.
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: Astatine
Symbol: At
Atomic Number: 85
Atomic Weight: (210)
Group: Halogens
Date Discovered: 1940
Discovered By: Dale R. Corson, K. R. MacKenzie, and Emilio Segrè
Name: Barium
Symbol: Ba
Atomic Number: 56
Atomic Weight: 137.328
Group: Alkaline earth metals
Date Discovered: 1808
Discovered By: Humphry Davy
Name: Berkelium
Symbol: Bk
Atomic Number: 97
Atomic Weight: (247)
Group: Actinide
Date Discovered: 1949
Discovered By: Glenn Seaborg, Stanley Thompson, and Albert Ghiorso
Name: Beryllium
Symbol: Be
Atomic Number: 4
Atomic Weight: 9.0122
Group: Alkaline earth metals
Date Discovered: 1798
Discovered By: Louis-Nicolas Vauquelin (isolated by Friedrich Wöhler and Antoine-Alexandre-Brutus Bussy 1828)
Name: Bismuth
Symbol: Bi
Atomic Number: 83
Atomic Weight: 208.9804
Group: Other metals
Date Discovered: prehistoric
Discovered By: unknown
Name: Bohrium
Symbol: Bh
Atomic Number: 107
Atomic Weight: (262)
Group: Transition metals
Date Discovered: 1976
Discovered By: Georgii Flerov and Yuri Oganessian (confirmed by German scientist Peter Armbruster and coworkers)
Name: Boron
Symbol: B
Atomic Number: 5
Atomic Weight: 10.81
Group: Nonmetals
Date Discovered: 1808
Discovered By: Humphry Davy, and independently by Joseph Gay-Lussac and Louis-Jacques Thénard
Name: Bromine
Symbol: Br
Atomic Number: 35
Atomic Weight: 79.904
Group: Halogens
Date Discovered: 1826
Discovered By: Antoine-Jérôme Balard
Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.
Periodic Table
.
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 most recent element to be synthesized was an element with 114 protons in the nucleus of each of its atoms. None of the artificially created elements with atomic numbers higher than 109 have yet been officially named.
Whereas the ordering of the elements is completely determined by their atomic numbers, the arrangement into vertical columns, called groups, is determined by a number of factors. These factors include chemical properties, physical properties, and the number of electrons thought to exist in the outer shells of the element’s atoms. (The electrons that surround the nucleus of an atom are arranged in concentric shells.)
The placement of elements into groups within the periodic table is not completely clear-cut. Some scientists disagree about minor differences in the placement of elements such as hydrogen and helium. Helium, which does not react with other elements, is usually placed in group 18, which houses the noble gases. This group also includes neon, argon, and krypton, all of which are also very unreactive. Scientists who group the elements based primarily on the number of outer-shell electrons place helium with elements such as magnesium, calcium, and barium among the alkaline earth metals of group 2. Elements in group 2 have two electrons in their outermost shell.
The periodic table has been published in various shapes and sizes, but the most commonly used modern form begins with a column of group 1 metals on the left-hand side, followed by a column of group 2 alkaline earth metals. These columns are followed by a block of 40 elements divided into ten columns of four elements each. The groups in this block, collectively called the transition metals, are numbered 3 through 12. Groups 13 through 18 make up the right-hand side of the table. A diagonal dividing line separates the nonmetals in the upper-right portion of this block, such as oxygen, carbon, and nitrogen, from the metals such as tin and lead in the lower left portion.
There is an additional block of 28 elements, divided into two rows of 14 elements each, that is usually placed beneath the main table. These are the rare earth elements, whose properties are all remarkably similar. They are so similar to one another that chemists have difficulty separating them when they occur together as mixtures. This additional block really belongs between the first block, consisting of groups 1 and 2, and the transition metal block. For convenience it is placed at the bottom of the table rather than in its proper place. Otherwise the periodic table would be very wide and would not lend itself to being represented on wall charts.
