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 atom, usually the mean or typical distance from the center of the nucleus to the outermost isolated electron. Since the boundary is not a well-defined physical entity, there are various non-equivalent definitions of atomic radius. Four widely used definitions of atomic radius are: Van der Waals radius, ionic radius, metallic radius and covalent radius. Typically, because of the difficulty to isolate atoms in order to measure their radii separately, atomic radius is measured in a chemically bonded state; however theoretical calculations are simpler when considering atoms in isolation. The dependencies on environment, probe, and state lead to a multiplicity of definitions.
Depending on the definition, the term may apply to atoms in condensed matter, covalently bonding 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.[1]
Electrons do not have definite orbits nor 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; these are referred to as atomic orbitals or electron clouds. 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),[2] and less than 1/1000 of the wavelength of visible light (400–700 nm).
The approximate shape of a molecule of ethanol, CH3CH2OH. 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.[citation needed]
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Cotton, F. A.; Wilkinson, G. (1988). Advanced Inorganic Chemistry (5th ed.). Wiley. p. 1385. ISBN 978-0-471-84997-1.
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Basdevant, J.-L.; Rich, J.; Spiro, M. (2005). Fundamentals in Nuclear Physics. Springer. p. 13, fig 1.1. ISBN 978-0-387-01672-6.
definitions of atomicradius. Four widely used definitions of atomicradius are: Van der Waals radius, ionic radius, metallic radius and covalent radius. Typically...
constant. The CODATA value of the Bohr radius (in SI units) is 5.29177210544(82)×10−11 m. In the Bohr model for atomic structure, put forward by Niels Bohr...
factor of about 26,634 (uranium atomicradius is about 156 pm (156×10−12 m)) to about 60,250 (hydrogen atomicradius is about 52.92 pm). The branch of...
Russian chemist Dmitri Mendeleev in 1863. Major periodic trends include atomicradius, ionization energy, electron affinity, electronegativity, valency and...
transferable to allow periodic trends to be recognized. As with other types of atomicradius, ionic radii increase on descending a group. Ionic size (for the same...
the inter-atomic distances, and angles) are known. For a single atom, it is the volume of a sphere whose radius is the van der Waals radius of the atom:...
until the early 20th century. The first calculated estimate of the atomicradius of hydrogen was published by physicist Arthur Haas in 1910 to within...
The covalent radius, rcov, is a measure of the size of an atom that forms part of one covalent bond. It is usually measured either in picometres (pm) or...
protons and neutrons in an atom make up a tiny atomic nucleus, and are collectively called nucleons. The radius of a nucleus is approximately equal to 1.07...
{m} } , while expressed in atomic units distances are on the order of 1 a 0 {\displaystyle 1a_{0}} (one Bohr radius, the atomic unit of length). An additional...
nucleus; this, in turn, leads to a decrease in atomicradius. In multi-electron atoms, the decrease in radius brought about by an increase in nuclear charge...
and the density functional theory. These models either depart from the atomic orbitals of neutral atoms that share their electrons, or (in the case of...
estimated by comparing the atomic radii of the atoms that form the bond to the length of bond itself. For example, the atomicradius of boron is estimated...
extracted from waste produced by nuclear reactors. It has the largest atomicradius of all elements whose radii have been measured or calculated, at about...
affects the atomicradius of the alkali metals is the number of electron shells. Since this number increases down the group, the atomicradius must also...
length that characterizes electron interactions in atomic-scale problems. The classical electron radius is given as r e = 1 4 π ε 0 e 2 m e c 2 = 2.8179403227...
The noble gas atoms, like atoms in most groups, increase steadily in atomicradius from one period to the next due to the increasing number of electrons...
face-centered cube; it has lattice parameter of 0.352 nm, giving an atomicradius of 0.124 nm. This crystal structure is stable to pressures of at least...
towards the nucleus and the atomicradius decreases. This can be used to explain a number of periodic trends such as atomicradius, first ionization energy...
speed of light (special relativity), small distances comparable to the atomicradius (quantum mechanics), and very high energies (relativity). In general...
gigametres, yielding a strain sensitivity of better than 1 part in 1020. "Atomicradius". WebElements: the periodic table on the web. Deza, Elena; Deza, Michel...
atomicradius. The high atomic number means that more electrons are needed to maintain a neutral charge and the short bond length and a small atomic radius...
square charge radius of about 0.8 fm. Protons and neutrons are both nucleons, which may be bound together by the nuclear force to form atomic nuclei. The...
similar properties which have similar chemistry are often found – the atomicradius, electronegativity, properties of compounds (and so forth) of the diagonal...
good conductor of electricity and heat. Due to having low atomic mass and large atomicradius, sodium is third-least dense of all elemental metals and...
Silicon is a chemical element; it has symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is...
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