as we go down a group, and the atoms grow in size. The same argument applies whether the valence shell orbitals are 's' or 'p'.

Moving from left to right across a period, electrons are added to the same shell. At the same time protons are being added to the nucleus. As the strength of the nucleus increases the valence electrons feel a larger attraction towards the centre of the nucleus. As the pull increases the electrons are drawn into a smaller orbit and the size of the atom decreases.

The main factor controlling these attractions is the way electrons in inner shells help offset, or partially neutralize, the positive charge of the nucleus. This means that the outermost electrons are exposed to only a fraction of the full nuclear charge.  This means that the effective nuclear charge experienced by the outermost electrons is much smaller than the number of protons in the nucleus.

For example in an atom of lithium there are three positive charges.  But there are two electrons in the 1s subshell.  These two electrons partially block the full +3 charge of the nucleus.  The outermost electron in the 2s subshell is held in orbit by an effective nuclear charge of only +1.

Across a row of transition elements, the size variations are less pronounced. This is because the outer shell configuration remains essentially the same while the inner shell is filled. From atomic numbers 21 to 30, for example, the outer electrons occupy the 4s subshell while the 3d subshell is gradually completed. As a result, the decrease in size with increasing atomic number is also more gradual.

Sizes of Ions
When an atom gains or loses electrons to form ions, significant changes take place.  Adding electrons creates an ion that is larger then the neutral atom; removing electrons produces an ion that is smaller then the neutral atom.  When electrons are added, mutual repulsions push them apart and they occupy a larger volume so the ion increases in size. By similar reasoning as we lose electrons we usually tend to lose shells. Removing a shell exposes the inner core of electrons, which naturally has a smaller volume than the neutral atom. Therefore positive ions always have a smaller size than the atoms from which they are formed.
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