Atom



Atom
This illustrates the Helium atom. The nucleus (upper right) is in reality spherically symmetric, although this is not always the case for more complicated nuclei.
Classification
Smallest recognized division of a chemical element
Properties
kg
Electric charge : zero
Diameter : (data page) 31 pm (Cs)
Number of atoms in the observable universe: ~1080[1]


In isotope of the element.

The concept of the atom as an indivisible component of matter was proposed by early Indian and Greek philosophers. Early chemists, such as Erwin Schrödinger and others to produce an accurate mathematical model of the structure and properties of the atom.[2]

Relative to everyday experience, atoms are miniscule objects with proportionately tiny masses that can only be observed individually using special instruments such as the magnetic moment.

History

Main articles: Atomic theory and Atomism

The concept that matter is composed of discrete units and can not be divided into any arbitrarily tiny or small quantities has been around for thousands of years, but these ideas were founded in abstract, philosophical reasoning rather than experimentation and empirical observation. The nature of atoms in philosophy varied considerably over time and between cultures and schools, and often had spiritual elements. Nevertheless, the basic idea of the atom was adopted by scientists thousands of years later because it elegantly explained new discoveries in the field of chemistry.[5]

The earliest references to the concept of atoms date back to ancient India in the 6th century BCE.[6] The Nyaya and Vaisheshika schools developed elaborate theories of how atoms combined into more complex objects (first in pairs, then trios of pairs).[7] The references to atoms in the West emerged a century later from Leucippus whose student, Democritus, systemized his views. In around 450 BCE, Democritus coined the term atomos, which meant "uncuttable". Though both the Indian and Greek concepts of the atom were based purely on philosophy, modern science has retained the name coined by Democritus.[5]

Antoine Lavoisier, to mean basic substances that could not be further broken down by the methods of chemistry.[9]

  In 1803, John Dalton used the concept of atoms to explain why elements always reacted in simple proportions, and why certain gases dissolved better in water than others. He proposed that each element consists of atoms of a single, unique type, and that these atoms could join to each other, to form chemical compounds.[10][11]

In 1827 a British botanist Robert Brown used a microscope to look at dust grains floating in water. He called their erratic motion "Brownian motion". J. Desaulx suggested in 1877 that the phenomenon was caused by the thermal motion of water molecules, and in 1905 Albert Einstein produced the first mathematical analysis of the motion, thus confirming the hypothesis.[12][13]

In 1897, JJ Thomson, through his work on cathode rays, discovered the electron and its subatomic nature, which destroyed the concept of atoms as being indivisible units. Later, Thomson also created a technique for separating different types of atoms through his work on ionized gases, which subsequently led to the discovery of isotopes.[14]

  Thomson believed that the electrons were distributed evenly throughout the atom, balanced by the presence of a uniform sea of positive charge. However, in 1909, the Rutherford model), with the electrons orbiting it like planets around a sun. In 1913, Niels Bohr added quantum mechanics into this model, which now stated that the electrons were locked or confined into clearly defined orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states.[15]

In 1926, spectral patterns of atoms bigger than hydrogen. Thus, the planetary model of the atom was discarded in favor of one that described orbital zones around the nucleus where a given electron is most likely to exist.[16][17]

In 1913, proton, by James Chadwick in 1932. Isotopes were then explained as elements with the same number of protons, but different numbers of neutrons within the nucleus.[19]

Around 1985, Steven Chu and co-workers at Bell Labs developed a technique for lowering the temperatures of atoms using Bose-Einstein condensation.[20]

Components

Subatomic particles

Main article: Subatomic particle

Though the word atom originally denoted a particle that cannot be cut into smaller particles, in modern scientific usage the 'atom' is composed of various neutron.

The electron is by far the least massive of these particles at 9.11×10-31 kg, with a negative electrical charge and a size that is so small as to be currently unmeasurable.[21] Protons have a positive charge and a mass 1,836 times that of the electron, at 1.6726×10-27 kg, although atomic binding energy changes can reduce this. Neutrons have no electrical charge and have a free mass of 1,839 times the mass of electrons,[22] or 1.6929x10-27 kg. Neutrons and protons have comparable dimensions—on the order of 2.5×10-15 m—although the 'surface' of these particles is not very sharply defined.[23]

In the Standard Model of physics, both protons and neutrons are composed of bosons, which are elementary particles that mediate physical forces.[24][25]

Nucleus

Main article: Atomic nucleus

All of the bound protons and neutrons in an atom make up a dense, massive nucleons. Although the positive charge of protons causes them to repel each other, they are bound together with the neutrons by a short-ranged attractive potential called the residual strong force. At distances smaller than 2.5 fm, the residual strong force is stronger than the coulomb force, so it is able to overcome the mutual repulsion between the protons in the nucleus. The radius of a nucleus is approximately equal to \begin{smallmatrix}1.2 \cdot \sqrt[3]{A}\end{smallmatrix} fm, where A is total number of nucleons. This is much smaller than the radius of the atom, which is on the order of 105 fm.[26]

Atoms of the same radioactive decay.

