Atom is the fundamental building block of all stuff, or what scientists like to call "matter". An individual atom is very small. In fact, the smallest type of atom, hydrogen, has a diameter of 10-8 cm. Every single object is composed of atoms. Our body is made up of many, many individual atoms. There are also many different types of atom. These different types are called elements. Examples of some elements are hydrogen, oxygen, and helium. Under normal conditions many atoms can stick together to form larger, different stuff. Scientists call material that results from the joining of different types of atoms "compounds". Atoms are not the smallest things there but are made up of still smaller stuff. Also, atoms can be broken apart into smaller constituents.
An early Greek who postulated the existence of atoms Democritus (460-357 BCE), who said the matter is made of different kinds of invisible atoms existing in otherwise empty space. Democritus made no attempt to think of a way to determine whether atoms exist or not. Evidently, the idea of atoms just appealed to him philosophically.
The idea that matter could be infinitely subdivided appealed to other Greek philosophers, among them Aristotle, seldom made any attempt to verify their hypotheses. Although Aristotle emphasized observation, he did not restrict his theoretical ideas to object he could observe. Aristotle was a great philosopher but his science left much to be desired. Many theories go beyond observation
In the 1800's an English chemist, John Dalton performed experiments with various chemicals that showed that matter, indeed, seem to consist of elementary lumpy particles (atoms). Although he did not know about their structure, he knew that the evidence pointed to something fundamental. It was discovered that many substances, called “compounds,” are made of simpler substance, called “elements,” which combine in definite proportions. The smallest amount of a compound is called a molecule, and the smallest amount of an element is called an atom. For example, water is a compound, made by combining two elements, hydrogen and oxygen, in definite proportions. By weight, there are eight parts oxygen for every part hydrogen. It was further discovered that there exists a smallest amount of water, a single molecule. This molecule is made of two atoms of hydrogen and one atom of oxygen. The reason that the oxygen in water weighs eight times as much as the hydrogen is that a single atom of oxygen weighs 16 times as much as a single atom of hydrogen. The number of each kind of atom combining into molecule is exact, but the fact that an atom of oxygen is 16 times as heavy as an atom of hydrogen is not exact.
There are more than 100 elements, which some are unstable and do not ordinary exist on the earth but decay in a process known as “radioactivity” into other elements. Most of the known unstable elements have been made artificially. The lightest stable element is hydrogen. The heaviest element naturally occurring on earth in sizeable amounts is uranium, which is unstable but is slow to decay that a good fraction of the amount that was on the earth when it was formed still exists.
An ancient pursuit of obscure origin was called alchemy, and had as one of its principal aims, the transmutation of so-called base elements, like lead, into gold. Isaac Newton was one of the greatest physicists who ever lived, but he devoted years of his life to alchemy. The alchemy of the ancient and medieval periods and of Newton really had no chance to succeed because the experiments were chemical. We now know that the nature of a chemical element is determined by the nuclei of its atoms, and so any transmutation of the elements must involve nuclear physics. The existence of atomic nuclei was not known until early in the 20th century. In 1905, Einstein calculated that if dust particles were struck randomly by molecules, they would undergo just the sort of motion...
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