Atomic radius - wikipedia Atomic radius Diagram of a helium atom, showing the electron probability density as shades of gray. The atomic radius of a chemical element is a measure of the size of its atoms, usually the mean or typical distance from the center of the nucleus to the boundary of the surrounding cloud of electrons. Since the boundary is not a well - defined physical entity, there are various non-equivalent definitions of atomic radius. Three widely used definitions of atomic radius are : Van der Waals radius, ionic radius, and covalent radius. Depending on the definition, the term may apply only to isolated atoms, or also to atoms in condensed matter, covalently bound in molecules, or in ionized and excited states ; and its value may be obtained through experimental measurements, or computed from theoretical models. The value of the radius may depend on the atom 's state and context. Electrons do not have definite orbits, or sharply defined ranges. Rather, their positions must be described as probability distributions that taper off gradually as one moves away from the nucleus, without a sharp cutoff. Moreover, in condensed matter and molecules, the electron clouds of the atoms usually overlap to some extent, and some of the electrons may roam over a large region encompassing two or more atoms. Under most definitions the radii of isolated neutral atoms range between 30 and 300 pm ( trillionths of a meter ), or between 0.3 and 3 ångströms. Therefore, the radius of an atom is more than 10,000 times the radius of its nucleus ( 1 -- 10 fm ), and less than 1 / 1000 of the wavelength of visible light ( 400 -- 700 nm ). The approximate shape of a molecule of ethanol, CH CH OH. Each atom is modeled by a sphere with the element 's Van der Waals radius. For many purposes, atoms can be modeled as spheres. This is only a crude approximation, but it can provide quantitative explanations and predictions for many phenomena, such as the density of liquids and solids, the diffusion of fluids through molecular sieves, the arrangement of atoms and ions in crystals, and the size and shape of molecules. Atomic radii vary in a predictable and explicable manner across the periodic table. For instance, the radii generally decrease along each period ( row ) of the table, from the alkali metals to the noble gases ; and increase down each group ( column ). The radius increases sharply between the noble gas at the end of each period and the alkali metal at the beginning of the next period. These trends of the atomic radii ( and of various other chemical and physical properties of the elements ) can be explained by the electron shell theory of the atom ; they provided important evidence for the development and confirmation of quantum theory. The atomic radii decrease across the Periodic Table because as the atomic number increases, the number of protons increases across the period, but the extra electrons are only added to the same quantum shell. Therefore, the effective nuclear charge towards the outermost electrons increases, drawing the outermost electrons closer. As a result, the electron cloud contracts and the atomic radius decrease. Contents 1 History 2 Definitions 3 Empirically measured atomic radius 4 Explanation of the general trends 4.1 Lanthanide contraction 4.2 d - Block contraction 5 Calculated atomic radii 6 See also 7 References History In 1920, shortly after it had become possible to determine the sizes of atoms using X-ray crystallography, it was suggested that all atoms of the same element have the same radii. However, in 1923, when more crystal data had become available, it was found that the approximation of an atom as a sphere does not necessarily hold when comparing the same atom in different crystal structures. Definitions Widely used definitions of atomic radius include : Van der Waals radius : in principle, half the minimum distance between the nuclei of two atoms of the element that are not bound to the same molecule. Ionic radius : the nominal radius of the ions of an element in a specific ionization state, deduced from the spacing of atomic nuclei in crystalline salts that include that ion. In principle, the spacing between two adjacent oppositely charged ions ( the length of the ionic bond between them ) should equal the sum of their ionic radii. Covalent radius : the nominal radius of the atoms of an element when covalently bound to other atoms, as deduced from the separation between the atomic nuclei in molecules. In principle, the distance between two atoms that are bound to each other in a molecule ( the length of that covalent bond ) should equal the sum of their covalent