Together, all of the electrons of an atom create a negative charge that balances the positive charge of the protons in the atomic nucleus. Electrons are extremely small compared to all of the other parts of the atom. The mass of an electron is almost 1, times smaller than the mass of a proton. Shells and Shapes Electrons are found in clouds that surround the nucleus of an atom.
Those clouds are specific distances away from the nucleus and are generally organized into shells. Because electrons move so quickly, it is impossible to see where they are at a specific moment in time. After years of experimentation, scientists discovered specific areas where electrons are likely to be found. The overall shape of the shells changes depending on how many electrons an element has. The higher the atomic number, the more shells and electrons an atom will have.
Hence, the concept of a dimensionless electron possessing these properties might seem inconsistent. The apparent paradox can be explained by the formation of virtual photons in the electric field generated by the electron. These photons cause the electron to shift about in a jittery fashion known as zitterbewegung , which results in a net circular motion with precession. This motion produces both the spin and the magnetic moment of the electron.
In atoms, this creation of virtual photons explains the Lamb shift observed in spectral lines. An electron can be bound to the nucleus of an atom by the attractive Coulomb force.
A system of one or more electrons bound to a nucleus is called an atom. If the number of electrons is different from the nucleus' electrical charge, such an atom is called an ion. The wave-like behavior of a bound electron is described by a function called an atomic orbital. Each orbital has its own set of quantum numbers such as energy, angular momentum and projection of angular momentum, and only a discrete set of these orbitals exist around the nucleus.
According to the Pauli exclusion principle each orbital can be occupied by up to two electrons, which must differ in their spin quantum number. Electrons can transfer between different orbitals by the emission or absorption of photons with an energy that matches the difference in potential.
Other methods of orbital transfer include collisions with particles, such as electrons, and the Auger effect. In order to escape the atom, the energy of the electron must be increased above its binding energy to the atom. This occurs, for example, with the photoelectric effect , where an incident photon exceeding the atom's ionization energy is absorbed by the electron. The orbital angular momentum of electrons is quantized.
Because the electron is charged, it produces an orbital magnetic moment that is proportional to the angular momentum. The net magnetic moment of an atom is equal to the vector sum of orbital and spin magnetic moments of all electrons and the nucleus.
The magnetic moment of the nucleus is negligible compared with that of the electrons. The magnetic moments of the electrons that occupy the same orbital so called, paired electrons cancel each other out.
The chemical bond between atoms occurs as a result of electromagnetic interactions, as described by the laws of quantum mechanics. The strongest bonds are formed by the sharing or transfer of electrons between atoms, allowing the formation of molecules.
Within a molecule, electrons move under the influence of several nuclei, and occupy molecular orbitals ; much as they can occupy atomic orbitals in isolated atoms.
A fundamental factor in these molecular structures is the existence of electron pairs. These are electrons with opposed spins, allowing them to occupy the same molecular orbital without violating the Pauli exclusion principle much like in atoms.
Different molecular orbitals have different spatial distribution of the electron density. For instance, in bonded pairs i. On the contrary, in non-bonded pairs electrons are distributed in a large volume around nuclei. If a body has more or fewer electrons than are required to balance the positive charge of the nuclei, then that object has a net electric charge. When there is an excess of electrons, the object is said to be negatively charged.
When there are fewer electrons than the number of protons in nuclei, the object is said to be positively charged. When the number of electrons and the number of protons are equal, their charges cancel each other and the object is said to be electrically neutral. A macroscopic body can develop an electric charge through rubbing, by the triboelectric effect. Independent electrons moving in vacuum are termed free electrons.
Electrons in metals also behave as if they were free. In reality the particles that are commonly termed electrons in metals and other solids are quasi-electrons— quasiparticles , which have the same electrical charge, spin and magnetic moment as real electrons but may have a different mass. When free electrons—both in vacuum and metals—move, they produce a net flow of charge called an electric current , which generates a magnetic field.
Likewise a current can be created by a changing magnetic field. These interactions are described mathematically by Maxwell's equations. At a given temperature, each material has an electrical conductivity that determines the value of electric current when an electric potential is applied. Examples of good conductors include metals such as copper and gold, whereas glass and Teflon are poor conductors. In any dielectric material, the electrons remain bound to their respective atoms and the material behaves as an insulator.
Most semiconductors have a variable level of conductivity that lies between the extremes of conduction and insulation. On the other hand, metals have an electronic band structure containing partially filled electronic bands. The presence of such bands allows electrons in metals to behave as if they were free or delocalized electrons. These electrons are not associated with specific atoms, so when an electric field is applied, they are free to move like a gas called Fermi gas through the material much like free electrons.
Because of collisions between electrons and atoms, the drift velocity of electrons in a conductor is on the order of millimeters per second. This occurs because electrical signals propagate as a wave, with the velocity dependent on the dielectric constant of the material.
Metals make relatively good conductors of heat, primarily because the delocalized electrons are free to transport thermal energy between atoms. However, unlike electrical conductivity, the thermal conductivity of a metal is nearly independent of temperature. This is expressed mathematically by the Wiedemann—Franz law, which states that the ratio of thermal conductivity to the electrical conductivity is proportional to the temperature.
The thermal disorder in the metallic lattice increases the electrical resistivity of the material, producing a temperature dependence for electrical current. When cooled below a point called the critical temperature , materials can undergo a phase transition in which they lose all resistivity to electrical current, in a process known as superconductivity.
In BCS theory, this behavior is modeled by pairs of electrons entering a quantum state known as a Bose—Einstein condensate. These Cooper pairs have their motion coupled to nearby matter via lattice vibrations called phonons , thereby avoiding the collisions with atoms that normally create electrical resistance.
Cooper pairs have a radius of roughly nm, so they can overlap each other. However, the mechanism by which higher temperature superconductors operate remains uncertain. Electrons inside conducting solids, which are quasi-particles themselves, when tightly confined at temperatures close to absolute zero , behave as though they had split into two other quasiparticles : spinons and holons.
The former carries spin and magnetic moment, while the latter electrical charge. According to Einstein's theory of special relativity , as an electron's speed approaches the speed of light , from an observer's point of view its relativistic mass increases, thereby making it more and more difficult to accelerate it from within the observer's frame of reference.
The speed of an electron can approach, but never reach, the speed of light in a vacuum, c. However, when relativistic electrons—that is, electrons moving at a speed close to c —are injected into a dielectric medium such as water, where the local speed of light is significantly less than c , the electrons temporarily travel faster than light in the medium.
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