# atomic theory

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## atomic theory

1. any theory in which matter is regarded as consisting of atoms, esp that proposed by John Dalton postulating that elements are composed of atoms that can combine in definite proportions to form compounds
2. the current concept of the atom as an entity with a definite structure

## Atomic theory

The study of the structure and properties of atoms based on quantum mechanics and the Schrödinger equation. These tools make it possible, in principle, to predict most properties of atomic systems. A stationary state of an atom is governed by a time-independent wave function which depends on the position coordinates of all the particles within the atom. To obtain the wave function, the time-independent Schrödinger equation, a second-order differential equation, has to be solved. The potential energy term in this equation contains the Coulomb interaction between all the particles in the atom, and in this way they are all coupled to each other. See Quantum mechanics

A many-particle system where the behavior of each particle at every instant depends on the positions of all the other particles cannot be solved directly. This is not a problem restricted to quantum mechanics. A classical system where the same problem arises is a solar system with several planets. In classical mechanics as well as in quantum mechanics, such a system has to be treated by approximate methods.

#### Independent particle model

As a first approximation, it is customary to simplify the interaction between the particles. In the independent particle model the electrons are assumed to move independently of each other in the average field generated by the nucleus and the other electrons. In this case the potential energy operator will be a sum over one-particle operators. The simplest wave function which will satisfy the resulting equation is a product of one-particle orbitals. To fulfill the Pauli exclusion principle, the total wave function must, however, be written in a form such that it will vanish if two particles are occupying the same quantum state. This is achieved with an antisymmetrized wave function, that is, a function which, if two electrons are interchanged, changes sign but in all other respects remains unaltered. The antisymmetrized product wave function is usually called a Slater determinant. See Exclusion principle

#### Hartree-Fock method

In the late 1920s, only a few years after the discovery of the Schrödinger equation, D. Hartree showed that the wave function to a good approximation could be written as a product of orbitals, and also developed a method to calculate the orbitals. Important contributions to the method were also made by V. Fock and J. C. Slater (thus, the Hartree-Fock method). The Hartree-Fock model thus gives the lowest-energy ground state within the assumption that the electrons move independently of each other in an average field from the nucleus and the other electrons.

To simplify the problem even further, it is common to add the requirement that the Hartree-Fock potential should be spherically symmetric. This leads to the central-field model and the so-called restricted Hartree-Fock method.

The Hartree-Fock method gives a qualitative understanding of many atomic properties. Generally it is, for example, able to predict the configurations occupied in the ground states of the elements. Electron binding energies are also given with reasonable accuracy.

#### Electron correlation

Correlation is commonly defined as the difference between the full many-body problem and the Hartree-Fock model. More specifically, the correlation energy is the difference between the experimental energy and the Hartree-Fock energy. There are several methods developed to account for electron correlation, including the configuration-interaction method, the multiconfiguration Hartree-Fock method, and perturation theory.

#### Strongly correlated systems

Although the Hartree-Fock model can qualitatively explain many atomic properties, there are systems and properties for which correlation is more important, such as negative ions, doubly-excited states, and some open-shell systems. If the interest is not in calculating the total energy of a state but in understanding some other properties, such as the hyperfine structure, effects beyond the central field model can be more important. See Hyperfine structure, Negative ion

#### Relativistic effects

The Schrödinger equation is a nonrelativistic wave equation. In heavy elements the kinetic energy of the electrons becomes very large, and calculations are based on the relativistic counterpart to the Schrödinger equation, the Dirac equation. It is possible to construct a Hartree-Fock model based on the Dirac equation, where the electron-electron interaction is given by the Coulomb interaction, a magnetic contribution, and a term which corrects for the finite speed (retardation) with which the interaction propagates. See Antimatter, Relativistic quantum theory

Radiative corrections, which arise when the electromagnetic field is quantized within the theory of quantum electrodynamics, For many-body systems, calculations of radiative effects are usually done within some independent-particle model, and the result is added to a correlated relativistic calculation based on the Dirac equation. See Atomic structure and spectra, Quantum electrodynamics

## atomic theory

[ə′täm·ik ′thē·ə·rē]
(chemistry)
The assumption that matter is composed of particles called atoms and that these are the limit to which matter can be subdivided.
References in periodicals archive ?
(31) See Crane, 9-10, on the fact that early modern atomic theory raised the threatening possibility that void space, or a vacuum, could exist, contrary to Aristotelian teaching.
Besides the "Atomic Theory," many theories have been modified through time, becoming even stronger in their explanation of naturally occurring objects, phenomena, or processes.
If a similar book had been written at the end of the last century, he points out, the author might have considered many fundamental questions in chemistry, atomic theory, and cosmology settled.
Pierre Gassendi is best known for his revival of the atomic theory of Epicurus, advanced as a hypothetical system for modern science.
The curriculum correlates to Florida's Sunshine State Standards and is divided into the following 21 units of study: (1) scientific method; (2) scientific measurements; (3) matter; (4) changes in matter; (5) introduction to the atom; (6) atomic theory; (7) structure of matter; (8) chemical equations; (9) solutions and suspensions; (10) acids, bases, and salts; (11) chemical reactions; (12) energy, work, force, and power; (13) forms of energy; (14) forces and motions; (15) machines; (16) magnetism; (17) electricity; (18) nuclear energy; (19) heat; (20) waves; and (21) science, society, and the world.
He was a friend of Henry Brougham and of John Dalton, and helped Dalton in his work leading to the atomic theory. He also worked on coal gas and its use in lighting, on disease and contagion and, perhaps not surprisingly, believed contagion was a chemical problem that should be dealt with by using chemical methods.
329), or the finer details of atomic theory? While the concentration upon a clear uncluttered text is admirable, both the price and the paucity of useful annotation would seem to alienate the very audience these texts are trying to reach.
How could this be reconciled with Boltzmann's major scientific achievement, the kinetic atomic theory of gases, a theory that explained the observationally apparent thermal features of macroscopic objects by positing invisible atoms as their constituents and a Newtonian, quasi-billiard ball, dynamics for their behaviour?
Even though atomic theory may change, "atoms are altogether safe" (p.
When he proposed his atomic theory in 1803, John Dalton improved on the Greeks' idea that all matter was composed of tiny, indivisible particles by introducing the concept of atomic weight.
These latter sciences do not contradict atomic theory, they simply find it irrelevant.
It is outstanding both as a scientific explanation of the poet's atomic theory and as a fine poem.

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