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Trends in Atomic Radius

Key Concepts

  • Atomic radius* increases down a Group, from top to bottom, of the Periodic Table.
  • Atomic radius* decreases across a Period, from left to right, of the Periodic Table#.

Trends in Atomic Radius in Groups of the Periodic Table

As you go down a Group in the Periodic Table from top to bottom, the number of energy levels or electron shells increases so the atomic radius of the elements increases.

Group 1 (IA, Alkali Metals)

Element Atomic
Number
(Z)
Symbol Simple
Electron
Configuration
No. Energy Levels
(electron shells)
Atomic
Radius
(pm)
Trend
lithium 3 Li 2,1 2 134 smallest
|
sodium 11 Na 2,8,1 3 154 |
potassium 19 K 2,8,8,1 4 196 |
rubidium 37 Rb 2,8,18,8,1 5 211 |
cesium 55 Cs 2,8,18,18,8,1 6 225 \/
largest
Atomic radius increases as you go down the Group 1 elements from top to bottom as an additional energy level (electron shell) is being added to each successive element.

Group 17 (VIIA, Halogens)

Element Atomic
Number
(Z)
Symbol Simple Electron Configuration No. Energy Levels
(electron shells)
Atomic Radius (pm) Trend
fluorine 9 F 2,7 2 71 (smallest)
|
chlorine 17 Cl 2,8,7 3 99 |
bromine 35 Br 2,8,18,7 4 114 |
iodine 53 I 2,8,18,18,7 5 133 \/
(largest)
Atomic radius increases as you go down Group 17 from top to bottom as an additional energy level (electron shell) is being added to each successive element.

Trends in Atomic Radius in Periods of the Periodic Table

In general, the atomic radius of elements decreases as you go across a Period from left to right.
As we go across a Period from left to right, electrons are being added to the same energy level, the valence shell.
The increased nuclear charge, due to the positively charged protons, attracts all the negatively charged electrons more strongly so all the electrons are drawn in closer to the nucleus, in other words, the radius of each successive atom gets smaller across a period of the Periodic Table.

Period 2

Element Li Be B C N O F Ne
Simple Electron Configuration 2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8
Energy Level being filled
(Valence Shell)
second
(L)
second
(L)
second
(L)
second
(L)
second
(L)
second
(L)
second
(L)
second
(L)
Nuclear Charge
(charge on all protons)
3+ 4+ 5+ 6+ 7+ 8+ 9+ 10+
Atomic Radius (pm) 134 90 82 77 75 73 71 69
General Trend (largest) ---- ----- ----- ----- ----- ----> (smallest)
Atomic radius generally decreases across Period 2 from left to right as the nuclear charge increases.

Period 3

Element Na Mg Al Si P S Cl Ar
Simple Electron Configuration 2,8,1 2,8,2 2,8,3 2,8,4 2,8,5 2,8,6 2,8,7 2,8,8
Energy Level being filled
(Valence Shell)
third
(M)
third
(M)
third
(M)
third
(M)
third
(M)
third
(M)
third
(M)
third
(M)
Nuclear Charge
(charge on all protons)
11+ 12+ 13+ 14+ 15+ 16+ 17+ 18+
Atomic Radius (pm) 154 130 118 111 106 102 99 97
General Trend (largest) ----- ---- ----- ----- ----- ----> (smallest)
Atomic radius generally decreases across Period 3 from left to right as the nuclear charge increases.

*The values for atomic radii provided here are approximations derived from interatomic-distance measurements.
#(We will be excluding transition metals, lanthanides and actinides from the disucssion)

Ionic Radius

  • Cations are smaller than their respective atoms as electrons are removed from the highest energy level (valence shell) reducing the radius of the cation.

  • Anions are larger than their respective atoms as electrons are added to the highest energy level (valence shell) the repulsion between the negatively charged electrons increases the ionic radius.

  • Down a Group of the Periodic Table from top to bottom, comparing ions with a similar charge, the ionic radius increases as the number of completed energy levels increases.

Group 1 (IA, alkali metals)

Element Symbol of Atom Atoms' Simple
Electron
Configuration
Atomic
Radius
(pm)
Symbol of Ion Ion's Simple
Electron
Configuration
Ionic
Radius*
(pm)
Trend
lithium Li 2,1 134 Li+ 2 74 smallest
|
sodium Na 2,8,1 154 Na+ 2,8 102 |
potassium K 2,8,8,1 196 K+ 2,8,8 138 |
rubidium Rb 2,8,18,8,1 211 Rb+ 2,8,18,8 149 |
cesium Cs 2,8,18,18,8,1 225 Cs+ 2,8,18,18,8 170 \/
largest
  • Ionic radius increases as you go down the Group 1 elements from top to bottom as an additional energy level (electron shell) is being added to each successive element.
  • Ionic radius is less than atomic radius for each element as the number of energy levels (electron shells) is reduced by removing electrons.

*Ionic radius based on a coordination number of 6

Group 17 (VIIA, halogens)

Element Symbol of Atom Atom's Simple Electron Configuration Atomic Radius (pm) Symbol of Ion Ion's Simple Electron Configuration Ionic Radius (pm) Trend
fluorine F 2,7 71 F- 2,8 131 (smallest)
|
chlorine Cl 2,8,7 99 Cl- 2,8,8 181 |
bromine Br 2,8,18,7 114 Br- 2,8,18,8 196 |
iodine I 2,8,18,18,7 133 I- 2,8,18,18,8 220 \/
(largest)
  • Ionic radius increases as you go down the Group 17 elements from top to bottom as an additional energy level (electron shell) is being added to each successive element.
  • Ionic radius is greater than atomic radius for each element as additional electrons are added to each energy level increasing the repulsion between electrons and expanding the radius of the ion.


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