Two of the most common methods of radius measurement are the Van-der-Waals radius which is
the contact distances between unbonded atoms in touching molecules or atoms and the Covalent
Radii obtained by measuring bond lengths between pairs of covalently-bonded atoms.
Van-der-Waals Radii
Covalent Radii
Atomic Size
The first of these properties is the atomic size. You know that each atom has a
nucleus inside and electrons zooming around outside the nucleus. It should seem
reasonable that the size of an atom depends on how far away its outermost (valence)
electrons are from the nucleus. If they are very close to the nucleus, the atom will be
very small. If they are far away, the atom will be quite a bit larger. So the atomic size
is determined by how much space the electrons take up.
Measuring the size of atoms is, in some ways, like measuring the size of cotton balls
or automobile tires. The value you get depends on the conditions under which they
are measured. A "free" cotton ball has a different size than when it is in the package.
The radius of the tire is different when measured to the top of the tire than when
measured to the bottom of the tire resting on the ground. Different values for the
sizes of atoms are obtained depending on both the method used and the conditions in
which the atoms find itself - free or bonded to other atoms. The following table gives
a variety of values collected from a variety of sources.Whichever set of values you
choose to use, note the trends.
Atomic Sizes (in Angstroms, which is 10-10 meter) from Various Sources
1.58
0.3
1.2
0.98
n.a.
4.10 2.80
1.52 1.12
2.34 1.82 1.50 1.30 1.14 1.02
0.88 0.77 0.70 0.66 0.64 n.a.
1.5 1.40 1.35 1.60
4.46 3.44
1.86 1.60
3.64 2.92 2.46 2.18 1.94 1.76
1.43 1.17 1.10 1.04 0.99 n.a.
1.9 1.85 1.80 1.92
5.54 4.46 4.18 4.00 3.84 3.70 3.58 3.44 3.34 3.24 3.14 3.06 3.62 3.04 2.66 2.44 2.24 2.06
2.31 1.97 1.60 1.46 1.31 1.25 1.29 1.26 1.25 1.24 1.28 1.33 1.22 1.22 1.21 1.17 1.14 n.a.
2.0 2.00 1.95 1.97
5.96 4.90 4.54 4.32 4.16 4.02 3.90 3.78 3.66 3.58 3.50 3.42 4.00 3.44 3.06 2.84 2.64 2.48
2.44 2.15 1.80 1.57 1.41 1.36 1.3 1.33 1.34 1.38 1.44 1.49 1.62 1.4 1.41 1.37 1.33 n.a.
2.2 2.20 2.15 2.17
6.68 5.56 5.48 4.32 4.18 4.04 3.94 3.84 3.74 3.66 3.58 3.52 4.16 3.62 3.26 3.06 2.86 2.68
2.62 2.17 1.88 1.57 1.43 1.37 1.37 1.34 1.35 1.38 1.44 1.52 1.71 1.75 1.46 1.4 1.4 n.a.
2.7 2.20 2.2
Atomic diameter computed using quantum mechanical calculations, Periodic Chart of the
Atoms (1979), Sargent-Welch
Atomic radii and covalent radii, "Chemical Systems," Chemical Bond Approach Project (1964),
McGraw-Hill
Van der Waals radii, Handbook of Chemistry and Physics, 65th Ed. (1984), CRC Press and
"Chemical Systems"
Practice with Comparing Atomic Size
Now try your hand at answering the following questions (also shown in exercise 6 in
your workbook). Check your answers below and then continue with the lesson.
For each of the following sets of atoms, decide which is larger, which is smaller, and
why.
a.
b.
c.
d.
e.
Li, C, F
Li, Na, K
Ge, P, O
C, N, Si
Al, Cl, Br
Answers for Comparing Atomic Sizes
Here are answers for the questions above.
a. Li, C, F
All are in the same period and thus have the same number of energy levels.
Therefore, the important factor is the nuclear charge. Li is the largest because it has
the smallest nuclear charge and pulls the electrons toward the nucleus less than the
others. F is the smallest because it has the largest nuclear charge and pulls the
electrons toward the nucleus more than the others.
b. Li, Na, K
All are in the same group and thus have the same effective nuclear charge.
Therefore, the important factor is the number of energy levels. Li is the smallest
because it uses the smallest number of electron energy levels. K is the largest
because it uses the largest number of electron energy levels.
c. Ge, P, O
All are in different groups and periods, therefore both factors must be taken into
account. Fortunately both factors reinforce one another. Ge is the largest because it
uses the largest number of energy levels and has the smallest effective nuclear
charge. O is the smallest because it uses the smallest number of energy levels and has
the largest effective nuclear charge.
d. C, N, Si
Not all are in the same group and period, so, again, both factors must be taken into
account. C and N tie for using the smallest number of energy levels, but N has a
higher effective nuclear charge. Therefore, N is the smallest. C and Si tie for having
the lowest effective nuclear charge, but Si uses more energy levels. Therefore, Si is
the largest.
e. Al, Cl, Br
Not all are in the same group and period, so, again, both factors must be taken into
account. Cl is the smallest because it has higher effective nuclear charge than Al and
uses fewer energy levels than Br. Which is largest is less straightforward. Al has a
lower effective nuclear charge (by four), but Br uses more energy levels (by one).
Because the difference in effective nuclear charge is larger, it should be the more
important factor in this case, making Al the largest.
Al and Br can also be compared to one another indirectly by comparing both to Cl.
Both Al and Br are larger than Cl. Al is larger than Cl because it has lower effective
nuclear charge (by four). Br is larger than Cl because it uses more energy levels (by
one). Because Al is larger than Cl by four "steps" and Br is larger than Cl by only one
"step", Al is likely the largest of the three.