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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)
General Trend	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)
General Trend	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.