radii. Metallic radius : the nominal radius of atoms of an element when joined to other atoms by metallic bonds. Bohr radius : the radius of the lowest - energy electron orbit predicted by Bohr model of the atom ( 1913 ). It is only applicable to atoms and ions with a single electron, such as hydrogen, singly ionized helium, and positronium. Although the model itself is now obsolete, the Bohr radius for the hydrogen atom is still regarded as an important physical constant. Empirically measured atomic radius The following table shows empirically measured covalent radii for the elements, as published by J.C. Slater in 1964. The values are in picometers ( pm or 1 × 10 m ), with an accuracy of about 5 pm. The shade of the box ranges from red to yellow as the radius increases ; gray indicates lack of data. Group ( column ) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Period ( row ) 25 He Li 145 Be 105 85 70 65 O 60 50 Ne Na 180 Mg 150 Al 125 Si 110 100 100 Cl 100 Ar 220 Ca 180 Sc 160 Ti 140 V 135 Cr 140 Mn 140 Fe 140 Co 135 Ni 135 Cu 135 Zn 135 Ga 130 Ge 125 As 115 Se 115 Br 115 Kr 5 Rb 235 Sr 200 Y 180 Zr 155 Nb 145 Mo 145 Tc 135 Ru 130 Rh 135 Pd 140 Ag 160 Cd 155 In 155 Sn 145 Sb 145 Te 140 140 Xe 6 Cs 260 Ba 215 * Hf 155 Ta 145 135 Re 135 Os 130 Ir 135 Pt 135 Au 135 Hg 150 Tl 190 Pb 180 Bi 160 Po 190 At Rn 7 Fr Ra 215 * * Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og Lanthanides * La 195 Ce 185 Pr 185 Nd 185 Pm 185 Sm 185 Eu 185 Gd 180 Tb 175 Dy 175 Ho 175 Er 175 Tm 175 Yb 175 Lu 175 Actinides * * Ac 195 Th 180 Pa 180 U 175 Np 175 Pu 175 Am 175 Cm Bk Cf Es Fm Md No Lr Explanation of the general trends A graph comparing the atomic radius of elements with atomic numbers 1 -- 100. Accuracy of ± 5 pm. The way the atomic radius varies with increasing atomic number can be explained by the arrangement of electrons in shells of fixed capacity. The shells are generally filled in order of increasing radius, since the negatively charged electrons are attracted by the positively charged protons in the nucleus. As the atomic number increases along each row of the periodic table, the additional electrons go into the same outermost shell ; whose radius gradually contracts, due to the increasing nuclear charge. In a noble gas, the outermost shell is completely filled ; therefore, the additional electron of next alkali metal will go into the next outer shell, accounting for the sudden increase in the atomic radius. The increasing nuclear charge is partly counterbalanced by the increasing number of electrons, a phenomenon that is known as shielding ; which explains why the size of atoms usually increases down each column. However, there is one notable exception, known as the lanthanide contraction : the 5d block of elements are much smaller than one would expect, due to the shielding caused by the 4f electrons. Essentially, atomic radius decreases across the periods due to an increasing number of protons. Therefore, there is a greater attraction between the protons and electrons because opposite charges attract, and more protons creates a stronger charge. The greater attraction draws the electrons closer to the protons, decreasing the size of the particle. Therefore, atomic radius decreases. Down the groups, atomic radius increases. This is because there are more energy levels and therefore a greater distance between protons and electrons. In addition, electron shielding causes attraction to decrease, so re electrons can go farther away from the positively charged nucleus. Therefore, size ( atomic radius ) increases. The following table summarizes the main phenomena that influence the atomic radius of an element : factor principle increase with... tend to effect on radius electron shells quantum mechanics principal and azimuthal quantum numbers increase atomic radius increases down each column nuclear charge attractive force acting on electrons by protons in nucleus atomic number decrease atomic radius decreases along each period shielding repulsive force acting on outermost shell electrons by inner electrons number of electrons in inner shells increase atomic radius reduces the effect of the 2nd factor Lanthanide contraction Main article : Lanthanide contraction The electrons in the 4f - subshell, which is progressively filled from cerium ( Z = 58 ) to lutetium ( Z = 71 ), are not particularly effective at shielding the increasing nuclear charge from the sub-shells further out. The elements immediately following the lanthanides have atomic radii which are smaller than would be expected and which are almost identical to the atomic radii of the elements immediately above them. Hence hafnium