27.1 - Strong nuclear force and the nucleus

What holds the particles of the nucleus together?The nucleus of the atom contains both protons and neutrons. Protons are positively charged and neutrons do not have any charge. The coulomb repulsion between protons as close together as they are in a nucleus is enormously strong and should tear the nucleus apart instantly. Why does the nucleus stay together? The fact that atomic nuclei are stable implies that another type of force must exist that is attractive and much stronger than the coulomb force. Read the text aloud
Evidence for the strong nuclear force
When Rutherford discovered in 1909 that the nucleus of the atom was tiny and had positive charge, the evidence shed no light on whether the nucleus had any structure or was one particle. In 1918, Rutherford discovered the proton by observing that hydrogen atoms are sometimes produced when nitrogen atoms are bombarded with alpha particles. A hydrogen nucleus contains a single proton. In 1932, James Chadwick discovered another electrically neutral elementary particle called the neutron and measured its mass to be nearly the same as the proton. Physicists then knew that there were individual protons and neutrons in the nucleus. They were then faced with the problem of what holds the nucleus together? Read the text aloud
The strong nuclear force overcomes the electric repulsion of protons in the nucleus
The primary evidence for the existence of a strong nuclear force is that otherwise the nucleus would immediately disintegrate from the powerful repulsion of positively charged protons. Since we observe that many nuclei do not fly apart, we deduce there must be a strong force attracting protons and neutrons. This force is called the strong nuclear force and it is a short-range attractive force acting between protons and neutrons (but not electrons). Modern particle physicists usually refer to the strong nuclear force as the strong interaction. Of course, knowing a force exists does not explain what causes the force! The next step in the development of nuclear physics was to develop a theory to explain the strong force.
Read the text aloud Show The four fundamental forces
In 1935, Japanese theorist Hideki Yukawa proposed a model in which the strong force would work through a hitherto unknown type of massive, elementary particle called the meson. Yukawa named his predicted particles after the Greek word mesos (μέσος)—which means “intermediate”—for the role mesons play in mediating the strong force. Experimental confirmation of Yukawa’s theory came in 1947 when an international team led by English physicist Cecil Power discovered the first meson, a pi meson (or pion). Read the text aloud
The strong interaction not only binds the atomic nucleus together, it also governs the interactions among quarks, the elementary particles from which protons and neutrons are made. The existence of quarks was independently proposed by Murray Gell-Mann and George Zweig in 1962. The first quark was experimentally confirmed in 1968 at the Stanford Linear Accelerator Center, providing additional evidence for the strong interaction and the theory behind it. Read the text aloud Show Where does the word quark come from?
Who discovered the strong nuclear force? Show

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