Scientists refer to the horizontal rows in the periodic table as periods. Periods vary in length. Moving through the table from top to bottom, the successive periods contain 2, 8, 8, 18, 18, 32, and 32 elements. These numbers correspond to the maximum number of electrons that can be accommodated in the largest electron shell in an atom of any element belonging to that period.
Elements, Chemical
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 and chemical properties occur at specific intervals (see Periodic Law). These groups of elements with similar physical and chemical properties are called families, examples of which are the alkaline earth metals, rare earth elements, halogens, and the noble gases.
The unit for atomic weight of the elements is one-twelfth of the weight of the carbon-12 atom, which is arbitrarily set at 12 (see Atom). The atomic number, weight, and chemical symbol of each of the known elements are given in the accompanying table. See articles on each element.
When two atoms have the same atomic number but different atomic weights, they are said to be isotopes. Many natural isotopes are known for some elements, whereas other elements occur in only one isotopic form. Hundreds of synthetic isotopes have been made. Some natural isotopes, and many synthetic ones, are unstable (see Isotope; Radioactivity).
The heavy transuranium elements are produced in particle accelerators by bombarding atomic nuclei with charged atomic nuclei or nuclear particles to form a heavier element. These superheavy elements are radioactive and decay into more stable, lighter elements rapidly. In 1996 physicists at the German National Laboratory for Heavy Ion Research in Darmstadt, Germany, created an element with 112 protons by bombarding lead with atoms of zinc. This element is named Ununbium (Uub). Some physicists speculate that a number of stable, superheavy elements may exist—elements with atomic numbers as high as 164. In 1998, scientists at Russia’s Joint Institute for Nuclear Research in Dubna announced they had created the first of these elements. After bombarding plutonium with atoms of calcium for weeks, they found evidence of one atom containing 114 protons that lasted for 30 seconds. Atoms of other heavy elements last for only a small fraction of a second.
related articles:
Chemistry overview
Inorganic Chemistry, study of noncarbon chemical compounds
Organic Chemistry, study of chemical compounds containing carbon
Periodic Law, predicting chemical properties
Periodic Table, arranging chemical elements in groups
alphabetic list of chemical elements
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 and chemical properties occur at specific intervals (see Periodic Law). These groups of elements with similar physical and chemical properties are called families, examples of which are the alkaline earth metals, rare earth elements, halogens, and the noble gases.
The unit for atomic weight of the elements is one-twelfth of the weight of the carbon-12 atom, which is arbitrarily set at 12 (see Atom). The atomic number, weight, and chemical symbol of each of the known elements are given in the accompanying table. See articles on each element.
When two atoms have the same atomic number but different atomic weights, they are said to be isotopes. Many natural isotopes are known for some elements, whereas other elements occur in only one isotopic form. Hundreds of synthetic isotopes have been made. Some natural isotopes, and many synthetic ones, are unstable (see Isotope; Radioactivity).
The heavy transuranium elements are produced in particle accelerators by bombarding atomic nuclei with charged atomic nuclei or nuclear particles to form a heavier element. These superheavy elements are radioactive and decay into more stable, lighter elements rapidly. In 1996 physicists at the German National Laboratory for Heavy Ion Research in Darmstadt, Germany, created an element with 112 protons by bombarding lead with atoms of zinc. This element is named Ununbium (Uub). Some physicists speculate that a number of stable, superheavy elements may exist—elements with atomic numbers as high as 164. In 1998, scientists at Russia’s Joint Institute for Nuclear Research in Dubna announced they had created the first of these elements. After bombarding plutonium with atoms of calcium for weeks, they found evidence of one atom containing 114 protons that lasted for 30 seconds. Atoms of other heavy elements last for only a small fraction of a second.
related articles:
Chemistry overview
Inorganic Chemistry, study of noncarbon chemical compounds
Organic Chemistry, study of chemical compounds containing carbon
Periodic Law, predicting chemical properties
Periodic Table, arranging chemical elements in groups
alphabetic list of chemical elements
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