http://www.ck12.org Chapter 2. Matter
Inside the atom, two types of subatomic particles have electric charge: electrons, which have an electric charge of
-1, and protons, which have an opposite but equal electric charge of +1. The model of an atom in theFigure2.62
shows both types of charged particles. Protons are found inside the nucleus at the center of the atom, and they give
the nucleus a positive charge. (There are also neutrons in the nucleus, but they have no electric charge.) Negative
electrons stay in the area surrounding the positive nucleus because of the electromagnetic force of attraction between
them.
FIGURE 2.62
Q:Why do you think protons cluster together in the nucleus of the atom instead of repelling each other because of
their like charges?
A:The electromagnetic force of repulsion between positively charged protons is overcome by a stronger force,
called the strong nuclear force.
Strong Nuclear Force
The strong nuclear force is a force of attraction between fundamental particles called quarks, which have a type of
charge called color charge. The strong nuclear force is transferred between quarks by fundamental force-carrying
particles called gluons. Both protons and neutrons consist of quarks. The exchange of gluons holds quarks together
within a proton or neutron. Excess, or residual, strong force holds together protons and neutrons in the nucleus.
The strong nuclear force is strong enough to overcome the electromagnetic force of repulsion pushing protons apart.
Both forces are represented in theFigure2.63.
The strong nuclear force works only over very short distances. As a result, it isn’t effective if the nucleus gets too
big. As more protons are added to the nucleus, the electromagnetic force of repulsion between them gets stronger,
while the strong nuclear force of attraction between them gets weaker. This puts an upper limit on the number of
protons an atom can have and remain stable. If atoms have more than 83 protons, the electromagnetic repulsion
between them is greater than the strong nuclear force of attraction between them. This makes the nucleus unstable,