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. Why don't the electrons fall into the nucleus under the influence of this force? The answer is the same answer we would give to the question of why the moon doesn't fall to the earth, or why the planets don't fall into the sun. (In these cases the force is gravity rather than electricity, but it is still an attractive force.) The electron is in an orbit, and so the effect of the attractive force is to make it go in a curved path - a circular (or possibly elliptical) path around the nucleus.

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.

Atoms, Molecules, and Matter in Bulk

In an atom the number of electrons equals the number of protons, N_e = N_p. It is possible to have a system with N_e not equal to N_p, but then it is not called an atom; it is called an ion. On earth most of the matter we come in contact with is made of atoms. However, in the sun and the stars most of the matter is in the form of ions. It is probable that most of the matter in the universe is in the form of ions. The number of neutrons in the nucleus is in many cases approximately equal to the number of protons.

A small number of atoms may combine to form a molecule such as water (H₂O) or carbon dioxide (CO₂. The arrangements and rearrangements of atoms in molecules form the domain of chemistry. A solid consists of a regular array or lattice containing a very large number molecules or atoms. A typical number of atoms in a piece of matter on a human scale is 10²⁴. Physical properties of matter include hardness, malleability, color, and melting point.

The important point here is that chemical and physical properties of matter depend only on the electron cloud surrounding the atoms. Therefore they depend only on the number of protons in the nucleus. This number is symbolized by Z, and is called the "atomic number".

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