Protons neutrons
atomic symbol
atomic number
chemical element
Atoms are tiny pieces of matter that make up all the substances around you. All atoms
contain one or more positively charged particles called
, which are
found in an atom's nucleus. In many atoms, the nucleus also contains
which have no electrical charge. There are also small, negatively
charged particles called electrons found outside the nucleus. Together, these three types
of particles make up the structure of atoms.
The number of protons in an atom is the atom's
When two atoms
have the same atomic number, they are are classified as the same
All known chemical elements are listed in the periodic table, where they are arranged by
atomic number. The periodic table contains information about each element, often
including a short version of the element's full name called its

< > according to newton's third law of motion, forces always act in equal but opposite pairs. another way of saying this is for every action, there is an equal but opposite reaction.< >

most historians agree that gabriel mouton, the vicar of st. paul’s church in lyons, france, is the “founding father” of the metric system. he proposed a decimal system of measurement in 1670. mouton based it on the length of one minute of arc of a great circle of the earth (now called a nautical mile, 1852 meters). he also proposed the swing-length of a pendulum with a frequency of one beat per second as the unit of length (about 25 cm). a pendulum beating with this length would have been fairly easy to produce, thus facilitating the widespread distribution of uniform standards. over the years, his work was revised, improved, and extended by a number of french scientists.

the political sponsor of weights and measures reform in the french revolutionary national assembly was the bishop of autun, better known as talleyrand. under his auspices, the french academy appointed several committees to carry out the work of developing a usable system of weights and measures for france. one of the committees recommended a decimalized measurement system based upon a length equal to one ten-millionth of the length of a quadrant of the earth’s meridian (i.e., one ten-millionth of the distance between the equator and the north pole).