Atom
A depiction of the atomic structure of the helium atom. The darkness of the electron cloud corresponds to the line-of-sight integral over the probability function of the 1s electron orbital. The magnified nucleus is schematic, showing protons in pink and neutrons in purple. In reality, the nucleus (and the wavefunction of each of the nucleons) is also spherically symmetric. (For more complicated nuclei this is not the case.)
Classification: Smallest recognized division of a chemical element
Properties:
Mass ≈ 1.67 × 10-27 to 4.52 × 10-25 kg
Electric charge: zero (if the number of electrons equal the number of protons in an atom)
Diameter: 50 pm(H) to 520 pm(Cs)
Number of atoms in the observable universe: 1080
In chemistry and physics, an atom (Greek ἄτομος or átomos meaning "indivisible") is the smallest particle still characterizing a chemical element. (átomos is usually translated as "indivisible" or "uncuttable." Until the advent of quantum mechanics, dividing a material object was invariably equated with cutting it.) Whereas the word atom originally denoted a particle that cannot be cut into smaller particles, the atoms of modern parlance are composed of subatomic particles:
electrons, which have a negative charge, a size which is so small as to be currently unmeasurable, and which are the least heavy (i.e., massive) of the three;
- protons, which have a positive charge, and are about 1836 times more massive than electrons; and
- neutrons, which have no charge, and are the same size as protons.
- Protons and neutrons make up a dense, massive atomic nucleus, and are collectively called nucleons. The electrons form the much larger electron cloud surrounding the nucleus.
Atoms can differ in the number of each of the subatomic particles they contain. Atoms of the same element have the same number of protons (called the atomic number). Within a single element, the number of neutrons may vary, determining the isotope of that element. The number of electrons associated with an atom is most easily changed, due to the lower energy of binding of electrons. The number of protons (and neutrons) in the atomic nucleus may also change, via nuclear fusion, nuclear fission, bombardment by high energy subatomic particles or photons, or certain (but not all) types of radioactive decay. In such processes which change the number of protons in a nucleus, the atom becomes an atom of a different chemical element.
Atoms are electrically neutral if they have an equal number of protons and electrons. Atoms which have either a deficit or a surplus of electrons are called ions. Electrons that are furthest from the nucleus may be transferred to other nearby atoms or shared between atoms. By this mechanism atoms are able to bond into molecules and other types of chemical compounds like ionic and covalent network crystals.
Atoms are the fundamental building blocks of chemistry, and are conserved in chemical reactions.
Atoms and molecules
History of the molecule
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 salts, rocks, and liquid and solid metals). Thus, while molecules are common on Venus (making up all of the atmosphere and most of the oceans), most of the mass of the Earth (much of the crust, and all of the mantle and core) is not made of identifiable molecules, but rather represents atomic matter in other networked arrangements, all of which lack the particular type of small-scale interrupted order (i.e., small, strongly-bound collections of atoms held to other collections of atoms by much weaker forces) that is associated with molecular matter.
Most molecules are made up of multiple atoms; for example, a molecule of water is a combination of two hydrogen atoms and one oxygen atom. The term "molecule" in gases has been used as a synonym for the fundamental particles of the gas, whatever their structure. This definition results in a few types of gases (for example inert elements that do not form compounds, such as neon), which has "molecules" consisting of only a single atom.
History of atom
Atomic Theory and Atomism
Philosophical atomic ruminations date back to the ancient Greeks and Indians in the fifth and sixth centuries BCE. It was the Greeks (Democritus; see below) who coined the term atomos, which meant "uncuttable".
The first philosophical statements relating to an idea similar to atoms was developed by Democritus in Greece in the fifth century BCE around 450 BCE. The idea was lost for centuries until scientific interest was rekindled during the Renaissance Period.