  The number of protons and neutrons in the atomic nucleus can be modified, although this can require very high energies because of the strong force. Nuclear fission is the opposite process, causing the nucleus to emit some amount of nucleons—usually through radioactive decay. The nucleus can also be modified through bombardment by high energy subatomic particles or photons. In such processes which change the number of protons in a nucleus, the atom becomes an atom of a different chemical element.[27][28]

The fusion of two nuclei that have lower atomic numbers than speed of light.[29]

The mass of the nucleus is less than the sum of the masses of the separate particles. The difference between these two values is binding energy of the nucleus. It is the energy that is emitted when the individual particles come together to form the nucleus.[26] The binding energy per nucleon increases with increasing atomic number until iron or nickel is reached.[30] For heavier nuclei, the binding energy begins to decrease. That means fusion processes with nuclei that have higher atomic numbers is an hydrostatic equilibrium of a star.) In atoms with high or very low ratios of protons to neutrons, the binding energy becomes negative, resulting in an unstable nucleus.[31]

Electron cloud

Main article: Electron cloud

The electrons in an atom are bound to the protons in the nucleus by the electromagnetic force. Electrons, as with other particles, have properties of both a particle and a wave. The electron cloud is a region where each electron resides within a type of three-dimensional standing wave inside the electrostatic potential well that surrounds the much smaller nucleus. This standing wave condition is characterized by an atomic orbital, which is an mathematical function that defines the probability that an electron will appear to be at a particular location when its position is measured. Only a discrete (or quantized) set of these orbitals exist around the nucleus, as other possible wave patterns produce interference effects that would destroy the standing wave.[32]

 

Each atomic orbital corresponds to a particular atomic spectral lines.[32]

The number of electrons associated with an atom is readily changed, due to the lower energy of binding of electrons when compared to the binding energy of the nucleus. Atoms are crystals.[33]

Properties

An element consists of all atoms that have the same number of protons in their nuclei. Each element can have multiple ununoctium.[35] All known isotopes of elements with atomic numbers greater than 82 are radioactive.[36][37]

Mass

Main article: Atomic mass

Because the large majority of an atom's mass comes from the protons and neutrons, the total number of these particles in an atom is called the carbon-12, which is approximately 1.66×10-27 kg.[38] Hydrogen, the atom with the lowest mass, has an atomic weight of 1.007825 u.[39] An atom has a mass approximately equal to the mass number times the atomic mass unit.[40]

The Avogadro constant (NA).[38]

Size

Main article: Atomic radius

Atoms lack a well-defined outer boundary, so the dimensions are usually described in terms of the distances between two nuclei when the atoms are bonded. The radius varies with the location of an atom on the atomic chart,[41] its chemical bond type, coordination number (which is the total number of neighbors of a central atom in a chemical compound) and spin state.[42] The smallest atom is helium with a radius of 31 pm, while the largest known is scanning tunneling microscope.

Various analogies have been used to demonstrate the minuteness of the atom. A typical human hair is about 1 million carbon atoms in width. An HIV virion is the width of 800 carbon atoms and contains about 100 million atoms total. An E. coli bacterium contains perhaps 100 billion atoms, and a typical human cell roughly 100 trillion atoms. A speck of dust might contain 3 trillion atoms. A single drop of water contains about 2 sextillion (2×1021) atoms of oxygen, and twice as many hydrogen atoms.[43] If an apple was magnified to the size of the Earth, then the atoms in the apple would be approximately the size of the original apple.[44]

Radioactive decay

Main article: Radioactive decay

 

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. Radioactivity can occur when the radius of a nucleus is large compared to the radius of the strong force, which only acts over distances on the order of 1 fm.[45]

There are three major forms of radioactive decay:[46]

  • atomic number.
  • neutrino. The electron or positron emissions are called beta particles. Beta decay changes the atomic number of the nucleus.
  • Gamma decay results from a change in the energy level of the nucleus to a lower state, resulting in the emission of electromagnetic radiation. This can occur following the emission of an alpha or a beta particle from radioactive decay.

A number of less common forms of radioactive decay which result in emission of some of these particles by other mechanisms, or different particles, are detailed in the main article above.

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. This is an exponential decay process that steadily decreases the proportion of the remaining isotope by 50% every half life.

Magnetic moment

Main articles: Electron magnetic dipole moment and Nuclear magnetic moment

Elementary particles possess a quantum mechanical property known as spin. This property is equivalent to the possession of angular momentum, giving this property a directional component, although the particles themselves can not be said to be rotating. Electrons in particular are "spin-½" particles, as are protons and neutrons. The spin of an atom is determined by the spins of its constituent components, and how the spin is distributed and arranged among the sub-atomic components.

The spin of an atom determines its ferromagnetic elements such as iron though, one of the electrons is unpaired and the atom can experience a net magnetic moment in the absence of an external magnetic field. When the magnetic moment of many ferromagnetic elements are lined up, the material can produce a measurable macroscopic field.

