Understanding Atomic Radius Trends: The 2 Key Principles

An atom is defined as one of the basic unit of any chemical element. Any chemical element from small to large weight has large atomic number has some definite atomic radius. The radius is defined as the distance between the centre of a circular substance to its outer perimeter. Hence, the atomic radius is the distance between the centre of nuclei and the electron in the outermost shell. The atomic radius are typically measured in very small SI unit of pico-meters, when 1 pm =  meters. The smallest average atomic radius of hydrogen (H) is about 25 pico-meters and caesium (Cs) has the largest average radius of about 260 pico-meters.

The Basics Of Atomic Radius Trend

There is an interesting trend about the atomic radius of elements in periodic table. The two main trends of atomic radius in the periodic table can be seen when the movement is from left to right i.e. moving within a period and other trend is when moving from top to bottom. The periodic table shown below gives the trend of increase of atomic radii in the direction of arrow. The very first atomic radius trend is that the size of atom decreases as we move from left to right through a period. Now, for each period of elements one new electron is being added to the very same shell. Now, at the time of addition of electron to an atom a proton is also added to the nucleus of the atom. The electrons are basically attracted to the nucleus and thus makes the radius of the atom small. For an example the atom of a carbon C has 6 atomic number of 6 electrons and 6 protons. Whereas, the fluorine atom contain 9 electrons and 9 protons. Hence, from the trend of atomic radius the atom of Carbon C has a larger radius than the radius of an atom of fluorine F as with the three extra protons of fluorine will attract the electrons with higher force than the Carbon atom and thus making the radius of Fluorine smaller than that of Carbon. This is indeed true as the atomic radius of the Carbon atom is about 70 pico-meters and the atomic radius of Fluorine is 50 pico-meters.

The second trend of atomic radius suggests that atomic radius increases as we move to the bottom of the periodic table from the top. As we move through each group the atom gets an additional orbit of electron and thus each orbit becomes further away from the nucleus of the atom and hence the atomic radius is increased. The valence band electrons that is the electrons in the outermost shell are not much attracted by the nucleus to make the atomic radius smaller due to the electron shielding effect. Electron shielding is basically an effect by which the inner shell electrons shields the electrons in the outer shell from electro-magnetic attraction of the positive charged nucleus to electrons. This attraction force is also reduced also when there are more electrons in one shell. Hence, due to electron shielding the electrons in valence band can’t get particularly close to the nucleus of the atom and as they can’t get any close thus the atom has a large radius. For example, the potassium (K) has a large radius than the sodium (Na) atom as the potassium atom has one extra orbit of electrons than the Na atom or the valence electrons are further away from the nucleus and hence the potassium has a large radius of an atom. The atomic radius of potassium atom is about 220 pm which is larger than the atomic radius of sodium which is 180 pm.

Empirical atomic radii:

The empirical atomic number table shows the element name, symbol and their corresponding atomic number and empirical atomic radius in pico-meters from which the two trends can be identified.

Atomic Number	Atomic Symbol	Name of Element	Empirical Atomic Radius (in pm)
1	H	Hydrogen	25
2	He	Helium	No data
3	Li	Lithium	145
4	Be	Beryllium	105
5	B	Boron	85
6	C	Carbon	70
7	N	Nitrogen	65
8	O	Oxygen	60
9	F	Fluorine	50
10	Ne	Neon	No data
11	Na	Sodium	180
12	Mg	Magnesium	150
13	Al	Aluminum	125
14	Si	Silicon	110
15	P	Phosphorus	100
16	S	Sulfur	100
17	Cl	Chlorine	100
18	Ar	Argon	No data
19	K	Potassium	220
20	Ca	Calcium	180
21	Sc	Scandium	160
22	Ti	Titanium	140
23	V	Vanadium	135
24	Cr	Chromium	140
25	Mn	Manganese	140
26	Fe	Iron	140
27	Co	Cobalt	135
28	Ni	Nickel	135
29	Cu	Copper	135
30	Zn	Zinc	135
31	Ga	Gallium	130
32	Ge	Germanium	125
33	As	Arsenic	115
34	Se	Selenium	115
35	Br	Bromine	115
36	Kr	Krypton	No data
37	Rb	Rubidium	235
38	Sr	Strontium	200
39	Y	Yttrium	180
40	Zr	Zirconium	155
41	Nb	Niobium	145
42	Mo	Molybdenum	145
43	Tc	Technetium	135
44	Ru	Ruthenium	130
45	Rh	Rhodium	135
46	Pd	Palladium	140
47	Ag	Silver	160
48	Cd	Cadmium	155
49	In	Indium	155
50	Sn	Tin	145
51	Sb	Antimony	145
52	Te	Tellurium	140
53	I	Iodine	140
54	Xe	Xenon	No data
55	Cs	Caesium	260
56	Ba	Barium	215
57	La	Lanthanum	195
58	Ce	Cerium	185
59	Pr	Praseodymium	185
60	Nd	Neodymium	185
61	Pm	Promethium	185
62	Sm	Samarium	185
63	Eu	Europium	185
64	Gd	Gadolinium	180
65	Tb	Terbium	175
66	Dy	Dysprosium	175
67	Ho	Holmium	175
68	Er	Erbium	175
69	Tm	Thulium	175
70	Yb	Ytterbium	175
71	Lu	Lutetium	175
72	Hf	Hafnium	155
73	Ta	Tantalum	145
74	W	Tungsten	135
75	Re	Rhenium	135
76	Os	Osmium	130
77	Ir	Iridium	135
78	Pt	Platinum	135
79	Au	Gold	135
80	Hg	Mercury	150
81	Tl	Thallium	190
82	Pb	Lead	180
83	Bi	Bismuth	160
84	Po	Polonium	190
85	At	Astatine	No data
86	Rn	Radon	No data
87	Fr	Francium	No data
88	Ra	Radium	215
89	Ac	Actinium	195
90	Th	Thorium	180
91	Pa	Protactinium	180
92	U	Uranium	175
93	Np	Neptunium	175
94	Pu	Plutonium	175
95	Am	Americium	175
96	Cm	Curium	No data
97	Bk	Berkelium	No data
98	Cf	Californium	No data
99	Es	Einsteinium	No data
100	Fm	Fermium	No data
101	Md	Mendelevium	No data
102	No	Nobelium	No data
103	Lr	Lawrencium	No data
104	Rf	Rutherfordium	No data
105	Db	Dubnium	No data
106	Sg	Seaborgium	No data
107	Bh	Bohrium	No data
108	Hs	Hassium	No data
109	Mt	Meitnerium	No data
110	Ds	Darmstadtium	No data
111	Rg	Roentgenium	No data
112	Cn	Copernicium	No data
113	Nh	Nihonium	No data
114	Fl	Flerovium	No data
115	Mc	Moscovium	No data
116	Lv	Livermorium	No data
117	Ts	Tennessine	No data
118	Og	Oganesson	No data