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# The Nucleus Help

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By — McGraw-Hill Professional
Updated on Sep 5, 2011

## Introduction to the Nucleus

The part of an atom that gives an element its identity is the nucleus . It is made up of two kinds of particles, the proton and the neutron . Both are extremely dense. Protons and neutrons have just about the same mass, but the proton has an electric charge, whereas the neutron does not.

## The Proton

Protons are too small to be observed directly, even with the most powerful microscopes. All protons carry a positive electric charge, and the charge on every proton is the same as the charge on every other. Every proton at rest has the same mass as every other proton at rest. Most scientists accept the proposition that all protons are identical, at least in our part of the universe, although they, like all other particles, gain mass if accelerated to extreme speeds. This increase in mass takes place because of relativistic effects; you’ll learn about this later.

While an individual proton is invisible and not massive enough to make much of an impact all by itself, a high-speed barrage of them can have considerable effects on matter. Protons are incredibly dense. If you could scoop up a level teaspoon of protons the way you scoop up a teaspoon of sugar—with the protons packed tightly together like the sugar crystals—the resulting sample would weigh tons in the Earth’s gravitational field. A stone made of solid protons would fall into the Earth and cut through the crustal rocks like a lead shot falls through the air.

## The Neutron

A neutron has a mass slightly greater than that of a proton. Neutrons have no electrical charge, and they are roughly as dense as protons. However, while protons last for a long time all by themselves in free space, neutrons do not. The mean life of a neutron is only about 15 minutes. This means that if you gathered up a batch of, say, 1 million neutrons and let them float around in space, you would have only about 500,000 neutrons left after 15 minutes. After 30 minutes, you would have approximately 250,000 neutrons remaining; after 45 minutes, there would be only about 125,000 neutrons left.

Neutrons can last a long time when they are in the nuclei of atoms. This is a fortunate thing because if it weren’t true, matter as we know it could not exist. Neutrons also can survive for a long time when a huge number of them are tightly squeezed together. This happens when large stars explode and then the remaining matter collapses under its own gravitation. The end product of this chain of events is a neutron star .

## The Simplest Elements

The simplest element, hydrogen, has a nucleus made up of only one proton; there are usually no neutrons. This is the most common element in the universe. Sometimes a nucleus of hydrogen has a neutron or two along with the proton, but this does not occur very often. These mutant forms of hydrogen do, nonetheless, play significant roles in atomic physics.

The second most abundant element in the universe is helium. Usually, this atom has a nucleus with two protons and two neutrons. Hydrogen is changed into helium inside the Sun, and in the process, energy is given off. This makes the Sun shine. The process, called atomic fusion or nuclear fusion , is also responsible for the terrific explosive force of a hydrogen bomb.

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