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. A-B , C-D , E-F-G-H , I-J-K-L-M , N-O-P , Q-R-S , T-U , V-W-X-Y-Z Name: Vanadium Symbol: V Atomic Number: 23 Atomic Weight: 50.9415 Group: Transition metals Date Discovered: 1801 Discovered By: Andrés del Rio (disputed), or Nils Sefström 1830 Name: Xenon Symbol: Xe Atomic Number: 54 Atomic Weight: 131.29 Group: Noble gases Date Discovered: 1898 Discovered By: William Ramsay and Morris Travers Name: Ytterbium Symbol: Yb Atomic Number: 70 Atomic Weight: 173.04 Group: Lanthanide Date Discovered: series 1878 Discovered By: Jean Charles de Marignac Name: Yttrium Symbol: Y Atomic Number: 39 Atomic Weight: 88.906 Group: Transition metals Date Discovered: 1794 Discovered By: Johan Gadolin Name: Zinc Symbol: Zn Atomic Number: 30 Atomic Weight: 65.409 Group: Transition metals Date Discovered: prehistoric Discovered By: unknown Name: Zirconium Symbol: Zr Atomic Number: 40 Atomic Weight: 91.224 Group: Transition metals Date Discovered: 1789 Discovered By: Martin Klaproth A-B , C-D , E-F-G-H , I...

. A-B , C-D , E-F-G-H , I-J-K-L-M , N-O-P , Q-R-S , T-U , V-W-X-Y-Z Name: Tantalum Symbol: Ta Atomic Number: 73 Atomic Weight: 180.948 Group: Transition metals Date Discovered: 1802 Discovered By: Anders Ekeberg Name: Technetium Symbol: Tc Atomic Number: 43 Atomic Weight: (98) Group: Transition metals Date Discovered: 1937 Discovered By: Carlo Perrier and Emilio Segrè Name: Tellurium Symbol: Te Atomic Number: 52 Atomic Weight: 127.60 Group: Nonmetals Date Discovered: 1782 Discovered By: Franz Müller Name: Terbium Symbol: Tb Atomic Number: 65 Atomic Weight: 158.9253 Group: Lanthanide Date Discovered: series 1843 Discovered By: Carl Mosander Name: Thallium Symbol: Tl Atomic Number: 81 Atomic Weight: 204.3833 Group: Other metals Date Discovered: 1861 Discovered By: William Crookes (isolated by William Crookes and Claude August Lamy, independently of each other, in 1862 Name: Thorium Symbol: Th Atomic Number: 90 Atomic Weight: 232.0381 Group: Actinide Date Discovered: series 1828 Discover...

. A-B , C-D , E-F-G-H , I-J-K-L-M , N-O-P , Q-R-S , T-U , V-W-X-Y-Z Name: Radium Symbol: Ra Atomic Number: 88 Atomic Weight: (226) Group: Alkaline earth metals Date Discovered: 1898 Discovered By: Marie Curie Name: Radon Symbol: Rn Atomic Number: 86 Atomic Weight: (222) Group: Noble gases Date Discovered: 1900 Discovered By: Friedrich Dorn Name: Rhenium Symbol: Re Atomic Number: 75 Atomic Weight: 186.207 Group: Transition metals Date Discovered: 1925 Discovered By: Walter Noddack, Ida Tacke, and Otto Berg Name: Rhodium Symbol: Rh Atomic Number: 45 Atomic Weight: 102.9055 Group: Transition metals Date Discovered: 1804 Discovered By: William Wollaston Name: Rubidium Symbol: Rb Atomic Number: 37 Atomic Weight: 85.4678 Group: Alkali metals Date Discovered: 1861 Discovered By: Robert Bunsen and Gustav Kirchhoff Name: Ruthenium Symbol: Ru Atomic Number: 44 Atomic Weight: 101.07 Group: Transition metals Date Discovered: 1827 Discovered By: G. W. Osann (isolated by Karl Klaus 1844) Name: Rut...

. A-B , C-D , E-F-G-H , I-J-K-L-M , N-O-P , Q-R-S , T-U , V-W-X-Y-Z Name: Neodymium Symbol: Nd Atomic Number: 60 Atomic Weight: 144.24 Group: Lanthanide Date Discovered: series 1885 Discovered By: Carl von Welsbach Name: Neon Symbol: Ne Atomic Number: 10 Atomic Weight: 20.1798 Group: Noble gases Date Discovered: 1898 Discovered By: William Ramsay and Morris Travers Name: Neptunium Symbol: Np Atomic Number: 93 Atomic Weight: (237) Group: Actinide Date Discovered: series 1940 Discovered By: Edwin McMillan and Philip Abelson Name: Nickel Symbol: Ni Atomic Number: 28 Atomic Weight: 58.6934 Group: Transition metals Date Discovered: 1751 Discovered By: Axel Cronstedt Name: Niobium Symbol: Nb Atomic Number: 41 Atomic Weight: 92.9064 Group: Transition metals Date Discovered: 1801 Discovered By: Charles Hatchett Name: Nitrogen Symbol: N Atomic Number: 7 Atomic Weight: 14.0067 Group: Nonmetals Date Discovered: 1772 Discovered By: Daniel Rutherford Name: Nobelium Symbol: No Atomic Number: 102 At...

