Periodic Table:

The periodic table of the chemical elements (also known as the periodic table or periodic table of the elements) is a tabular display of the 118 known chemical elements organized by selected properties of their atomic structures. Elements are presented by increasing atomic number, the number of protons in an atom's atomic nucleus. While rectangular in general outline, gaps are included in the horizontal rows (known as periods) as needed to keep elements with similar properties together in vertical columns (known as groups), e.g. alkali metals, alkali earths, halogens, noble gases.

Although there were precursors, the current presentation's invention is generally credited to Russian chemist Dmitri Mendeleev’s, who developed a version of the now-familiar tabular presentation in 1869 to illustrate recurring ("periodic") trends in the properties of the then-known elements. The layout of the table has been refined and extended over time, as new elements have been discovered, and new theoretical models have been developed to explain chemical behavior.

Since the periodic table accurately predicts the abilities of various elements to combine into chemical compounds, use of the periodic table is now ubiquitous within the academic discipline of chemistry, providing a useful framework to classify, systematize, and compare many of the many different forms of chemical behavior. The table has found many applications not only in chemistry and physics, but also in such diverse fields as geology, biology, materials, engineering, agriculture, medicine, nutrition, environmental health, and astronomy. Its principles are especially important in chemical engineering.

One of the strengths of Mendeleev's presentation is that the original version accurately predicted some of the properties of then-undiscovered elements expected to fill gaps in his arrangement. For example: "eka-aluminium", expected to have properties intermediate between Aluminium and indium, was discovered with said properties in 1875 and named gallium. No gaps remain in the current 118-element periodic table; all elements from hydrogen to Plutonium except technetium, promethium and neptunium exist in the Earth in macroscopic or recurrently produced trace quantities. The three said exceptions do exist naturally, but only in trace amounts as the result of rare nuclear processes from decay of heavy elements. Every element through Copernicus, element 112, has been isolated, characterized, and named, and elements 113 through 118 have been synthesized in laboratories around the world.

While plutonium is now included among the 91 regularly occurring natural elements, and technetium, promethium, and neptunium also occur naturally in transient trace amounts, these four elements were first identified and characterized from technologically produced samples. Numerous synthetic radio nuclides of various naturally occurring elements have been produced as well.

Production of additional synthetic elements beyond atomic number 118 is being pursued; whether the next elements will neatly fill an eighth period or require modifications to the overall patterns of the present periodic table remains unknown.

Organizing principles

The main value of the periodic table is the ability to predict the chemical properties of an element based on its location on the table. It should be noted that the properties vary differently when moving vertically along the columns of the table than when moving horizontally along the rows. 111

The layout of the periodic table demonstrates recurring ("periodic") chemical properties. Elements are listed in order of increasing atomic number (i.e., the number of protons in the atomic nucleus). Rows are arranged so that elements with similar properties fall into the same columns (groups or families). According to quantum mechanical theories of electron configuration within atoms, each row (period) in the table corresponded to the filling of a quantum shell of electrons. There are progressively longer periods further down the table, grouping the elements into s-, p-, d- and f-blocks to reflect their electron configuration.

Elements, natural and synthetic.

Only chemical elements, not mixtures, compounds, or subatomic particles, are included in the periodic table. Each element has a single entry, even if it has multiple isotopes.

As of June 2011, the periodic table includes 118 chemical elements whose discoveries have been confirmed. Of these, 91 are regularly occurring primordial or recurrently produced elements found naturally on the Earth, at least in transient trace amounts, and three others occur naturally, but only incidentally. The 24 other known elements (those from americium through ununoctium) are synthetic, produced by human technology but not regularly or incidentally occurring naturally. Various synthetic elements, as well as synthetic isotopes of naturally occurring elements, are now also present in the environment from such sources as nuclear weapons explosions, nuclear waste processing, and disposal of materials including industrial and medical nucleotides. For example, americium and its decay product neptunium are incidentally present in household and commercial waste from disposal of unwanted americium-containing smoke detectors.

Formal naming of the chemical elements is overseen by the International Union of Pure and Applied Chemistry (IUPAC). Provisional names, such as un untrium, ununquadium, or un unpentium, are provided for elements that have been discovered but not yet been formally named; these names are based on the three digits of their atomic numbers.

Atomic number.

By definition, each chemical element has a unique atomic number, the number of protons in its nucleus. Different atoms of many elements have different numbers of neutrons, which differentiates between isotopes of an element. For example, all atoms of hydrogen have one proton, and no atoms of any other element have exactly one proton. On the other hand, a hydrogen atom can have one or two neutrons in its nucleus, or none at all, yet all of these cases are isotopes of hydrogen, not instances of some other element. (A hydrogen atom with no neutrons in addition to its sole proton is called protium, one with one neutron in addition to its proton is called deuterium, and one with two additional neutrons, tritium.

In the modern periodic table, the elements are placed progressively in each row (period) from left to right in the sequence of their atomic numbers, with each new row starting with the next atomic number following the last number in the previous row. No gaps or duplications exist. Since the elements can be uniquely sequenced by atomic number, conventionally from lowest to highest , sets of elements are sometimes specified by such notation as "through", "beyond", or "from ... through", as in "through iron", "beyond uranium", or "from lanthanum through lutetium". The terms "light" and "heavy" are sometimes also used informally to indicate relative atomic numbers (not densities!), as in "lighter than carbon" or "heavier than lead", although technically the weight or mass of atoms of an element (their atomic weights or atomic masses) do not always increase monotonically with their atomic numbers.

