|Themes > Science > Physics > Atomic Physics > Atomic Properties|
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.)
The proton and neutron are spherical, about
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
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.
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".