By Don Lincoln, Ph.D., University of Notre Dame
Protons and neutrons are the famous heavy particles of an atom nucleus. The nucleus is significantly smaller than the atom, but very dense and heavy. The electrons orbit the nucleus far away, creating the considerable empty space in the atom. Is the heavy nucleus compactly filled with solid particles? No. The nucleus also has significant empty space inside.
Most of an atom’s mass is concentrated in its nucleus. The atom nucleus is surrounded by extremely small and light electrons that spin around in the atom but do not disturb the significant empty space between the nucleus and the atom boundaries. To get a clear image of the atom nucleus, one must first know what goes on with the electrons and the rest of the atom.
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Electrons around the Atom Nucleus
The electrons have their specific orbits, namely, S, P, D, F, and then G. Each orbit is a spherical shell, and the names are derived from the spectrum of light emitted from these shells, describing the character of the spectral lines. Another fact about the electrons is that they are everywhere that quantum laws allow.
In a carbon atom, for example, there are six electrons. Two of them occupy a spherical shell at the center of the atom, and the remaining four are distributed in a mixture of spherical shells and three lobed shells. At the same time, they have a negative electric charge. Thus, the nucleus is surrounded by something like clouds of a negative charge, and electrons that are everywhere they can be, but not filling the empty spaces. The electrons help make the molecules.
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Electron Bonds among Atoms
If two carbon atoms get close enough, their closest electrons interact and create a single bond. This bond is called the sigma bond in chemistry. The clouds then bend over and connect, creating a ‘pi bond’ that looks somehow like trees connecting top branches over a street.
The bonds get more and more complicated in different situations, and that is beyond the scope of this article. Nevertheless, one thing remains constant in all of these bonds: electrons are still dispersed in a relatively huge cloud around a very dense nucleus, and there is still much empty space. The electric fields and the electron clouds hold this vast emptiness together. What is inside the atom nucleus, then?
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Protons and Neutrons
The nucleus of an atom is made up of protons, discovered in 1920 by Ernest Rutherford, and neutrons, discovered in 1932 by James Chadwick. Both protons and neutrons are like little spheres with a radius of 10-15m or a quadrillionth of a meter. Both particles are somehow stuck together in the nucleus.
When the atom is heated, it radiates visible light, but when the nucleus is heated up, it emits gamma rays. Gamma is 100,000 or even a million times more energetic than the visible light. Many models try to describe how the nucleus particles behave, but the shell model has been the most successful so far. It puts the protons and neutrons in different energy shells to describe the gamma emission. Does this mean the protons and neutrons are solid little marbles?
Inside the Protons and Neutrons
In the 1950s, science realized that protons and neutrons were made of smaller particles. Some years later, in 1964, American physicist Murray Gell-Mann introduced quarks. He did not know how many types of quarks exist, but today there are at least six quarks discovered: up, down, charm, strange, top, and bottom.
The up and down quarks are inside protons and neutrons. The other four have a very short lifetime and are just found in particle accelerators. The proton contains two up quarks and one down quark. On the other hand, the neutron has one up quark and two down quarks. What holds these quarks together in the one-femtometer sphere is the strongest force discovered so far: the strong force.
If the proton is considered as a basketball, each quark would be smaller than a grain of sand. Consequently, much of the proton and neutron is also empty space, while the quarks zoom around at near the speed of light.
The small nuclei hold particles so small that our strongest and most precise equipment cannot see. At the same time, they hold the strongest force ever in the empty spaces and the super-tiny particles called quarks.
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Common Questions about Atom Nucleus
An atom nucleus is made up of protons and neutrons. Protons bear a positive charge equal to the electrons orbiting around, and the neutrons bear no charge but weigh the same as the protons.
The atom nucleus creates the force required to keep the atom together and in order. It is the heaviest part of the atom and is very dense. The size of the nucleus against the whole atom is like a marble in a football field.
The nucleus of the atom bears a positive electric charge. However, the atom is neutral as the number of electrons with a negative charge is equal to the protons in the nucleus.
The nucleus of an atom is the atom’s dense and heavy core with a positive charge. It is considerably smaller than the atom but is much heavier than the rest of it.