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.
Atomic Number
According to modern atomic theory, every proton in the universe is exactly like every other. Neutrons are all alike too. The number of protons in an element’s nucleus, the atomic number , gives that element its identity.
The element with three protons is lithium , a light metal that reacts easily with gases such as oxygen or chlorine. Lithium always has three protons; conversely, any element with three protons in its nucleus must be lithium. The element with four protons is beryllium , also a metal. Carbon has six protons in its nucleus, nitrogen has seven, and oxygen has eight. In general, as the number of protons in an element’s nucleus increases, the number of neutrons also increases. Elements with high atomic numbers, such as lead, are therefore much more dense than elements with low atomic numbers, such as carbon. Perhaps you’ve compared a lead shot with a piece of coal of similar size and noticed this difference.
If you could somehow add two protons to the nucleus of every atom in a sample of carbon, you would end up with an equal number of atoms of oxygen. However, this is much easier said than done, even with a single atom. It is possible to change one element into another; the Sun does it all the time, fusing hydrogen into helium. The process is far from trivial, though. In ancient times, alchemists tried to do this; the most well-known example of their pursuits was the quest to turn lead (atomic number 82) into gold (atomic number 79). As far as anyone knows, they never succeeded. It was not until the 1940s, when the first atomic bombs were tested, that elements actually were “morphed” by human beings. The results were quite different from anything the alchemists ever strove for.
Table 9-1 lists all the known elements in alphabetical order, with the names of the elements in the first column, the standard chemical symbols in the second column, and the atomic numbers in the third column.
Table 9-1 The Chemical Elements in Alphabetical Order by Name, Including Chemical Symbols and Atomic Numbers 1 through 118 (As of the time of writing, there were no known elements with atomic numbers 113, 115, or 117.)
Isotopes
In the individual atoms of a given element, such as oxygen, the number of neutrons can vary. Regardless of the number of neutrons, however, the element keeps its identity based on the atomic number. Differing numbers of neutrons result in various isotopes for a specific material element.
Each element has one particular isotope that is found most often in nature. However, all elements have more than one isotope, and some have many. Changing the number of neutrons in an element’s nucleus results in a difference in the mass, as well as a difference in the density, of the element. Thus, for example, hydrogen containing a neutron or two in the nucleus, along with the proton, is called heavy hydrogen . The naturally occurring form of uranium has three more neutrons in its nucleus than the type that is notorious for use in atomic weapons.
Adding or taking away neutrons from the nucleus of an element is not quite as farfetched a business as adding or taking away protons, but it is still a task generally relegated to high-energy physics. You can’t simply take a balloon filled with air, which is approximately 78 percent nitrogen, and make it more massive by injecting neutrons into the nitrogen nuclei.
Atomic Mass
The atomic mass , sometimes called the atomic weight , of an element is approximately equal to the sum of the number of protons and the number of neutrons in the nucleus. This quantity is formally measured in atomic mass units (amu), where 1 amu is equal to exactly 1/12 the mass of the nucleus of the carbon isotope having six neutrons. This is the most common isotope of carbon and is symbolized 12 C or carbon-12. Any proton or any neutron has a mass of approximately 1/12 amu, but neutrons are a little more massive than protons.
Elements are uniquely defined by their atomic numbers, but the atomic mass of an element depends on the particular isotope of that element. A well-known isotope of carbon, 14 C, is found in trace amounts in virtually all carbon-containing substances. This fact has proven quite useful to geologists and archaeologists. The isotope 14 C is radioactive, whereas 12 C is not. The radioactivity of 14 C diminishes with time according to a well-known, predictable mathematical function. This makes it possible for researchers to determine when carbon-containing compounds were created and thus to find out how old various rocks, fossils, and artifacts are.
In nuclear reactions capable of producing energy, such as the reactions that take place inside stars, atomic bombs, and nuclear power plants, a certain amount of mass is always given up—and converted into energy—in the transactions between the atoms. This amount of mass can be exceedingly small yet produce an enormous burst of energy. The first person to formalize this relation was Albert Einstein, using his famous equation
E = mc 2
where E is the energy produced in joules, m is the total mass in kilograms lost during the reaction, and c is the speed of light in meters per second. The value of c 2 is gigantic: approximately 90 quadrillion meters squared per second squared (9 × 10 16 m 2 /s 2 ). This is why so much energy can be produced by an atomic reaction between two elemental samples of modest mass.
An excellent source of information concerning all the known elements, including atomic number, atomic mass, and various other characteristics, can be found at the following Web site:
http://www.chemicalelements.com/
If you have a computer with Internet access, it would be a good idea spend a while exploring this Web site right now.
The Nucleus Practice Problem
Problem
Suppose that the nucleus of an oxygen atom, which has eight protons and usually has eight neutrons, were split exactly in two. What element would be the result? How many atoms of this element would there be? Neglect, for simplicity, any energy that might be involved in the reaction.
Solution
This reaction would produce two atoms of beryllium, each with four protons and four neutrons. This would not be the most common isotope, however; it turns out that beryllium usually has five neutrons in its nucleus.
Practice problems of these concepts can be found at: Particles of Matter Practice Test
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