User:Anthony.Sebastian/SebastianSandbox/Chemical elements

Introduction

Following the convention of the International Union of Physical and Applied Chemistry (IUPAC), 'chemical elements', in one sense of the term, refers to species, or types, of atoms. Familiar species of atoms include oxygen, copper, gold, and mercury — among the 94 naturally occurring species of atoms on Earth. See tables below. The distinguishing characteristic of a species of atoms is the number of protons in the nucleus of its atoms, referred to as the atomic number, Z, unique for each species of atom and for each corresponding chemical element.

In another sense of the term,'chemical elements' refers to chemical 'substances', specifically to 'pure substances', which in the case of chemical elements each is composed of a population of atoms solely of a single species, or type, of atoms, again as distinguished by its atomic number, Z. In this sense of a 'chemical element', sometimes the term 'elementary substance' is used, but most often 'chemical element' is used for both senses. Familiar examples of such 'pure' chemical substances are segments of wire made solely of copper atoms, and rolls of aluminum foil made solely of aluminum atoms. This second sense, or concept, renders 'chemical element' a somewhat more tangible particular, defining it in terms of a substance.

Historical note regarding the definition of chemical element

Prior to John Dalton's development and advocation of a quantitative atomic theory at the turn of the 19th century, an element had been defined as a substance that neither physical nor chemical methods could decompose into simpler substances. Some introductory chemistry textbooks still give that older primary definition, as does the current (2009) edition of the Encyclopedia Britannica Online. Since one cannot know whether future technology will provide a chemical or physical method to further simplify a presumed 'pure' substance, that older concept cannot be definitive. Moreover the modern definitions of chemical elements described above do not accord with the pre-Daltonian definition, as physical methods can now separate a species of chemical element into 'varieties' called isotopes, all with the same number of protons in the nucleus, but differing numbers of neutrons in the nucleus (see below).

Elementary facts about chemical elements

Among the 94 naturally occurring chemical elements, the atoms of the element hydrogen, Z=1, have the fewest number of protons, and those of the element plutonium, Z=94, have the greatest number of protons.

As protons each carry a positive charge, Z gives the positive charge of the nucleus in units of the so-called elementary charge, symbolized e. It is known that Z electrons (of charge −e, or negative e, and of mass much smaller than the proton) "orbit" the nucleus of an atom, so that an atom as a whole is electrically neutral, with its mass concentrated in the nucleus.

The names of the elements are of historical origin and may differ among languages for an element. The atomic number (Z), on the other hand, is universally the unique designator of an element, as is its international chemical symbol consisting of one or two letters.

In addition to the 94 elements that occur naturally on Earth, about 23 other known elements that do not occur naturally on Earth have been man-made and are characterized by their constituent atoms having very short life times and being radioactive. The exclusively man-made elements on Earth run from Z = 95 to 118.

People from all walks of everyday life know something about many different chemical elements, even if they do not recognize them as such. They include: helium (He), used to make party balloons float, lithium (Li), used to make batteries for cellphones, oxygen (O), in the air we breathe, neon (Ne), in 'neon' lights, sodium (Na), which is present in table salt that nutritionists advise using sparingly in foods and aluminum (Al), used as foil for wrapping leftovers.

All matter directly perceptible by the human senses — whether solid, liquid or gas — is composed of one or more elements. Typically, elements are found in nature in the form of populations of atoms, often with the atoms of other elements, as compounds (e.g., iron ore, a population of unit compounds each of iron and oxygen atoms, oxides of iron, primarily the minerals called magnetite and hematite), or as mixtures. Some elements are abundant on Earth. For example, the elements hydrogen and oxygen, as the compound water, H₂O, make up the bulk of Earth's oceans, seas, lakes, rivers, and ponds, and make up the bulk (mass) of living cells and multicellular oganisms. For another example, the element carbon supplies the backbone of numerous species of essential compounds of all animal and plant life on Earth as well of all the fossil fuels (natural gas, petroleum and coal), which are the remains of plant material that once lived. Some compounds may consist of one element only, for instance a nugget of pure gold is made up solely of gold atoms arranged in crystalline form. Very often gold is not pure but an alloy — a mixture — of the elements copper, silver, and gold. Oxygen gas consists of entities [see molecule] each having two oxygen atoms chemically bonded to each other, hence the gas consists of the element oxygen only.

Some of the 94 elements , such as the gas neon, are very rare on Earth. Some elements are stable, and will live as long as the universe, while some, known as the radioactive elements, have finite life times and decay into other elements while emitting radiation. For example, plutonium is a well-known radioactive element.