Although there are countless types of matter all around us, this complexity is composed of various combinations of some 100 chemical elements. The names of some of these elements will be familiar to almost everyone. Elements such as hydrogen, chlorine, silver, and copper are part of our everyday knowledge. Far fewer people have heard of selenium or rubidium or hassium.
The periodic table has its origins in the early 19th century, when John Dalton first calculated the relative weights of atoms and compounds. Although the method for calculating atomic weights was disputed for another 50 years, in the long run atomic weights provided the key to organizing the elements into the periodic table.
Chemistry and physics overlap at the atomic and nuclear level. Appropriately, several of the pioneers in the study of atomic and nuclear structure are more commonly identified as physicists, but the line between chemist and physicist at this level is hard to draw, and the Nobel prizes for this kind of work are granted in both categories.
Determining the structure of the biomolecule deoxyribonucleic acid (DNA) was arguably the most important chemical discovery of the 20th century. This structure implied how genetic material is passed along from generation to generation, and from cell to cell in a living being. Soon scientists were studying how these replications happen in finest detail, making possible a new understanding of heredity and of hereditary disease. In turn, these discoveries led to a new biotechnology beyond antibiotics.
The earliest record of man's interest in chemistry was approximately 3,000 B.C, in the fertile crescent. At that time, chemistry was more an art than a science. Tablets record the first known chemists as women who manufactured perfumes from various substances. Ancient Egyptians produced certain compounds such as those used in mummification. By 1000 B.C,
a compound is a substance containing more than one element. Since the concept of an element is macroscopic and the distinction between elements and compounds was recognized long before the existence of physical atoms was accepted, the concept of a compound must also be a macroscopic one that makes no assumptions about the nature of the ultimate .
A chemical property, on the other hand, describes a chemical change: the interaction of one substance with another , or the change of one substance into another. Iron rusts in a moist environment, unrefrigerated milk turns sour, wood burns in air, photographs bleach when exposed to sunlight for a long time, dynamite explodes - each of these is a chemical property because each involves chemical change. During chemical changes, substances are actually changed into other substances. The simultaneous disappearance of some substances (called the reactants) and appearance of others (the products) is characteristic in chemical change (chemical reaction). Chemical changes are generally characterized by pronounced internal structural rearrangements.
One of the goals of chemistry is to be able to describe the properties of matter in terms of its internal structure, the arrangement and interrelationship of its parts. This word, structure, sometimes refers to the physical arrangement of particles, such as atoms or molecules in space. At other times it is used to indicate some other arrangement, such as the arrangement of energy levels of an electron in an atom. The structure of matter determines its properties. Properties can be classed as either physical or chemical. A physical property of a substance can be characterized without specific reference to any other substance and usually describes the response of the substance to some external influence, such as heat, light, force, electricity, etc. Physical properties include boiling point, melting point, thermal (heat) conductivity, colour, refractive index, viscosity, reflectivity, hardness, tensile strength, and electrical conductivity.
At the most fundmental level, chemistry can be organized along the lines shown here.
Dynamics refers to the details of that rearrangements of atoms that occur during chemical change, and that strongly affect the rate at which change occurs.
Energetics refers to the thermodynamics of chemical change, relating to the uptake or release of heat. More importantly, this aspect of chemistry controls the direction in which change occurs, and the mixture of substances that results.
Composition and structure define the substances that are results of chemical change. Structure refers specifically to the relative arrangements of the atoms in space. The extent to which a given structure can persist is itself determined by energetics and dynamics.
Synthesis, strictly speaking, refers to formation of new (and usually more complex) substances from simpler ones, but in the present context we use it in the more general sense to denote the operations required to bring about chemical change and to isolate the desired products.
chemistry is the study of matter and the changes it can undergo. Chemists sometimes refer to matter as ‘stuff’, and indeed so it is. Matter is anything that has mass and occupies space. Which is to say, anything you can touch or hold. Common usage might have us believe that ‘chemicals’ are just those substances in laboratories or something that is not a natural substance. Far from it, chemists believe that everything is made of chemicals.