The nucleus of an atom can also have a net spin. Normally the alignment of these nuclei are aligned in random directions because of 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.

Energy levels

Main articles: Atomic spectral line

When an electron is bound to an atom, it has a potential energy that is inversely proportional to its distance from the nucleus. This is measured by the amount of energy needed to unbind the electron from the atom, and is usually given in units of electron volts (eV). In the quantum mechanical model, a bound electron can only occupy a set of states centered on the nucleus, and each state corresponds to a specific energy level. The lowest energy state of a bound electron is called the ground state, while an electron at a higher energy level is in an excited state.[47]

In order for an electron to transition between two different states, it must absorb or emit a electromagnetic spectrum. Each atom has a characteristic spectrum that depends on its nuclear charge, subshells filled by electrons and the electromagnetic interactions between the electrons.

  When a continuous spectrum of energy is passed through a gas or plasma, some of the energy is absorbed by atoms, causing electrons to change their energy level. These excited electrons spontaneously emit this energy as a photon, travelling in a random direction, and so drop back to lower energy levels. Thus the atoms behave like a filter that form a series of dark Spectroscopic measurements of the strength and width of the various spectral lines allow the composition and physical properties of a substance to be determined.[48]

If a bound electron is in an excited state, an interacting photon with the proper energy can cause lasers, which can emit a coherent beam of light energy in a narrow frequency band.[49]

When an atom is in an external magnetic field, spectral lines become split into three or more components; a phenomenon called the Zeeman effect. This is caused by the interaction of the magnetic field with the magnetic moment of the atom and its electrons.[50]

Valence

Main article: Valence (chemistry)

The outermost electron shell of an atom in its uncombined state is known as the valence shell, and the electrons in that shell are called valence electrons. The number of valence electrons determines the bonding behavior with other atoms. Atoms tend to chemically react with each other in a manner that will fill their outer valence shells.

The noble gases.

Identification

 

The local density of states.

An atom can be inductively coupled plasma mass spectrometry, both of which use a plasma to vaporize samples for analyzation.[51]

A more area-selective method is doping species.

Spectra of excited states can be used to analyze the atomic composition of distant stars. Specific light wavelengths that are contained in the observed light from stars can be separated out and related to the quantized transitions in free gas atoms. These colors can be replicated using a gas discharge lamp containing the same element.[53] Helium was discovered in this way in the spectrum of our sun 23 years before it was found on earth.[54]

Applications

Historically single atoms have been prohibitively small for any scientific applications. Recently devices have been constructed that use a single metal atom connected through organic laser cooling in a cavity to gain a better physical understanding of matter.[56]

Origin and current state

Nucleosynthesis

Main article: Nucleosynthesis

The first nuclei of elements one through five, including most of the nuclear fusion to generate atoms up to iron.

Some atoms such as lithium-6 are generated in space through cosmic ray spallation. Elements heavier than iron were generated in supernovae through the lead, formed largely through the radioactive decay of heavier elements.

Earth

Main article: History of the molecule

Most of the atoms that currently make up the Earth and all its inhabitants were present in their current form in the nebula that collapsed out of a molecular cloud to form the solar system. The rest are the result of radioactive decay, and their relative proportion can be used to determine the age of the earth through alpha-decay.

There are a few trace atoms on Earth that were not present at the beginning (i.e. not "primordial"), nor are results of radioactive decay. neutron capture in uranium ore.

Most of the atoms at the surface of the Earth are bound into various molecules. For gases and certain molecular liquids and solids (such as water and sugar), molecules are the smallest division of matter which retains chemical properties; however, there are also many solids and liquids which are made of atoms, but do not contain discrete molecules such as metals. Thus, most of the mass of the Earth—much of the crust, and all of the mantle and core—is not made of identifiable molecules. Rather the atomic matter forms networked arrangements, all of which lack the particular type of small-scale interrupted order that is associated with molecular matter. That is, they form small, strongly bound collections of atoms held to other collections of atoms by much weaker forces.

Most molecules are made up of multiple atoms; for example, a molecule of water is a combination of two neon), which has 'molecules' consisting of only a single atom.[57]

Rare forms

It has been hypothesized that an "unbihexium, has 126 protons and 184 neutrons.

Each particle of matter has a corresponding antimatter particle with the opposite electrical charge. Thus the Antihydrogen, the antimatter counterpart of hydrogen, was first produced at the CERN laboratory in Geneva in 1995.

Other muonic atom. Atoms such as these can be used to test the fundamental predictions of physics.[58]

See also

References

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  3. ^ A carat is 200 milligrams. Avogadro constant defines 6×1023 atoms per mole.
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General references

  • Kenneth S. Krane, Introductory Nuclear Physics (1987)


  • Atomic sizes
  • How Atoms Work
  • Wikibooks FHSST Physics Atom:The Atom
  • Wikibooks Atomic structure
  • Science aid - atomic structure A guide to the atom for teens.
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