Atomic radio

Diagram of a helium atom, showing the probability distribution of the situation of electrons by a gray shade. In the center, the nucleus of the atom, with two protons and two neutrons.

The atomic radius identifies the average distance between two nuclei of the same element linked together. Using the atomic radius it is possible to determine the size of the atom.

Depending on the definition, the term can be applied to atoms in condensed matter, covalent bond in molecules, or in ionized and excited state; and its value can be obtained by experimental measurements or calculated from theoretical models. The value of the radius can depend on the state and context of the atom.

Electrons do not have defined orbits and do not have clearly defined ranges. Rather, their positions should be described as probability distributions that gradually decrease as one moves further from the nucleus, without a sharp limit; these are called atomic orbitals or electron clouds. Also, in condensed matter and molecules, the electron clouds of atoms usually overlap to some degree, and some of the electrons may wander across a large region spanning two or more atoms.

History

In 1920, shortly after it was already possible to determine the sizes of atoms using X-ray diffraction, it was suggested that all atoms of the same element have the same radius. However, in 1923, when there was With more data available, it was determined that the approximation of an atom as a sphere does not necessarily hold when comparing the same atom in crystals with different structures.

Definitions

Approximate form of an ethanol molecule, CH₃CH₂OH. Each atom is represented by a sphere with Van der Waals radius corresponding to the element (usual color code: black carbon; red oxygen; white hydrogen).

Widely used definitions of atomic radius include:

• Van der Waals Radio: in principle, half of the minimum distance between the nuclei of two atoms of the element that are not linked to the same molecule.
• Ionic radio: the nominal radius of the ions of an element in a specific ionization state, deduced from the separation of atomic nuclei in crystalline salts that include ion. In principle, the separation between two adjacent opposite load ions should be equal to the sum of their ionic radios.
• Covalent radio: the nominal radius of the atoms of an element when they have covalent link with other atoms, as deduced from the separation between the atomic nuclei in the molecules. In principle, the distance between two atoms that are attached to each other in one molecule (the length of that covalent link) must be equal to the sum of their covalent radios.
• Metallic radio: the nominal radius of atoms of an element when joining other atoms by metallic link.^{[chuckles]required]}
• Bohr Radio: the radius of the orbit of the lower-energy electron predicted by the Bohr model of the atom (1913). It applies only to atoms and ions with a single electron, such as hydrogen, helium simply ionized, and positronium. Although the model itself is already obsolete, Bohr's radio for the hydrogen atom is considered an important physical constant.

Properties

• In the same group, the atomic radio increases from top to bottom with the amount of energy levels. As the energy level is higher, the atomic radio is higher.
• In the same period, the atomic radius decreases from left to right, as, when going to the right, the atomic number (Z) increases in one unit as it moves from one element to another, that is, there is an increase in nuclear load so the electrons are attracted.
• The atomic radius can be covalent or metallic. The distance between nuclei of "vecino" atoms in a molecule is the sum of its covalent radios, while metallic radio is half the distance between nuclei of "vecino" atoms in metallic crystals. Usually, by atomic radio it is to understand covalent radio.

Atomic radius values

The following table shows the values in angstroms published by J. C. Slater, with an uncertainty of 0.12 Å: