atom is the basic building block of ordinary matter. Atoms can be combined into molecules, but they cannot be divided into anything smaller by ordinary methods. The word atom is derived from the Greek word atomos, meaning “indivisible.”Each individual atom is made up of smaller particles called electrons, protons, & neutrons. These are called subatomic particles. At the center of an atom is a densely packed nucleus. The nucleus consists of protons & neutrons. Protons carry a positive electrical charge, while neutrons carry no electric charge. Together, protons & neutrons are called nucleons. Surrounding the nucleus is a cloud of negatively charged electrons.Atoms can have different numbers of protons & electrons, but they are all roughly the same size. Approximately 50,000,000 atoms lined up in a row would measure only about 0.4 inch (1 centimeter) long. A convenient unit of measurement for atomic sizes is the angstrom (Å), named after the Swedish physicist &ers Jonas Ångström. An angstrom is one ten-billionth of a meter, which means that each typical atom is 2Å across. As small as atoms are, they are immense compared to their nuclei. If an atom were a football field, the nucleus would be the size of a marble. Most of the atom consists of empty space.The single most important thing to know about an atom is how many protons it has in its nucleus. This is known as its atomic number. The atomic number determines what kind of atom it is. Every atom is associated with a specific chemical element as the smallest unit of that element, & each chemical element has a unique atomic number. For instance, hydrogen has an atomic number of 1 because it has one proton in its nucleus. No other element has an atomic number of 1.Another property of atoms is their atomic weight. This is roughly equal to the total number of protons & neutrons in an atom. Atoms that have the same atomic number but different atomic weights are called isotopes. Carbon-12, the ordinary form of carbon, has six protons & six neutrons per atom. Carbon-14 is an isotope with eight neutrons per atom. It still has six protons. If it did not have six protons it would not be carbon.An ordinary atom has an equal number of protons & electrons. Thus the positive & negative charges are balanced. Some atoms, however, lose or gain electrons in chemical reactions or in collisions with other particles. Atoms with a net charge, from either the gain or loss of electrons, are called ions. If a neutral atom loses an electron, it becomes a positive ion. If it gains an electron, it becomes negative ion.
The idea of the atom as the basic unit of all matter began with ancient Greek philosophers in the 5th century BC. In about 430 BC Democritus of Abdera gave the name atomos to the basic building blocks of matter. Democritus believed that atoms were uniform, solid, & hard & that they could not be compressed or destroyed. Although the atomic theories of the Greeks are important both historically & philosophically, they were not based on observations of nature, measurements, tests, or experiments.It was not until the 19th century that scientists began to underst& the true properties of atoms. Between 1803 & 1808, the British chemist & physicist John Dalton developed the idea of atomic weights. In 1811, an Italian scientist named Amedeo Avogadro declared that atoms differ from molecules. Later in the century scientists learned how the physical properties of the chemical elements were related to their atomic weights.The major subatomic particles were discovered between 1897 & 1932. Starting in the 1960s, physicists tried to look inside the subatomic particles to see what they were made of. By the mid-1980s the scientists had concluded that the subatomic particles were in turn made up of substances they called quarks & leptons. The world of quarks is strange indeed. Various types are called up quarks, down quarks, top quarks, bottom quarks, charmed quarks—& strange quarks. In the years that followed scientists continued to study atoms to learn more about what they are made of & how they act.