. A-B , C-D , E-F-G-H , I-J-K-L-M , N-O-P , Q-R-S , T-U , V-W-X-Y-Z Name: Indium Symbol: In Atomic Number: 49 Atomic Weight: 114.818 Group: Other metals Date Discovered: 1863 Discovered By: Ferdinand Reich and Hieronymus Richter Name: Iodine Symbol: I Atomic Number: 53 Atomic Weight: 126.9045 Group: Halogens Date Discovered: 1811 Discovered By: Bernard Courtois Name: Iridium Symbol: Ir Atomic Number: 77 Atomic Weight: 192.217 Group: Transition metals Date Discovered: 1804 Discovered By: Smithson Tennant Name: Iron Symbol: Fe Atomic Number: 26 Atomic Weight: 55.845 Group: Transition metals Date Discovered: prehistoric Discovered By: unknown Name: Krypton Symbol: Kr Atomic Number: 36 Atomic Weight: 83.798 Group: Noble gases Date Discovered: 1898 Discovered By: William Ramsay and Morris Travers Name: Lanthanum Symbol: La Atomic Number: 57 Atomic Weight: 138.9055 Group: Lanthanide Date Discovered: series 1839 Discovered By: Carl Mosander Name: Lawrencium Symbol: Lr Atomic Number: 103 Atom...

. A-B , C-D , E-F-G-H , I-J-K-L-M , N-O-P , Q-R-S , T-U , V-W-X-Y-Z Name: Einsteinium Symbol: Es Atomic Number: 99 Atomic Weight: (252) Group: Actinide Date Discovered: series 1952 Discovered By: Albert Ghiorso and coworkers Name: Erbium Symbol: Er Atomic Number: 68 Atomic Weight: 167.26 Group: Lanthanide Date Discovered: series 1843 Discovered By: Carl Mosander Name: Europium Symbol: Eu Atomic Number: 63 Atomic Weight: 151.966 Group: Lanthanide Date Discovered: series 1901 Discovered By: Eugène Demarçay Name: Fermium Symbol: Fm Atomic Number: 100 Atomic Weight: (257) Group: Actinide Date Discovered: series 1955 Discovered By: Albert Ghiorso and coworkers Name: Fluorine Symbol: F Atomic Number: 9 Atomic Weight: 18.9984 Group: Halogens Date Discovered: 1771 Discovered By: Karl Scheele (isolated by Henri Moissan 1886) Name: Francium Symbol: Fr Atomic Number: 87 Atomic Weight: (223) Group: Alkali metals Date Discovered: 1939 Discovered By: Marguérite Perey Name: Gadolinium Symbol: Gd Ato...

. A-B , C-D , E-F-G-H , I-J-K-L-M , N-O-P , Q-R-S , T-U , V-W-X-Y-Z Name: Cadmium Symbol: Cd Atomic Number: 48 Atomic Weight: 112.412 Group: Transition metals Date Discovered: 1817 Discovered By: Friedrich Strohmeyer Name: Calcium Symbol: Ca Atomic Number: 20 Atomic Weight: 40.078 Group: Alkaline earth metals Date Discovered: 1808 Discovered By: Humphry Davy Name: Californium Symbol: Cf Atomic Number: 98 Atomic Weight: (251) Group: Actinide Date Discovered: 1950 Discovered By: Glenn Seaborg, Stanley Thompson, Kenneth Street, Jr., and Albert Ghiorso Name: Carbon Symbol: C Atomic Number: 6 Atomic Weight: 12.011 Group: Nonmetals Date Discovered: prehistoric Discovered By: unknown Name: Cerium Symbol: Ce Atomic Number: 58 Atomic Weight: 140.115 Group: Lanthanide Date Discovered: 1804 Discovered By: Jöns Berzelius and Wilhelm Hisinger, and independently by Martin Klaproth Name: Cesium Symbol: Cs Atomic Number: 55 Atomic Weight: 132.9054 Group: Alkali metals Date Discovered: 1860 Discover...

