C1 : 1.2-What is an Atom? & Sub-Atomic Particles ?

A more accurate depiction of an atom, showing it is mostly empty space (grey area) traversed by rapidly moving electrons (blue dots, drawn much larger than to scale) with the heavy nucleus (red and white dot at center, drawn larger than to scale) at center. Its shape is like that of a rural community, with expanses of uninhabited land, a few scattered farm houses, and a small village with closely packed houses at its center.

Atoms are the basic units of matter and the defining structure of element. Atoms are made up of three particles: protons, neutrons and electrons.

Protons and neutrons are heavier than electrons and reside in the center of the atom, which is called the nucleus. Electrons are extremely lightweight and exist in a cloud orbiting the nucleus. The electron cloud has a radius 10,000 times greater than the nucleus.

Protons and neutrons have approximately the same mass. However, one proton weighs more than 1,800 electrons. Atoms always have an equal number of protons and electrons, and the number of protons and neutrons is usually the same as well. Adding a proton to an atom makes a new element, while adding a neutron makes an isotope, or heavier version, of that atom.

Electron
1. Electrons are present outside the nucleus of an atom. 
2. Electrons are negatively charged. 
3. The mass of an electron is considered to negligible.
Proton 
1. Protons are present in the nucleus of an atom. 
2. Protons are positively charged. 
3. The mass of a proton is approximately 2000 times as the mass of an electron.
Neutron
1. Neutrons are present in the nucleus of an atom. 
2. Neutrons are neutral. 
3. The mass of neutron is nearly equal to the mass of a proton.

SOURCE : http://chemwiki.ucdavis.edu/

The Bohr model is outdated, but it depicts the three basic subatomic particles in a comprehensible way. Electron clouds are more accurate representations of where electrons are found. Darker areas represent where the electrons are more likely to be found, and lighter areas represent where they are less likely to be found.

Particle	Mass (gram)	Charge (Coulomb)	Charge (units)
Electron (e)	9.1 x 10-28	-1.6 x 10-19	-1
Proton (p)	1.67 x 10-27	+1.6 x 10-19	+1
Neutron(n)	1.67 x 10-27	0	0

• The positive charge of protons cancels the negative charge of the electrons. Neutrons have no charge.
• With regard to mass, protons and neutrons are very similar, and have a much greater mass than electrons. Compared with neutrons and protons, the mass of an electron is usually negligible. 

Protons

Protons were discovered by Ernest Rutherford in the year 1919, when he performed his gold foil experiment. He projected alpha particles (helium nuclei) at gold foil, and the positive alpha particles were deflected. He concluded that protons exist in a nucleus and have a positive nuclear charge. The atomic number or proton number is the number of protons present in an atom. The atomic number determines an element (e.g., the element of atomic number 6 is carbon). 

Electrons

Electrons were discovered by Sir John Joseph Thomson in 1897. After many experiments involving cathode rays, J.J. Thomson demonstrated the ratio of mass to electric charge of cathode rays. He confirmed that cathode rays are fundamental particles that are negatively-charged; these cathode rays became known as electrons. 

Electrons are located in an electron cloud, which is the area surrounding the nucleus of the atom. There is usually a higher probability of finding an electron closer to to the nucleus of an atom. Electrons can abbreviated as e-. Electrons have a negative charge that is equal in magnitude to the positive charge of the protons. However, their mass is considerably less than that of a proton or neutron (and as such is usually considered insignificant). Unequal amounts of protons and electrons create ions: positive cations or negative anions.

Neutrons

Neutrons were discovered by James Chadwick in 1932, when he demonstrated that penetrating radiation incorporated beams of neutral particles. Neutrons are located in the nucleus with the protons. Along with protons, they make up almost all of the mass of the atom. The number of neutrons is called the neutron number and can be found by subtracting the proton number from the atomic mass number. The neutrons in an element determine the isotope of an atom, and often its stability. The number of neutrons is not necessarily equal to the number of protons.

Different Elements—Different Numbers of Protons, Electron & Neutron

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HISTORY OF ATOM

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***EXRTA...INFO...

Protons and neutrons were each discovered to be made of elementary particles called “quarks.” The two most common types of quarks are called up and down, and come in three varieties (called red, green, and blue). When you add in the electron and the electron neutrino, you get a family of eight elementary particles. All of the atoms in the periodic table can be explained with just those eight particles. That’s a lot simpler than 117!

Timeline of particle discoveries

From Wikipedia, the free encyclopedia

This is a timeline of subatomic particle discoveries, including all particles thus far discovered which appear to beelementary (that is, indivisible) given the best available evidence. It also includes the discovery of composite particles andantiparticles that were of particular historical importance.

