Chemistry is the branch of science concerned with the properties, structure, and composition of substances and their reactions with one another. There are two main divisions, organic and inorganic. Inorganic chemistry is concerned with the elements and all their compounds except those of carbon, which are the subject of organic chemistry. Physical chemistry studies their physical properties and structures and the relations between energy and physical and chemical change. Analytical chemistry is concerned with determining the composition of substances.
Origins of Chemistry
Its origins lie partly in ancient technology (eg metallurgy and soap manufacture), partly in mediaeval speculation on methods of obtaining gold (alchemy), and partly in early attempts to improve medicines (iatrochemistry). Antoine Lavoisier is usually considered the father of modern chemistry, with his distinction between elements and compounds formed from elements, and his insistence that chemical reactions are quantitative in nature.
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With the development of the atomic theory of John Dalton, chemistry evolved rapidly in the 19th-c. Organic chemistry originated from the isolation of medicinal compounds from animals and plants, and inorganic chemistry from the study of minerals. They developed mainly into the synthesis and study of molecular compounds and of ionic compounds respectively, but are now often indistinguishable.
Physical chemistry (studying the relationship of physical properties to chemical composition, structure, and reactivity) and analytical chemistry (studying the composition of material) also developed in parallel, particularly with 20th-c developments in spectroscopic and electrochemical methods. Modern chemistry has also been advanced by the elucidation of the nature of chemical
Chemistry today is the basis of a worldwide industry concerned with almost every aspect of life, including food, fuel, clothing, building materials, and medicines. While the growth of this industry has had some undeniably bad ecological effects, chemistry provides the key to the improvement of the environment and world living standards.
Organic chemistry originated with the isolation of medical compounds from animals and plants. It has expanded to include the reactions of carbon based compounds (which are 100 times more numerous than non-carbon based compounds) and the study of molecules.
An atom is the smallest piece of an element that has the element’s chemical properties. Chemistry deals with the interaction of atoms to form molecules; Chemists use this knowledge to guide them in their work in studying all existing chemical compounds and in making new ones.
Even living systems are made up of fundamental particles and, as studied in biology, biophysics and biochemistry, they follow the same types of laws as the simpler particles traditionally studied by a physicist.
Molecules and Crystals
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Atoms join or ‘bond’ together to form molecules by sharing their electrons. Chemical bonding is essentially the interaction of electrons from one atom with the electrons of another atom.
They might join together with others of the same kind. Oxygen molecules contain two identical oxygen atoms. Water molecules each contain two hydrogen atoms and one oxygen atom. When the atoms are of different elements, the substance formed is called a compound. For example, when two hydrogen atoms join together with an oxygen atom, water is formed – H2O.
Chemically bonding occurs when two particles can exchange or combine their outer electrons in such a way that is energetically favorable. An energetically favorable state can be seen as analogous to the way a dropped rock has a natural tendency to fall to the floor.
When two atoms are close to each other and their electrons are of the correct type, it is more energetically favorable for them to come together and share electrons (become “bonded”) than it is for them to exist as individual, separate atoms.
When the bond occurs, the atoms become a compound. Like the rock falling to the floor, they “fall” together naturally.
Molecules in turn can join up in regular patterns, and these are what we know as crystals. Salt is made of molecules of sodium chloride (sodium, Na, and chlorine, Cl) or NaCl.
Molecules can get very complicated. Some proteins contain thousands of atoms per molecule, largely carbon, oxygen, hydrogen, and nitrogen. One of the most complex is DNA (deoxyrobonucleic acid) which contains genetic information of living things, such as how our bodies grow.
The properties and reactions of chemical elements and compounds (color, melting temperature, hardness, solubility, etc) are due to the electrical charges on atoms and also to how they are arranged.
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When writing names of elements, a chemist usually uses abbreviations, since they are quicker to write than the names. The abbreviations that we use are called symbols. None of the symbols contain more than two letters; the first one is always capitalized and the second, if any, is always lower case.
Examples: Hydrogen (H), Carbon (C), Oxygen (O), etc. If the second letter of the symbol is capitalized, then it represents a molecule; for example CO represents a molecule of Carbon Monoxide.
The symbols for all known elements are given in the
Periodic Table of the Elements.
A chemical formula is one or more symbols that represents a substance. The symbols in a formula identify the elements in the substance. NaCl is the formula for sodium chloride (common table salt) and so identifies the elements sodium and chlorine as the constituents of salt.
Subscripts are used in formulas to indicate the relative numbers of atoms of each type in the compound, but only if more than one atom of a given element is there. The formula for water, H20, shows that each molecule contains two atoms of hydrogen and one atom of oxygen.
Compounds differ from elements because they contain more than one type of atom. For example,
both hydrogen and oxygen are elements because they contain only one type of atom; however, as water contains both hydrogen and oxygen atoms, it is a compound.
Any given compound is formed using a specific “recipe” of two or more different elements. According to the Law of Definite Composition (or the Law of Definite Proportions), any given compound always contains the same elements in the same mass proportions.
For example, any water molecule (H2O) has a mass that is 11.21% hydrogen and 88.79% oxygen. Similarly, any molecule of sodium chloride (NaCl) is composed of 39.34% sodium and 60.66% chlorine by mass.
It is also possible for two elements to combine with one another in different ratios and form more than one compound. This relationship is known as the Law of Multiple Proportions. For example, the elements carbon and oxygen can combine with each other to form two different compounds: carbon monoxide (CO) and carbon dioxide (CO2).