Atomic Radii
المؤلف:
University of Missouri System
المصدر:
Organic Chemistry ii
الجزء والصفحة:
.................
4-11-2020
2392
Atomic Radii
The atomic radius is an indication of the size of an atom. Although the concept of a definite radius of an atom is a bit fuzzy, atoms behave as if they have a certain radius. Such radii can be estimated from various experimental techniques, such as the x-ray crystallography of crystals.
As you go down a column of the periodic table, the atomic radii increase. This is because the valence electron shell is getting a larger and there is a larger principal quantum number, so the valence shell lies physically farther away from the nucleus. This trend can be summarized as follows: as ↓PT, atomic radius↑ where PT stands for periodic table. Going across a row on the periodic table, left to right, the trend is different. This is because although the valence shell maintains the same principal quantum number, the number of protons—and hence the nuclear charge—is increasing as you go across the row. The increasing positive charge leads to a larger effective nuclear charge which casts a tighter grip on the valence electrons, so as you go across the periodic table, the atomic radii decrease. Again, we can summarize this trend as follows: as →PT, atomic radius↓
Figure 1.1 “Atomic Radii Trends on the Periodic Table” shows spheres representing the atoms of the s and p blocks from the periodic table to scale, showing the two trends for the atomic radius.
Example 9
Referring only to a periodic table and not to Figure 1.1 “Atomic Radii Trends on the Periodic Table”, which atom is larger in each pair?
- Si or S
- S or Te
Solution
- Si is to the left of S on the periodic table, so it is larger because as you go across the row, the atoms get smaller.
- S is above Te on the periodic table, so Te is larger because as you go down the column, the atoms get larger.
Test Yourself
Referring only to a periodic table and not to Figure 8.19 “Atomic Radii Trends on the Periodic Table”, which atom is smaller, Ca or Br?
Answer
Br
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