. A-B , C-D , E-F-G-H , I-J-K-L-M , N-O-P , Q-R-S , T-U , V-W-X-Y-Z Name: Actinium Symbol: Ac Atomic Number: 89 Atomic Weight: (227) Group: Actinide Date Discovered: 1899 Discovered By: André Debierne Name: Aluminum Symbol: Al Atomic Number: 13 Atomic Weight: 26.9815 Group: Other metals Date Discovered: 1824 Discovered By: Hans Oersted (also attributed to Friedrich Wöhler 1827) Name: Americium Symbol: Am Atomic Number: 95 Atomic Weight: 243 Group: Actinide Date Discovered: 1944 Discovered By: Glenn Seaborg, Ralph James, Leon Morgan, and Albert Ghiorso Name: Antimony Symbol: Sb Atomic Number: 51 Atomic Weight: 121.760 Group: Other metals Date Discovered: prehistoric Discovered By: unknown Name: Argon Symbol: Ar Atomic Number: 18 Atomic Weight: 39.948 Group: Noble gases Date Discovered: 1894 Discovered By: John Rayleigh and William Ramsay Name: Arsenic Symbol: As Atomic Number: 33 Atomic Weight: 74.9216 Group: Nonmetals Date Discovered: prehistoric Discovered By: unknown Name: Astatin...

. Periodic Table, table of the chemical elements arranged to illustrate patterns of recurring chemical and physical properties. Elements, such as oxygen, iron, and gold, are the most basic chemical substances and cannot be broken down by chemical reactions . All other substances are formed from combinations of elements. The periodic table provides a means of arranging all the known elements and even those yet to be discovered. See Alphabetic list of Chemical Elements ARRANGEMENT OF THE TABLE The elements within the modern periodic table are arranged from left to right, top to bottom, in order of increasing atomic number. An element’s atomic number is the number of protons in its nucleus. There are 92 naturally occurring elements, ranging from hydrogen, which has atomic number 1, to uranium, whose atomic number is 92. The periodic table also includes artificially created elements, whose atomic numbers are higher than 92. These additions must be prepared through nuclear reactions. The ...

Elements, Chemical, substances that cannot be decomposed or broken into more elementary substances by ordinary chemical means. Elements were at one time believed to be the fundamental substances but are now known to consist of a number of different elementary particles . More than 100 chemical elements are known to exist in the universe, although several of these, the so-called transuranium elements , have not been found in nature, and can only be produced artificially. See also Chemistry . Chemical elements are classified as metals and nonmetals. The atoms of metals are electropositive and combine readily with the electronegative atoms of the nonmetals. A group of elements called metalloids, intermediate in properties between the metals and the nonmetals, are sometimes considered a separate class. When the elements are arranged in the order of their atomic numbers (a number proportional to the net positive charge on the nucleus of an atom of an element), elements of similar physical a...

Proton, elementary particle that carries a positive electric charge and, along with the electron and the neutron , is one of the building blocks of all atoms. Elementary particles are the smallest parts of matter that scientists can isolate. The proton is one of the few elementary particles that is stable—that is, it can exist by itself for a long period of time. Protons and neutrons are the building blocks of the atomic nucleus, the center of the atom. Electrons form the outer part of the atom. Protons have a positive electrical charge of 1.602 x 10-19 coulomb. This charge is equal but opposite to the negative charge of the electron. Neutrons have no electrical charge. Protons have a mass of 1.67 x 10-27 kg and, along with neutrons, they account for most of the mass in atoms. Atoms contain an equal number of protons and electrons so that every atom has an overall charge of zero.(See also Atom and Electricity ) The number of protons in the nucleus of an atom determines what kind of che...

Neutron, electrically neutral elementary particle that is part of the nucleus of the atom. Elementary particles are the smallest parts of matter that scientists can isolate. The neutron is about 10-13 cm in diameter and weighs 1.6749 x 10-27 kg. See also Atom . Neutrons and protons bind tightly together to create atomic nuclei. The number of protons an atom contains determines which chemical element it is, ranging from 1 proton for hydrogen to 92 for uranium, the largest naturally occurring element. Each atom usually contains about as many neutrons as protons, but different atoms of the same element may have different numbers of neutrons. Atoms that differ only in the number of neutrons are called isotopes . For example, most atoms of the simplest element, hydrogen, have a nucleus containing only a single proton. In natural hydrogen, however, 0.015 percent of the atoms have a neutron in addition to the proton. This isotope is called heavy hydrogen or deuterium. An element usually has ...

Electron, negatively charged particle found in an atom. Electrons, along with neutrons and protons , comprise the basic building blocks of all atoms. The electrons form the outer layer or layers of an atom, while the neutrons and protons make up the nucleus, or core, of the atom. Electrons, neutrons, and protons are elementary particles —that is, they are among the smallest parts of matter that scientists can isolate. The electron carries a negative electric charge of –1.602 x 10-19 coulomb and has a mass of 9.109 x 10-31 kg. See also Atom . Electrons are responsible for many important physical phenomena, such as electricity and light , and for physical and chemical properties of matter. Electrons form electric currents by flowing in a stream and carrying their negative charge with them. All electrical devices, from flashlights to computers, depend on the movement of electrons. Electrons also are involved in creating light. The electrons in the outer layers of the atom sometimes lose e...

