Chemical Elements - chemistry. I INTRODUCTION Chemical Elements, substance made up only of atoms that all have the same atomic number. The atomic number is the number of protons in the atom's nucleus: element atomic number 1 hydrogen has one proton in the nucleus and the element atomic number 92 uranium has 92 protons. Ninety-four natural elements have been detected in the universe. More than 110 elements have been identified, with some created only in laboratories as artificial elements. Under certain conditions, one element may be changed into another element through processes that add or remove protons from a nucleus. Although the number of protons in the nucleus of a particular chemical element is always the same, the number of neutrons can vary, creating isotopes of that element that have different atomic masses and physical properties. The chemical properties of an element are mainly determined by the number of electrons in the outermost shell of an atom of that element. The number of electrons is the same as the number of protons in the nucleus of an element. 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 also Chemistry. Atoms of a single element may combine to form molecules of that element--two atoms of oxygen combine to form oxygen molecules, eight atoms of sulfur combine to form sulfur molecules. Atoms of different elements may combine to form molecules of chemical compounds--two hydrogen atoms combine with one oxygen atom to form molecules of water. Unlike chemical compounds, elements cannot be broken down into simpler substances by ordinary heat, light, electricity, or chemical reactions. Like other forms of matter, most elements can exist as gases, liquids, or solids, depending on the pressure and temperature. Scientists use one- and two-letter symbols for each element. In some cases these symbols are based on old Greek or Latin names for the elements and don't correspond to their common English names. For example, hydrogen is H, but gold is Au, from Latin aurum. II ORIGIN OF ELEMENTS The process of building up the nucleus of an element from protons and neutrons is called nucleosynthesis. This process only takes place under extreme conditions of pressure and temperature. The first period of nucleosynthesis occurred in the early moments of the universe following the big bang, creating hydrogen and helium. When the first stars formed, another type of nucleosynthesis began in the cores of stars, fusing hydrogen into helium, and releasing energy. The fusion processes continued to create the elements carbon (6 protons), nitrogen (7 protons), and oxygen (8 protons) on up to iron (26 protons). The fusion of iron does not release energy and stars that reach this stage suffer a core collapse, usually exploding as supernovas. When supernovas explode, the extreme conditions can create even heavier elements, up to uranium. Other processes can also change the number of protons in a nucleus, transforming one element into another. Radioactive decay can break a heavy element into lighter elements. In space impacts of high energy protons called cosmic rays can break off nuclei of lithium (3 protons), beryllium (4 protons), and boron (5 protons) from atoms of carbon, nitrogen and oxygen. Only tiny amounts of elements heavier than uranium (92 protons)--neptunium (93 protons) and plutonium (94 protons)--are known to exist in nature. All other transuranium elements have been made artificially in laboratories by bombarding heavy atoms with neutrons or with charged particles and atomic nuclei accelerated to high energy. III ABUNDANCE OF ELEMENTS The relative abundance of elements in the universe reflects how the elements were formed. The big bang created the protons that form all the hydrogen nuclei in the universe. Hydrogen accounts for about 73 percent of the ordinary matter in the universe by mass, helium about 25 percent, and all other elements less than 2 percent. By number of atoms, about 90 percent of the atoms in the universe are hydrogen, about 9 percent are helium, and all the other elements account for less than 1 percent. Processes in stars, and particularly supernova explosions, determined the abundance of other elements. Oxygen (8 protons) is the most abundant, followed by carbon (6 protons), nitrogen (7 protons), neon (10 protons), silicon (14 protons), magnesium (12 protons), and iron (26 protons). The particular abundance of elements found on Earth reflects the processes that formed our planet from material that became the Sun and the solar system. For example, Earth was not massive enough to retain helium from the original material that formed the solar system. The helium now found in Earth is the result of radioactive decay of minerals that release alpha particles, which have 2 protons and 2 neutrons, the same as helium nuclei. IV CLASSIFICATION OF ELEMENTS Chemical elements are classified broadly as metals and nonmetals. The atoms of metals are electropositive--they tend to gain electrons--and combine readily with the electronegative atoms of the nonmetals, which tend to lose electrons. 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. The main families are: alkali metals, alkaline earth metals, halogens, noble gases, nonmetals, other metals, transition elements, rare earth elements, and the actinide series. Alkali metals are a series of six chemical elements in a group on the periodic table. They give up an electron easily, and react violently with water to form hydrogen gas and hydroxides, or strong bases. The alkali metals include lithium, sodium, potassium, rubidium, cesium, and francium. Alkaline earth metals are a series of six chemical elements in a group on the periodic table. They are less reactive than the alkali metals, but reactive enough not to be found free in nature. The alkaline earth metals include beryllium, magnesium, calcium, strontium, barium, and radium. Halogens are a group of five closely related chemically active elements--fluorine, chlorine, bromine, iodine, and astatine. Noble gases, also called inert gases, are a group of six gaseous chemical elements in a group on the periodic table. Most of them are chemically inactive. They are helium, neon, argon, krypton, xenon, and radon. The nonmetals are the elements hydrogen, boron, carbon, nitrogen, oxygen, silicon, phosphorus, sulfur, arsenic, selenium, and tellurium. The other metals are aluminum, antimony, bismuth, gallium, germanium, indium, lead, polonium, thallium, and tin. Transition elements (also called transition metals) are a series of 30 chemical elements that share similar chemical properties. They have atomic numbers 21 to 30, 39 to 48, and 71 to 80, and include titanium, iron, copper, zinc, gold, and mercury. The rare earth elements series (or rare earth metals) include the elements with atomic numbers 57 through 71, and include lanthanum, cerium, promethium, samarium, europium, gadolinium, ytterbium, and lutetium. (Yttrium [atomic no. 39] and scandium [atomic no. 21] are sometimes included in the group of rare earth elements.) The elements cerium (atomic no. 58) through lutetium (atomic no. 71) are commonly known as the lanthanide series. The actinide series is 14 radioactive elements in the periodic table with atomic numbers 89 through 102. The actinide series is related to rare earth elements. They include actinium, thorium, uranium, neptunium, plutonium, americium, curium, berkelium, californium, and einsteinium. The atomic number, weight, and chemical symbol of each of the known elements are given in the accompanying table. See articles on each element. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