has virtually the same atomic radius ( and chemistry ) as zirconium, and tantalum has an atomic radius similar to niobium, and so forth. The effect of the lanthanide contraction is noticeable up to platinum ( Z = 78 ), after which it is masked by a relativistic effect known as the inert pair effect. Due to lanthanide contraction, the 5 following observations can be drawn : The size of Ln ions regularly decreases with atomic number. According to Fajans ' rules, decrease in size of Ln ions increases the covalent character and decreases the basic character between Ln and OH ions in Ln ( OH ), to the point that Yb ( OH ) and Lu ( OH ) can dissolve with difficulty in hot concentrated NaOH. Hence the order of size of Ln is given : La > Ce >...,... > Lu. There is a regular decrease in their ionic radii. There is a regular decrease in their tendency to act as a reducing agent, with increase in atomic number. The second and third rows of d - block transition elements are quite close in properties. Consequently, these elements occur together in natural minerals and are difficult to separate. D - block contraction Main article : d - block contraction The d - block contraction is less pronounced than the lanthanide contraction but arises from a similar cause. In this case, it is the poor shielding capacity of the 3d - electrons which affects the atomic radii and chemistries of the elements immediately following the first row of the transition metals, from gallium ( Z = 31 ) to bromine ( Z = 35 ). Calculated atomic radii The following table shows atomic radii computed from theoretical models, as published by Enrico Clementi and others in 1967. The values are in picometres ( pm ). Group ( column ) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Period ( row ) 53 He 31 Li 167 Be 112 87 67 56 O 48 42 Ne 38 Na 190 Mg 145 Al 118 Si 111 98 88 Cl 79 Ar 71 243 Ca 194 Sc 184 Ti 176 V 171 Cr 166 Mn 161 Fe 156 Co 152 Ni 149 Cu 145 Zn 142 Ga 136 Ge 125 As 114 Se 103 Br 94 Kr 88 5 Rb 265 Sr 219 Y 212 Zr 206 Nb 198 Mo 190 Tc 183 Ru 178 Rh 173 Pd 169 Ag 165 Cd 161 In 156 Sn 145 Sb 133 Te 123 115 Xe 108 6 Cs 298 Ba 253 * Hf 208 Ta 200 193 Re 188 Os 185 Ir 180 Pt 177 Au 174 Hg 171 Tl 156 Pb 154 Bi 143 Po 135 At Rn 120 7 Fr Ra * * Rf Db Sg Bh Hs Mt Ds Rg Cn Nh Fl Mc Lv Ts Og Lanthanides * La Ce Pr 247 Nd 206 Pm 205 Sm 238 Eu 231 Gd 233 Tb 225 Dy 228 Ho Er 226 Tm 222 Yb 222 Lu 217 Actinides * * Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr See also Atomic radii of the elements ( data page ) Chemical bond Covalent radius Bond length Steric hindrance Kinetic diameter References Jump up ^ Cotton, F.A. ; Wilkinson, G. ( 1988 ). Advanced Inorganic Chemistry ( 5th ed. ). Wiley. p. 1385. ISBN 978 - 0 - 471 - 84997 - 1. Jump up ^ Basdevant, J. - L. ; Rich, J. ; Spiro, M. ( 2005 ). Fundamentals in Nuclear Physics. Springer. p. 13, fig 1.1. ISBN 978 - 0 - 387 - 01672 - 6. Jump up ^ Bragg, W.L. ( 1920 ). `` The arrangement of atoms in crystals ''. Philosophical Magazine. 6. 40 ( 236 ) : 169 -- 189. doi : 10.1080 / 14786440808636111. Jump up ^ Wyckoff, R.W.G. ( 1923 ). `` On the Hypothesis of Constant Atomic Radii ''. Proceedings of the National Academy of Sciences of the United States of America. 9 ( 2 ) : 33 -- 38. Bibcode : 1923PNAS... 9... 33W. doi : 10.1073 / pnas. 9.2. 33. PMC 1085234. PMID 16576657. ^ Jump up to : Pauling, L. ( 1945 ). The Nature of the Chemical Bond ( 2nd ed. ). Cornell University Press. LCCN 42034474. Jump up ^ Bohr, N. ( 1913 ). `` On the Constitution of Atoms and Molecules, Part I. -- Binding of Electrons by Positive Nuclei '' ( PDF ). Philosophical Magazine. 6. 26 ( 151 ) : 1 -- 24. doi : 10.1080 / 14786441308634955. Retrieved 8 June 2011. Jump up ^ Bohr, N. ( 1913 ). `` On the Constitution of Atoms and Molecules, Part II. -- Systems containing only a Single Nucleus '' ( PDF ). Philosophical Magazine. 6. 26 ( 153 ) : 476 -- 502. doi : 10.1080 / 14786441308634993. Retrieved 8 June 2011. Jump up ^ Slater, J.C. ( 1964 ). `` Atomic Radii in Crystals ''. Journal of Chemical Physics. 41 ( 10 ) : 3199 -- 3205. Bibcode : 1964JChPh... 41.3199 S. doi : 10.1063 / 1.1725697. ^ Jump up to : Jolly, W.L. ( 1991 ). Modern Inorganic Chemistry ( 2nd ed. ). McGraw - Hill. p. 22. ISBN 978 - 0 - 07 - 112651 - 9. Jump up ^ Clementi, E. ; Raimond, D.L. ; Reinhardt, W.P. ( 1967 ). `` Atomic Screening Constants from SCF Functions. II. Atoms with 37 to 86 Electrons ''. Journal of Chemical Physics. 47 ( 4 ) : 1300 -- 1307. Bibcode : 1967JChPh... 47.1300 C. doi : 10.1063 / 1.1712084. 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Question: what are the two factors that affect atomic radius
Answer: atom's state