More specifically, the inclusion criteria are:

• Elementary particles from the Standard Model of particle physics that have so far been observed. The Standard Model is the most comprehensive existing model of particle behavior. All Standard Model particles including the Higgs boson have been verified, and all other observed particles are combinations of two or more Standard Model particles.
• Antiparticles which were historically important to the development of particle physics, specifically the positron andantiproton. The discovery of these particles required very different experimental methods from that of their ordinary matter counterparts, and provided evidence that all particles had antiparticles—an idea that is fundamental to quantum field theory, the modern mathematical framework for particle physics. In the case of most subsequent particle discoveries, the particle and its anti-particle were discovered essentially simultaneously.
• Composite particles which were the first particle discovered containing a particular elementary constituent, or whose discovery was critical to the understanding of particle physics.

Time	Event
1800	William Herschel discovers "heat rays"
1801	Johann Wilhelm Ritter made the hallmark observation that invisible rays just beyond the violet end of the visible spectrum were especially effective at lightening silver chloride-soaked paper. He called them "oxidizing rays" to emphasize chemical reactivity and to distinguish them from "heat rays" at the other end of the invisible spectrum (both of which were later determined to be photons). The more general term "chemical rays" was adopted shortly thereafter to describe the oxidizing rays, and it remained popular throughout the 19th century. The terms chemical and heat rays were eventually dropped in favor of ultraviolet and infrared radiation, respectively.[1]
1895	Discovery of the ultraviolet radiation below 200 nm, named vacuum ultraviolet (later identified as photons) because it is strongly absorbed by air, by the German physicist Victor Schumann[2]
1895	X-ray produced by Wilhelm Röntgen (later identified as photons)[3]
1897	Electron discovered by J. J. Thomson[4]
1899	Alpha particle discovered by Ernest Rutherford in uranium radiation[5]
1900	Gamma ray (a high-energy photon) discovered by Paul Villard in uranium decay[6]
1911	Atomic nucleus identified by Ernest Rutherford, based on scattering observed by Hans Geiger and Ernest Marsden[7]
1919	Proton discovered by Ernest Rutherford[8]
1932	Neutron discovered by James Chadwick[9] (predicted by Rutherford in 1920[10])
1932	Antielectron (or positron), the first antiparticle, discovered by Carl D. Anderson[11] (proposed by Paul Dirac in 1927 and by Ettore Majorana in 1928)
1937	Muon (or mu lepton) discovered by Seth Neddermeyer, Carl D. Anderson, J.C. Street, and E.C. Stevenson, usingcloud chamber measurements of cosmic rays[12] (it was mistaken for the pion until 1947[13])
1947	Pion (or pi meson) discovered by C. F. Powell's group (predicted by Hideki Yukawa in 1935[14])
1947	Kaon (or K meson), the first strange particle, discovered by George Dixon Rochester and Clifford Charles Butler[15]
1947	Λ0 discovered during a study of cosmic-ray interactions[16]
1955	Antiproton discovered by Owen Chamberlain, Emilio Segrè, Clyde Wiegand, and Thomas Ypsilantis[17]
1956	Electron antineutrino detected by Frederick Reines and Clyde Cowan (proposed by Wolfgang Pauli in 1930 to explain the apparent violation of energy conservation in beta decay)[18] At the time it was simply referred to asneutrino since there was only one known neutrino.
1962	Muon neutrino (or mu neutrino) shown to be distinct from the electron neutrino by a group headed by Leon Lederman[19]
1964	Xi baryon discovery at Brookhaven National Laboratory[20]
1969	Partons (internal constituents of hadrons) observed in deep inelastic scattering experiments between protons andelectrons at SLAC;[21][22] this was eventually associated with the quark model (predicted by Murray Gell-Mann andGeorge Zweig in 1964) and thus constitutes the discovery of the up quark, down quark, and strange quark.
1974	J/ψ meson discovered by groups headed by Burton Richter and Samuel Ting, demonstrating the existence of thecharm quark[23][24] (proposed by James Bjorken and Sheldon Lee Glashow in 1964[25])
1975	Tau discovered by a group headed by Martin Perl[26]
1977	Upsilon meson discovered at Fermilab, demonstrating the existence of the bottom quark[27] (proposed byKobayashi and Maskawa in 1973)
1979	Gluon observed indirectly in three-jet events at DESY[28]
1983	W and Z bosons discovered by Carlo Rubbia, Simon van der Meer, and the CERN UA1 collaboration[29][30](predicted in detail by Sheldon Glashow, Mohammad Abdus Salam, and Steven Weinberg)
1995	Top quark discovered at Fermilab[31][32]
1995	Antihydrogen produced and measured by the LEAR experiment at CERN[33]
2000	Tau neutrino first observed directly at Fermilab[34]
2011	Antihelium-4 produced and measured by the STAR detector; the first particle to be discovered by the experiment
2012	A particle exhibiting most of the predicted characteristics of the Higgs boson discovered by researchers conducting the Compact Muon Solenoid and ATLAS experiments at CERN's Large Hadron Collider[35]

See also[edit]

• List of mesons
• List of baryons
• List of particles