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Try again? Cited by. Back to tab navigation Download options Please wait Article type: Other. DOI: Download Citation: J. General and physical chemistry J. Twenty-six elements have only a single stable isotope, while the largest number of stable isotopes observed for any element is ten, for the element tin. Elements 43 , 61 , and all elements numbered 83 or higher have no stable isotopes.
Stability of isotopes is affected by the ratio of protons to neutrons, and also by the presence of certain "magic numbers" of neutrons or protons that represent closed and filled quantum shells. These quantum shells correspond to a set of energy levels within the shell model of the nucleus; filled shells, such as the filled shell of 50 protons for tin, confers unusual stability on the nuclide.
Of the known stable nuclides, only four have both an odd number of protons and odd number of neutrons: hydrogen-2 deuterium , lithium-6 , boron and nitrogen Also, only four naturally occurring, radioactive odd—odd nuclides have a half-life over a billion years: potassium , vanadium , lanthanum and tantalumm. Most odd—odd nuclei are highly unstable with respect to beta decay , because the decay products are even—even, and are therefore more strongly bound, due to nuclear pairing effects.
The large majority of an atom's mass comes from the protons and neutrons that make it up. The total number of these particles called "nucleons" in a given atom is called the mass number. It is a positive integer and dimensionless instead of having dimension of mass , because it expresses a count. An example of use of a mass number is "carbon," which has 12 nucleons six protons and six neutrons.
The actual mass of an atom at rest is often expressed using the unified atomic mass unit u , also called dalton Da. This unit is defined as a twelfth of the mass of a free neutral atom of carbon , which is approximately 1. However, this number will not be exactly an integer except in the case of carbon see below. As even the most massive atoms are far too light to work with directly, chemists instead use the unit of moles. One mole of atoms of any element always has the same number of atoms about 6.
Atoms lack a well-defined outer boundary, so their dimensions are usually described in terms of an atomic radius. This is a measure of the distance out to which the electron cloud extends from the nucleus. Atomic radii may be derived from the distances between two nuclei when the two atoms are joined in a chemical bond. The radius varies with the location of an atom on the atomic chart, the type of chemical bond, the number of neighboring atoms coordination number and a quantum mechanical property known as spin.
When subjected to external forces, like electrical fields , the shape of an atom may deviate from spherical symmetry.
The deformation depends on the field magnitude and the orbital type of outer shell electrons, as shown by group-theoretical considerations. Aspherical deviations might be elicited for instance in crystals , where large crystal-electrical fields may occur at low-symmetry lattice sites. However, individual atoms can be observed using a scanning tunneling microscope.
Every element has one or more isotopes that have unstable nuclei that are subject to radioactive decay, causing the nucleus to emit particles or electromagnetic radiation. The most common forms of radioactive decay are:  . Other more rare types of radioactive decay include ejection of neutrons or protons or clusters of nucleons from a nucleus, or more than one beta particle. An analog of gamma emission which allows excited nuclei to lose energy in a different way, is internal conversion —a process that produces high-speed electrons that are not beta rays, followed by production of high-energy photons that are not gamma rays.
A few large nuclei explode into two or more charged fragments of varying masses plus several neutrons, in a decay called spontaneous nuclear fission. Each radioactive isotope has a characteristic decay time period—the half-life —that is determined by the amount of time needed for half of a sample to decay. Elementary particles possess an intrinsic quantum mechanical property known as spin. This is analogous to the angular momentum of an object that is spinning around its center of mass , although strictly speaking these particles are believed to be point-like and cannot be said to be rotating.
In an atom, electrons in motion around the nucleus possess orbital angular momentum in addition to their spin, while the nucleus itself possesses angular momentum due to its nuclear spin.
The magnetic field produced by an atom—its magnetic moment —is determined by these various forms of angular momentum, just as a rotating charged object classically produces a magnetic field. However, the most dominant contribution comes from electron spin. Due to the nature of electrons to obey the Pauli exclusion principle , in which no two electrons may be found in the same quantum state , bound electrons pair up with each other, with one member of each pair in a spin up state and the other in the opposite, spin down state. Thus these spins cancel each other out, reducing the total magnetic dipole moment to zero in some atoms with even number of electrons.
In ferromagnetic elements such as iron, cobalt and nickel, an odd number of electrons leads to an unpaired electron and a net overall magnetic moment. The orbitals of neighboring atoms overlap and a lower energy state is achieved when the spins of unpaired electrons are aligned with each other, a spontaneous process known as an exchange interaction.
When the magnetic moments of ferromagnetic atoms are lined up, the material can produce a measurable macroscopic field. Paramagnetic materials have atoms with magnetic moments that line up in random directions when no magnetic field is present, but the magnetic moments of the individual atoms line up in the presence of a field.
The nucleus of an atom will have no spin when it has even numbers of both neutrons and protons, but for other cases of odd numbers, the nucleus may have a spin. Normally nuclei with spin are aligned in random directions because of thermal equilibrium.
However, for certain elements such as xenon it is possible to polarize a significant proportion of the nuclear spin states so that they are aligned in the same direction—a condition called hyperpolarization. This has important applications in magnetic resonance imaging. The potential energy of an electron in an atom is negative , its dependence of its position reaches the minimum the most absolute value inside the nucleus, and vanishes when the distance from the nucleus goes to infinity , roughly in an inverse proportion to the distance. An energy level can be measured by the amount of energy needed to unbind the electron from the atom, and is usually given in units of electronvolts eV.
The lowest energy state of a bound electron is called the ground state, i. For an electron to transition between two different states , e. Electrons jump between orbitals in a particle-like fashion. For example, if a single photon strikes the electrons, only a single electron changes states in response to the photon; see Electron properties. The energy of an emitted photon is proportional to its frequency , so these specific energy levels appear as distinct bands in the electromagnetic spectrum.
When a continuous spectrum of energy is passed through a gas or plasma, some of the photons are absorbed by atoms, causing electrons to change their energy level. Those excited electrons that remain bound to their atom spontaneously emit this energy as a photon, traveling in a random direction, and so drop back to lower energy levels.
Thus the atoms behave like a filter that forms a series of dark absorption bands in the energy output. An observer viewing the atoms from a view that does not include the continuous spectrum in the background, instead sees a series of emission lines from the photons emitted by the atoms. Spectroscopic measurements of the strength and width of atomic spectral lines allow the composition and physical properties of a substance to be determined. Close examination of the spectral lines reveals that some display a fine structure splitting. This occurs because of spin—orbit coupling , which is an interaction between the spin and motion of the outermost electron.
This is caused by the interaction of the magnetic field with the magnetic moment of the atom and its electrons. Some atoms can have multiple electron configurations with the same energy level, which thus appear as a single spectral line. The interaction of the magnetic field with the atom shifts these electron configurations to slightly different energy levels, resulting in multiple spectral lines.
If a bound electron is in an excited state, an interacting photon with the proper energy can cause stimulated emission of a photon with a matching energy level. For this to occur, the electron must drop to a lower energy state that has an energy difference matching the energy of the interacting photon.