

Size

The atom is about 10-10 meters (or 10-8 centimeters) in size. This means a row of 108 (or 100,000,000) atoms would stretch a centimeter, about the size of your fingernail. Atoms of different elements are different sizes, but 10-10 m can be thought of as a rough value for any atom. It is also a good approximation to think of atoms as spherical in shape, although they are not always so. The atom with the smallest mass is the hydrogen atom; its mass is about 10-27 kg. The masses of other atoms go up to about 200 times this. The nucleus of an atom is about 10-15 m in size; this means it is about 10-5 (or 1/100,000) of the size of the whole atom. A good comparison of the nucleus to the atom is like a pea in the middle of a racetrack. (10-15 m is typical for the smaller nuclei; larger ones go up to about 10 times that.)



Mass

Although it is very small, the nucleus is massive compared to the rest of the atom. Typically the nucleus contains more than 99.9% of the mass of the atom. (Hence the numbers given above for masses of atoms also apply approximately to the nucleus alone.) Nuclei are usually spherical in shape, although some are spheroidal (egg-shaped).



Subatomic particles



The nucleus is made up of protons and neutrons bound together by attractive forces. The outer volume of the atom (which means most of the atom) is occupied by electrons. An electron itself is small (its size is not known, but we do know that it is smaller than a nucleus), but it occupies the space of the atom by constantly whirling around in a kind of orbit around the nucleus. The proton and neutron are spherical, about

10-15 m in radius. The proton and neutron have almost the same mass - the neutron's is slightly larger. These masses are more than 2000 times the mass of the electron. That is why the nucleus has most the atom's mass.



Forces inside the Atom

The electron has a negative electric charge; the proton has a positive electric charge of exactly the same strength; the neutron has no electric charge. Like charges repel and unlike charges attract. The nucleus is a dense ball of positive charge in the center of the atom and it exerts an attractive force on the electrons, thereby holding them as part of the atom. In other words, atoms would not exist if it were not for the electric force.



Stability of the Atom

Why don't the electrons fall into the nucleus under the influence of this force? The atom looks superficially like the solar system, with its planets in orbit around the central sun. But the reason for the atom's stability -- the fact that the electron's orbit does not collapse -- lies in the fundamental nature of quantum mechanics, the science that supersedes Newton's mechanics in the world of the atom. The electron can exist only in one of a discrete set of "energy states", and the lowest energy state is stable. The electron jumps from one state to another when it receives or emits a quantum of energy in the form of light (or other form of electromagnetic radiation).



The Nuclear Force

Inside the nucleus all the electric forces are repulsive because the protons repel each other, and the neutrons don't feel any electric force. How then is the nucleus held together? There is another force, called the nuclear force , that is mostly attractive and acts between two protons, between two neutrons, and between a neutron and proton. In nuclei this force is stronger than the repulsive electric force and so nuclei are held together.