Atoms - Introduction
Any sample of matter is either an element or is composed of elements. An
element is a substance that cannot be resolved into any other substances. Sulfur,
copper, carbon, oxygen, iron are examples of chemical elements. An atom is the
smallest particle, which has a nucleus and one or more electrons that move about
the nucleus. The nucleus lies at the center of the atom and consists of two types of
particles, protons and neutrons.
Almost all the mass of an atom is in the nucleus. Protons bear a positive
electric charge. Electrons carry a negative charge, equal in magnitude and opposite
to the charge of a proton. Neutrons are not charged (neutral). Protons and neutrons
are nearly the same size and weight. An electron is 1/1836 the mass of a proton
and is very much smaller. Atoms are extremely small, on the order of a few ten
billionths of a meter.
An atom as a whole is electrically neutral. A negative cloud of electrons
surrounds the positive nuclear core. A neutral atom has equal numbers of electrons
and protons. Almost all nuclei contain roughly equal numbers of neutrons and
protons. Since all nuclei contain protons, all nuclei are positively charged.
An atom with an excess or deficit of electrons is charged and is called an
ion. Ions have very different properties than the neutral atom and are very
important in reactions. A body made of sodium atoms is a highly reactive soft
metal. The sodium ions in table salt have given up a single electron and have very
different properties.
The various elements are distinguished by the number of protons in the
nucleus of their atoms. The number of protons in the nucleus is called the atomic
number. There are ninety-two naturally occurring elements, each with a unique
atomic number. Several other elements have been synthesized. The nucleus of the
smallest atom, the commonest form of hydrogen, consists of a single proton and
no neutrons. All other nuclei contain neutrons as well as protons.
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Almost all hydrogen nuclei consist of only a proton. A very small number
of hydrogen atoms have a proton and a neutron in the nucleus making it twice as
heavy as the commonest form. This is known as deuterium. An even smaller
number of hydrogen atoms have two neutrons and a proton in the nucleus. This is
called tritium. All atoms with one proton are hydrogen atoms.
So the hydrogen nucleus can have 0, 1, or 2 neutrons. All three have the
same chemical properties. The variations of nuclear weights of an element due to
different numbers of neutrons are called isotopes. Some are unstable and
radioactive.
The mass of an atom is expressed in atomic mass units (amu). The
reference used presently is to assign the value of 12.000 amu to the most common
isotope of carbon that contains 12 particles, 6 protons and 6 neutrons. Measured
atomic weights are averages including the various isotopes.