An Overview of Chemistry

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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.

Inorganic chemistry studies the preparation, properties, and reactions of all chemical elements and compounds except those that are carbon based.

The study of chemistry began as the combination of ancient technologies, such as metallurgy and soap making, the medieval attempts to make gold, and efforts to improve medicine. Antoine Lavoisier is considered to be the father of modern chemistry because of his distinction between elements and compounds and his insistance that chemical reactions are quantitative. John Dalton's development of the atomic theory caused chemistry to evolve quickly. During the 20th century, two new divisions have developed: physical chemistry, the study of the relationship of physical properties to chemical composition, structure, and reactivity; and analytical chemistry, the study of the composition of material.

Atomic Structure

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

Different chemical elements have different kinds of atoms; the atoms of any particular element are all identical to each other (except for the posibility of having electrical charge).

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.

Chemical Formulae

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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

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).