Father of modern atomism was a Jesuit priest, Rudjer Boscovich, who based his atomism mostly on classical mechanics (Newtonian mechanics) and published it in 1758. The theory was further developed by Amedeo Avogadro, his brother Johann Avogadro and the developers of the kinetic theory of gases such as James Clerk Maxwell and physicist Ludwig Boltzmann. Boscovich was regarded as the father of modern atomic theory by Faraday, Mendeleev, Maxwell, and Kelvin, who observed that his and the work of others' were "all developments of Boscovich's theory pure and simple."
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 compound chemicals.
In 1897, JJ Thomson, through his work on cathode rays, discovered the electron and its subatomic nature (i.e., its lightness compared with the mass of atoms), which destroyed the concept of atoms as being indivisible units. Later, Thomson also discovered the existence of isotopes through his work on ionized gases.
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 gold foil experiment was interpreted by Ernest Rutherford as suggesting that the positive charge of an atom and most of its mass was concentrated in a nucleus at the center of the atom (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.
In 1926, Erwin Schrodinger, using Louis DeBroglie's 1924 proposal that all particles behave to an extent like waves, developed a mathematical model of the atom that described the electrons as three-dimensional waveforms, rather than point particles. A consequence of using waveforms to describe electrons, pointed out by Werner Heisenberg a year later, is that it is mathematically impossible to obtain precise values for both the position and momentum of a particle at any point in time; this became known as the uncertainty principle. In this concept, for any given value of position one could only obtain a range of probable values for momentum, and vice versa. Although this model was difficult to visually conceptualize, it was able to explain many observations of atomic behavior that previous models could not, such as certain structural and 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.
Atoms and the Big Bang Theory
Immediately after the start of the Big Bang, space expanded incredibly quickly for a very short time. This process, which lasted for the minutest fraction of a second, is called inflation. After that, expansion began to slow down and different kinds of subatomic particles including quarks and electrons made their appearance. Just one millionth of a second after the birth of the universe, the quarks had clumped together to form new particles called protons and neutrons. After a hundred seconds or so, some of the protons and nearly all of the neutrons gathered into bunches, consisting of two protons and two neutrons. Eventually, each bunch, or atomic nucleus, captured two electrons to form a helium atom, and each remaining proton captured a single electron to form a hydrogen atom. The first building blocks of matter had been born.
In models of the Big Bang, Big Bang nucleosynthesis predicts that within one to three minutes of the Big Bang almost all atomic material in the universe was created. During this process, nuclei of hydrogen and helium formed abundantly, but almost no elements heavier than lithium. Hydrogen makes up approximately 92% of the atoms in the universe (by number, not mass); helium makes up less than 7%; and all other elements make up less than 1% (see Abundance of the chemical elements). However, although nuclei (fully-ionized atoms) were created, neutral atoms themselves could not form in the intense heat.
Big Bang chronology of the atom continues to approximately 380,000 years after the Big Bang when the cosmic temperature had dropped to just 3,000 K. It was then cool enough to allow the nuclei to capture electrons. This process is called recombination, during which the first neutral atoms took form. Once atoms become neutral, they only absorb photons of a discrete absorption spectrum. This allows most of the photons in the universe to travel unimpeded for billions of years. These photons are still detectable today in the cosmic microwave background radiation.
After Big Bang nucleosynthesis, no heavier elements could be created until the formation of the first stars. These stars fused heavier elements through stellar nucleosynthesis during their lives and through supernova nucleosynthesis as they died. The seeding of the interstellar medium by heavy elements eventually allowed the formation of terrestrial planets like the Earth.
Atom size comparisons
Various analogies have been used to demonstrate the minuteness of the atom:
- A human hair is about 1 million carbon atoms wide.
- A single drop of water contains about 2 sextillion atoms of oxygen (2 followed by 21 zeros, 2×1021) and twice as many hydrogen atoms.
- 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 3x1012 (3 trillion) atoms.
- The number of atoms in 12 grams of charcoal (about 6 x 1023) is more than 1,400,000 times the age of the universe in seconds.
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