Atom, tiny basic building block of matter. All the material on Earth is composed of various combinations of atoms. Atoms are the smallest particles of a chemical element that still exhibit all the chemical properties unique to that element. A row of 100 million atoms would be only about a centimeter long. See also Chemical Element . Understanding atoms is key to understanding the physical world. More than 100 different elements exist in nature, each with its own unique atomic makeup. The atoms of these elements react with one another and combine in different ways to form a virtually unlimited number of chemical compounds. When two or more atoms combine, they form a molecule . For example, two atoms of the element hydrogen (abbreviated H) combine with one atom of the element oxygen (O) to form a molecule of water (H20). Since all matter—from its formation in the early universe to present-day biological systems—consists of atoms, understanding their structure and properties plays a vit...

. Elementary Particles, in physics, particles that cannot be broken down into any other particles. The term elementary particles also is used more loosely to include some subatomic particles that are composed of other particles. Particles that cannot be broken further are sometimes called fundamental particles to avoid confusion. These fundamental particles provide the basic units that make up all matter and energy in the universe. Scientists and philosophers have sought to identify and study elementary particles since ancient times. Aristotle and other ancient Greek philosophers believed that all things were composed of four elementary materials: fire , water , air , and earth . People in other ancient cultures developed similar notions of basic substances. As early scientists began collecting and analyzing information about the world, they showed that these materials were not fundamental but were made of other substances. In the 1800s British physicist John Dalton was so sure he ha...

Force, in physics, any action or influence that accelerates an object. Force is a vector , which means that it has both direction and magnitude. When several forces act on an object, the forces can be combined to give a net force. The net force acting on an object, the object's mass , and the acceleration of the object are all related to each other by Newton's second law of motion, named after English physicist and mathematician Isaac Newton . The second law of motion states that the acceleration an object experiences multiplied by the mass of the object is equal to the net force acting on an object. Thus, if a given force acts on two objects of different mass, the object with a larger mass will have a lower acceleration. See Mechanics . An object experiences a force when it is pushed or pulled by another object. For example, shoving a stationary shopping cart applies a force that causes the shopping cart to accelerate. An object can also experience a force because of the infl...

. Gravitation, the force of attraction between all objects that tends to pull them toward one another. It is a universal force , affecting the largest and smallest objects, all forms of matter , and energy . Gravitation governs the motion of astronomical bodies. It keeps the moon in orbit around the earth and keeps the earth and the other planets of the solar system in orbit around the sun. On a larger scale, it governs the motion of stars and slows the outward expansion of the entire universe because of the inward attraction of galaxies to other galaxies. Typically the term gravitation refers to the force in general, and the term gravity refers to the earth's gravitational pull. Gravitation is one of the four fundamental forces of nature, along with electromagnetism and the weak and strong nuclear forces, which hold together the particles that make up atoms. Gravitation is by far the weakest of these forces and, as a result, is not important in the interactions of atoms and nuclea...

Quantum Theory, in physics, description of the particles that make up matter and how they interact with each other and with energy. Quantum theory explains in principle how to calculate what will happen in any experiment involving physical or biological systems, and how to understand how our world works. The name “quantum theory” comes from the fact that the theory describes the matter and energy in the universe in terms of single indivisible units called quanta (singular quantum). Quantum theory is different from classical physics. Classical physics is an approximation of the set of rules and equations in quantum theory. Classical physics accurately describes the behavior of matter and energy in the everyday universe. For example, classical physics explains the motion of a car accelerating or of a ball flying through the air. Quantum theory, on the other hand, can accurately describe the behavior of the universe on a much smaller scale, that of atoms and smaller particles. The rules o...

Mass (physics), in physics, amount of matter that a body contains, and a measure of the inertial property of that body, that is, of its resistance to change of motion (see Inertia ). Mass is different from weight, which is a measure of the attraction of the earth for a given mass (see Gravitation ). Inertial mass and gravitational mass are identical. Weight, although proportional to mass, varies with the position of a given mass relative to the earth; thus, equal masses at the same location in a gravitational field will have equal weights. A mass in interstellar space may have nearly zero weight. A fundamental principle of classical physics is the law of conservation of mass, which states that matter cannot be created or destroyed. This law holds true in chemical reactions but is modified in cases where atoms disintegrate and matter is converted to energy or energy is converted to matter (see Nuclear Energy ; X Ray ). The theory of relativity , initially formulated in 1905 by the Germa...