The significance of atomic numbers to the organization of the periodic table was not appreciated until the existence and properties of protons and neutrons became understood. Mendeleev's periodic tables instead used atomic weights, information determinable to fair precision in his time, which worked well enough in most cases to give a powerfully predictive presentation far better than any other comprehensive portrayal of the chemical elements' properties.

Chemistry:

Chemistry is the science of matter (in particular matter that is composed of chemical elements), especially its properties, structure, composition, behavior, reactions, interactions and the changes it undergoes.

Chemistry is sometimes called "the central science" because it connects physics with other natural sciences such as astronomy, geology and biology.

Physics also studies matter, but includes subatomic matter and the properties of non-matter entities such a selector, even when it does not interact with atoms. Thus, physics is the science of study of the laws governing all forces and particles in nature, including forces such as gravitation, the weak force and the strong force, all of which are outside the province of chemistry.

Chemistry, which focuses primarily upon atoms and their interactions with other atoms, is a branch of physical science but not a branch of physics. However, chemistry utilizes physics. For example, chemistry uses quantities like energy and entropy in relation to the spontaneity of chemical processes. It also explains the structure and properties of atomic matter (matter made from chemical elements) as a consequence of the physical properties of chemical substances and their interactions. For example, steel is harder than iron because its atoms are bound together in a more rigid crystalline lattice; wood burns or undergoes rapid oxidation because it can react spontaneously with oxygen in a chemical reaction above a certain temperature; sugar and salt dissolve in water because their molecular/ionic properties are such that dissolution is preferred under the ambient conditions. Synthesis is the major aspect that separates chemistry from physics and biology as sciences. Chemistry includes the knowledge (science) to design and make more complex substances from simpler ones. These new substances might then be analyzed for their physical or biological properties.

The etymology of the word chemistry has been much disputed. The genesis of chemistry can be traced to certain practices, known as alchemy, which had been practiced for several millennia in various parts of the world, particularly the Middle East.

Theory:

Traditional chemistry starts with the study of elementary particles, atoms, molecules, substances, metals, crystals and other aggregates of matter. In solid, liquid, and gas states, whether in isolation or combination. The interactions, reactions and transformations that are studied in chemistry are a result of interaction either between different chemical substances or between matter and energy. Such behaviors are studied in a chemistry laboratory using various forms of laboratory glassware.

A chemical reaction is a transformation of some substances into one or more other substances. It can be symbolically depicted through a chemical equation. The number of atoms on the left and the right in the equation for a chemical transformation is most often equal. The nature of chemical reactions a substance may undergo and the energy changes that may accompany it are constrained by certain basic rules, known as chemical laws.

Energy and entropy considerations are invariably important in almost all chemical studies. Chemical substances are classified in terms of their structure, phase as well as their chemical compositions. They can be analyzed using the tools of chemical analysis, e.g. spectroscopy and chromatography. Scientists engaged in chemical research are known as chemists.

History of Chemistry:

By 1000 BC, ancient civilizations used technologies that would eventually form the basis of the various branches of chemistry. Examples include extracting metals from ores, making pottery and glazes, fermenting beer and wine, making pigments for cosmetics and painting, extracting chemicals from plants for medicine and perfume, making cheese, dying cloth, tanning leather, rendering fat into soap, making glass, and making alloys like bronze.

Early attempts to explain the nature of matter and its transformations failed. The proto science of chemistry, Alchemy, was also unsuccessful in explaining the nature of matter. However, by performing experiments and recording the results the alchemist set the stage for modern chemistry. This distinction begins to emerge when a clear differentiation was made between chemistry and alchemy by Robert Boyle in his work The Sceptical Chymist (1661). Chemistry then becomes a full-fledged science when Antoine Lavisher develops his law of conservation of mass, which demands careful measurements and quantitative observations of chemical phenomena. So, while both alchemy and chemistry are concerned with the nature of matter and its transformations, it is only the chemists who apply the scientific method. The history of chemistry is intertwined with the history of thermodynamics, especially through the work of Willard Gibbs.

Arguably the first chemical reaction used in a controlled manner was fire. However, for millennia fire was simply a mystical force that could transform one substance into another (burning wood, or boiling water) while producing heat and light. Fire affected many aspects of early societies. These ranged from the simplest facets of everyday life, such as cooking and habitat lighting, to more advanced technologies, such as pottery, bricks, and melting of metals to make tools.

Philosophical attempts to rationalize why different substances have different properties (color, density, smell), exist in different states (gaseous, liquid, and solid), and react in a different manner when exposed to environments, for example to water or fire or temperature changes, led ancient philosophers to postulate the first theories on nature and chemistry. The history of such philosophical theories that relate to chemistry can probably be traced back to every single ancient civilization. The common aspect in all these theories was the attempt to identify a small number of primary elements that make up all the various substances in nature. Substances like air, water, and soil/earth, energy forms, such as fire and light, and more abstract concepts such as ideas, anther, and heaven, were common in ancient civilizations even in absence of any cross-fertilization; for example in Greek, Indian, Mayan, and ancient Chinese philosophies all considered air, water, earth and fire as primary elements.

Atomism can be traced back to ancient Greece and ancient India. Greek atomism dates back to 440 BC, as what might be indicated by the book De Rerum Natura (The Nature of Things) written by the Roman Lucretius in 50 BC. In the book was found ideas traced back to Democritus and Leucippus, who declared that atoms were the most indivisible part of matter. This coincided with a similar declaration by Indian philosopher Kanada in his Vaisheshika sutras around the same time period. In much the same fashion he discussed the existence of gases. What kanada declared by sutra, Democritus declared by philosophical musing. Both suffered from a lack of empirical data. Without scientific proof, the existence of atoms was easy to deny. Aristotle opposed the existence of atoms in 330 BC.