atomic radius

(noun)

A measure of the size of an atom. Assuming atoms have a spherical shape, the radius of the sphere describes the size of the atom.

Related Terms

  • quantum theory
  • noble gas
  • electron shell
  • period

Examples of atomic radius in the following topics:

  • Atomic Radius

    • The atomic radius is one such characteristic that trends across a period and down a group of the periodic table.
    • Depending on context, the term atomic radius may apply only to isolated atoms, or also to atoms in condensed matter, covalently bound in molecules, or in ionized and excited states.
    • Under some definitions, the value of a radius may depend on the atom's state and context.
    • The way atomic radius varies with increasing atomic number can be explained by the arrangement of electrons in shells of fixed capacity.
    • A chart showing the atomic radius relative to the atomic number of the elements.
  • Atomic Size

    • The atomic radius of a chemical element is a measure of the size of its atoms.
    • These trends in atomic radii (as well as trends in various other chemical and physical properties of the elements) can be explained by considering the structure of the atom.
    • an increase in atomic size because of additional repulsions between electrons,
    • In a noble gas, the outermost level is completely filled; therefore, the additional electron that the following alkali metal (Group I) possesses will go into the next principal energy level, accounting for the increase in the atomic radius.
    • Therefore, atomic size, or radius, increases as one moves down a group in the periodic table.
  • General Trends in Chemical Properties

    • Specifically, elements are presented by increasing atomic number.
    • Elements in the same group show patterns in atomic radius, ionization energy, and electronegativity.
    • Elements in the same period show trends in atomic radius, ionization energy, electron affinity, and electronegativity.
    • Moving left to right across a period, from the alkali metals to the noble gases, atomic radius usually decreases.
    • The decrease in atomic radius also causes ionization energy to increase from left to right across a period: the more tightly bound an element is, the more energy is required to remove an electron.
  • Nuclear Size and Density

    • Nuclear size is defined by nuclear radius; nuclear density can be calculated from nuclear size.
    • Nuclear size is defined by nuclear radius, also called rms charge radius.
    • The problem of defining a radius for the atomic nucleus is similar to the problem of atomic radius, in that neither atoms nor their nuclei have definite boundaries.
    • This gives a charge radius for the gold nucleus ($A=197$) of about 7.5 fm.
    • Nuclear density is the density of the nucleus of an atom, averaging about $4 \cdot 10^{17} \text{kg/}\text{m}^3$.
  • Bond Lengths

    • The covalent radius of an atom is determined by halving the bond distance between two identical atoms.
    • Based on data for the H2 molecule, the covalent radius of H is 37 pm.
    • Generally, when we consider a bond between a given atom and a varying atomic bonding partner, the bond length decreases across a period in the periodic table, and increases down a group.
    • This trend is identical to that of the atomic radius.
    • A bond between two atoms can be thought of as a spring with two balls attached to it.
  • Multielectron Atoms

    • Atoms with more than one electron are referred to as multielectron atoms.
    • Atoms with more than one electron, such as Helium (He) and Nitrogen (N), are referred to as multielectron atoms.
    • Hydrogen is the only atom in the periodic table that has one electron in the orbitals under ground state.
    • For example, consider a sodium cation, a fluorine anion, and a neutral neon atom.
    • As a consequence, the sodium cation has the largest effective nuclear charge and, therefore, the smallest atomic radius.
  • Ionic Radius

    • Ionic radius (rion) is the radius of an ion, regardless of whether it is an anion or a cation.
    • When an atom loses an electron to form a cation, the lost electron no longer contributes to shielding the other electrons from the charge of the nucleus; consequently, the other electrons are more strongly attracted to the nucleus, and the radius of the atom gets smaller.
    • Similarly, when an electron is added to an atom, forming an anion, the added electron repels other electrons, resulting in an increase in the size of the atom.
    • Relative sizes of atoms and ions.
    • Identify the general trends of the ionic radius size for the periodic table.
  • The Periodic Table of Elements

    • In the periodic table, elements are presented in order of increasing atomic number (the number of protons).
    • Elements in the same period show trends in atomic radius, ionization energy, and electron affinity.
    • Atomic radius usually decreases from left to right across a period.
    • This occurs because each successive element has an added proton and electron, which causes the electron to be drawn closer to the nucleus, decreasing the radius.
    • Here is the complete periodic table with atomic numbers, groups, and periods.
  • Energy of a Bohr Orbit

    • To be more general, we note that this analysis is valid for any single-electron atom.
    • So, if a nucleus has $Z$ protons ($Z=1$ for hydrogen, $Z=2$ for helium, etc.) and only one electron, that atom is called a hydrogen-like atom.
    • This is consistent with the planetary model of the atom.
    • The smallest possible value of $r$ in the hydrogen atom is called the Bohr radius and is equal to 0.053 nm.
    • The energy of the $n$-th level for any atom is determined by the radius and quantum number:
  • The Rutherford Model

    • The Rutherford model is a model of the atom named after Ernest Rutherford.
    • Thomson's so-called "plum pudding model" of the atom was incorrect.
    • This central volume also contained the bulk of the atom's mass .
    • From purely energetic considerations of how far particles of known speed would be able to penetrate toward a central charge of 100 e, Rutherford was able to calculate that the radius of his gold central charge would need to be less than $3.4 \cdot 10^{-14}$ meters.
    • This was in a gold atom known to be about $10^{-10}$ meters in radius; a very surprising finding, as it implied a strong central charge less than $\frac{1}{3000}$th of the diameter of the atom.
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