Protons, neutrons, and the atomic number

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. These are extremely dense. A teaspoonful of either of these particles, packed tightly together, would weigh tons. Protons and neutrons have just about the same mass, but the proton has an electric charge while the neutron does not.

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 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 fusion, is also responsible for the terrific explosive force of a hydrogen bomb.

Every proton in the universe is just 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 ithium, a light metal that reacts easily with gases such as oxygen or chlorine. The element with four protons is beryllium, also a metal. In general, as the number of protons in an element’s nucleus increases, the number of neutrons also increases. Elements with high atomic numbers, like lead, are therefore much denser than elements with low atomic numbers, like carbon. Perhaps you’ve compared a lead sinker with a piece of coal of similar size, and noticed this difference.

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Atoms

All matter is made up of countless tiny particles whizzing around. These particles are extremely dense; matter is mostly empty space. Matter seems continuous because the particles are so small, and they move incredibly fast.

Even people of ancient times suspected that matter is made of invisible particles. They deduced this from observing things like water, rocks, and metals. These substances are much different from each other. But any given material—copper, for example—is the same wherever it is found. Even without doing any complicated experiments, early physicists felt that substances could only have these consistent behaviors if they were made of unique types, or arrangements, of particles. It took centuries before people knew just how this complicated business works. And even today, there are certain things that scientists don’t really know. For example, is there a smallest possible material particle?

There were some scientists who refused to believe the atomic theory, even around the year of 1900. Today, practically everyone accepts the theory. It explains the behavior of matter better than any other scheme.

Eventually, scientists identified 92 different kinds of fundamental substances in nature, and called them elements. Later, a few more elements were artificially made. Each element has its own unique type of particle, known as its atom. Atoms of different elements are always different. The slightest change in an atom can make a tremendous difference in its behavior.

You can live by breathing pure oxygen, but you can’t live off of pure nitrogen. Oxygen will cause metal to corrode, but nitrogen will not. Wood will burn furiously in an atmosphere of pure oxygen, but will not even ignite in pure nitrogen. Yet both are gases at room temperature and pressure; both are colorless, both are odorless, and both are just about of equal weight. These substances are so different because oxygen has eight protons, while nitrogen has only seven.

There are many other examples in nature where a tiny change in atomic structure makes a major difference in the way a substance behaves.

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